Effects of Hydroponic Cultivation on Baby Plant Characteristics of Tetragonia tetragonioides (Pallas) O. Kunze at Harvest and During Storage as Minimally Processed Produce

Abstract

Tetragonia tetragonioides, or New Zealand spinach, is a widespread halophyte native to eastern Asia, Australia, and New Zealand, and naturalized in some Mediterranean regions. This underutilized vegetable is consumed for its leaves, raw or cooked. For the first time, we investigated the feasibility of using whole baby plants (including stems and leaves) as raw material for ready-to-eat (RTE) vegetable production. Our study assessed Tetragonia’s suitability for hydroponic cultivation over two cycles (autumn–winter and spring). We investigated the impact of increasing nutrient rates (only water, half-strength, and full-strength nutrient solutions) and plant densities (365, 497, and 615 plants m⁻² in the first trial and 615 and 947 plants m⁻² in the second) on baby plant production. We also analyzed the plants’ morphological and biochemical characteristics, and their viability for cold storage (21 days at 4 °C) as a minimally processed product. Tetragonia adapted well to hydroponic cultivation across both growing periods. Nevertheless, climatic conditions, plant density, and nutrient supply significantly influenced plant growth, yield, nutritional quality, and post-harvest storage. The highest plant density combined with the full-strength nutrient solution resulted in the highest yield, especially during spring (1.8 kg m⁻²), and favorable nutritional characteristics (β-carotene, Vitamin C, Fe, Cu, Mn, and Zn). Furthermore, Tetragonia baby plants proved suitable for minimal processing, maintaining good quality retention for a minimum of 14 days, thus resulting in a viable option for the RTE vegetable market.

1. Introduction

In the last decades, the consumer’s demand for fresh leafy vegetables and ready-to-eat products has greatly increased as they experienced a wide change in eating habits arising from the need for a healthier lifestyle [1]. Fruits and vegetables, and among them leafy vegetables, are considered an important component of a healthy diet due to their content in vitamins, minerals, antioxidants, and other important phytonutrients [2]. The increasing consumer demand for diverse and nutritious fresh-cut vegetables has prompted researchers and growers to explore unconventional crops, such as underutilized vegetable crops, that can meet these needs. Underutilized vegetable crops are all of the crops which are not traded widely or farmed commercially on a large scale according to the definition gave by Jaenicke and Hoeschle [3]: “species with underexploited potential for contributing to food security, health (nutritional/medicinal), income generation, and environmental services”. The reasons for the low spread of underutilized vegetables, although highly appreciated in some geographic areas, can be found in the low availability of propagation material and in the lack of information on agronomic management techniques and on their nutritional and medicinal importance [4]. Therefore, it would be necessary to start research activities on the management, use, and improvement of these genetic resources to offer a wider choice to farmers and consumers and, at the same time, ensure food and nutritional security.

Tetragonia tetragonioides (Pallas) O. Kunze, commonly known as New Zealand spinach, is a halophytic herbaceous plant recognized for its distinct flavor and impressive nutritional profile. Native to Australia, New Zealand, and eastern Asia, it is now widely distributed throughout the world and it is naturalized in some Mediterranean areas [5,6,7]. Historically utilized in traditional cuisines as well as to treat many diseases, its leaves can be eaten raw as a salad vegetable or cooked in soup preparations [8]. T. tetragonoides is valued for its nutritional profile, which includes high levels of vitamins (A, C, and E), minerals (K, P, Na, Ca, Mg, Fe, and Zn), and dietary fiber, and for its richness in other bioactive compounds [9,10]. Furthermore, Tetragonia has anti-inflammatory, anti-ulcerogenic, and antioxidant activities [11]. Its ability to thrive in saline and poor soil conditions and its heat resistance make it particularly advantageous in the context of climate change, where soil salinity and heat stress are becoming increasingly pressing issues [12]. The adaptability of this species suggests its potential for cultivation in diverse agro-ecological and climatic conditions and with different cultivation techniques such as hydroponics.

Hydroponic systems consistently support strong growth, high yields, and efficient resource use for many vegetables, making them a viable and sustainable alternative to traditional soil-based farming [13]. Hydroponics is especially effective for leafy vegetables due to their fast growth cycles, high water content, and adaptability. Lettuce, for example, thrives in hydroponic systems, achieving high yields and superior quality compared to conventional methods [14]. Several studies show that hydroponic systems can match or exceed soil-based cultivation in terms of fresh weight, the number of leaves, and root development for various leafy greens. Different hydroponic methods (NFT, DWC, ebb-and-flow, and aeroponics) are all commonly used and effective for leafy vegetable production. The hydroponic cultivation systems are characterized by high resource use efficiency and sustainability. These systems may reduce the use of water by 70–90% compared to soil-based agriculture, gaining particular interest in regions with water scarcity in terms of quantity or quality [15]. They also allow for year-round production, reduced pest and disease incidence, and minimal need for weeding or spraying. Although hydroponic systems can have higher initial and operational costs, they offer advantages in urban and resource-limited settings and can be profitable with quality-based pricing or for high-value crops such as minimally processed leafy vegetables [16]. Hydroponic cultivation could be highly suitable also for wild native or underutilized leafy vegetables, as found for Amaranthus cruentus [17], Portulaca oleracea [18], and Urospermum picroides [19]. These cultivation systems can easily enable their cultivation in controlled environments with improved yield, resource efficiency, and quality traits.

Despite the advantages and the potential of New Zealand spinach, systematic research on the agronomic practices necessary for the effective cultivation of T. tetragonioides remains limited, especially as regards its suitability for hydroponic cultivation and fresh-cut vegetable production and its post-harvest physiology and shelf life.

This paper aims to address this gap by evaluating the suitability of Tetragonia tetragonioides to hydroponic cultivation for baby plant production and its quality characteristics and shelf life as a minimally processed vegetable.

2. Materials and Methods

2.1. Tetragonia Baby Plant Production

The research was conducted in two different growing seasons (autumn–winter 2022–2023 and spring 2024) in an unheated greenhouse at the Agricultural, Food, and Forest Sciences Department (SAAF) of the University of Palermo (38°6′28″ N 13°21′3″ E; altitude 49 m). The fruits of T. tetragonioides (hard capsule containing about 4–6 seeds) (Franchi Sementi, Grassobbio, Italy) were sown (18 November 2022 and 1 March 2024) into polystyrene trays with different cell volumes, to set different plant densities. The trays were filled with a commercial substrate (Tappeti erbosi, Vigorplant Italia srl, Fombio, Italy), a mix of peat, bark humus, and silica sand fertilized with 850 g m⁻³ of a mineral fertilizer NPK (pH 6.0–6.8; EC 0.10–0.20 dS m⁻¹). In the first growing season, three plant densities were tested based on the expected size of Tetragonia baby plants and the range of plant density tested in other studies [19]: 365, 497, and 615 plants m⁻² (trays with a cell volume of 58, 32, and 28 mL, respectively). Based on the results of the first experiment, the second growing season tested higher plant densities of 615 and 947 plants per square meter (trays with a cell volume of 28 and 18 mL, respectively). The trays were kept in a dark room at 25 °C until emergence (7 days after sowing) and were then moved on fixed benches in the greenhouse. After the emergence, the plantlets were thinned to one per cell by selecting seedlings with similar morphological traits.

The plants were cultivated in a hydroponic ebb and flow system using three concentrations of nutrient solutions (NSs) to explore the species’ response to different nutrient availability: 100% (full-strength—FS), 50% (half-strength—HS), and 0% (control with only water—C). The FS NS was prepared according to Miceli et al. [20], by adding the following to tap water (pH 7.6; electrical conductivity (EC) 500 μS cm⁻¹): 19 mM NO₃⁻, 1.25 mM NH₄⁺, 2 mM H₂PO₄⁻, 11 mM K⁺, 4.5 mM Ca²⁺, 1 mM Mg²⁺, 1.1 mM SO₄²⁻, 40 μM Fe³⁺, 30 μM BO₃³⁻, 5 μM Mn²⁺, 4 μM Zn²⁺, 0.75 μM Cu²⁺, and 0.50 µM Mo [21].

During the first cultivation period (autumn–winter 2022–2023), the average minimum and maximum temperatures inside the greenhouse were 10.0 ± 0.4 and 24.0 ± 0.5 °C and ranged from 6.0 to 29.7 °C, respectively, whereas the relative humidity was 68.5 ± 1.6% and varied from 52.0% to 100%. During spring 2024, the average air temperatures were 13.1 ± 0.4 (night) and 30.1 ± 0.5 °C (day) and varied from 8.9 to 35.6 °C, while the relative humidity was 54.6 ± 1.0% on average and ranged from 41.0% to 63.3%.

2.2. Agronomical and Morpho-Physiological Parameters

The amount of water and NS consumed during plant growth was measured and used to calculate the water use efficiency (WUE) and the nitrogen use efficiency (NUE) as follows: WUE (g DW L⁻¹ H₂O) = plant dry weight (g)/H₂O (L); NUE (g DWg⁻¹ N) = plant dry weight (g)/supplied N (g) (supplied N = initial N content of the substrate + N supplied with sub fertigation) [22].

The plants of T. tetragonioides were harvested by cutting the baby plants at the base when they had 5–7 leaves and soon before axillary bud shooting (10 January 2023 and 5 April 2024), and total baby plant yield was calculated.

Soon after harvest, 30 plants for each replicate were randomly sampled for morphological and biochemical analysis. Leaf color was measured on one expanded leaf for each plant using a colorimeter (CR-400, Minolta corporation, Ltd., Osaka, Japan) that recorded the CIELab parameters (L*, a* and b*). These parameters were used to calculate hue angle (h°) and chroma (C*) as h° = 180 + arctan(b*/a*) [23] and C* = (a*² + b*²)^1/2. Then, each plant was divided into leaves, stem, and roots to determine the fresh weight and dry weight after drying at 65 °C for 72 h. Before drying, the leaf area was measured by scanning leaves at 300 dpi (Epson Perfection 4180 Photo, Seiko Epson Corp., Suwa, Japan) to obtain digital images that were analyzed with the ImageJ 1.52a software (National Institutes Health, Bethesda, MD, USA). The specific leaf area (SLA cm² g⁻¹ DW) was estimated as leaf area/leaf dry weight.

2.3. Chemical Determinations

The dried samples (four replicated samples for each treatment) were also used for chemical determinations. Ash content was determined through the procedure described in AOAC [24] while the Kjeldahl method was used for protein determination [25]. The crude fiber was determined by the Weende method [25]. The free carbohydrate content was obtained with the anthrone method reported in Loewus [25]. The contents of K, Na, Ca, Mg, and Fe, were determined using atomic absorption spectroscopy following wet mineralization while P was determined using a colorimetric method [26]. The vitamins A and E involved a phase of extraction with an organic solvent from the food matrix, a phase of separation, and, successively, identification and quantification in the HPLC system [25]. Riboflavin (Vit. B2) was extracted in an autoclave with a solution of diluted H₂SO₄ and later, after enzymatic treatment, was determined through HPLC (for the fluorescent spectra) [25]. Niacin (Vit. B3) was extracted from the sample in an acidic solution at 12–18 °C for 30 min and measured through a microbiological method [25]. The thiamine content (Vit. B1) was obtained through extraction in 0.1 N HCl and oxidation by thiochromium and analysis in HPLC using fluorometric detection [25]. Ascorbic acid (Vit. C) was determined according to procedures previously described by Barros et al. [27]. The oxalic acid content was determined by high-performance liquid chromatography [28]. The samples of the spring harvest were analyzed only for mineral and oxalic acid content.

2.4. Minimal Processing and Cold Storage

Once harvested, the plants obtained with FS and HS nutrient solutions (only FS in the first cultivation season) were immediately transferred to the laboratory of Vegetable Analysis of the SAAF Department and minimally processed for fresh-cut production. The baby plants were selected, retaining only those free from defects, yellowing, or decay. Then, they were washed in tap water for 5 min two times and dried by manual centrifugation for 1 min with a handheld salad spinner. Samples of 50 g from each treatment were directly packed in polyethylene (PE) bags, sealed with a hot bar (Laica VT3112, Vicenza, Italy), and stored at 4 °C for 21 days. At the end of processing and every 7 days (0, 7, 14, and 21 days of cold storage), three replicated samples for each treatment were randomly selected and weight loss, leaf color, and overall quality were evaluated.

Weight loss (g 100 g⁻¹ of initial fresh weight FW) was estimated by weighing each sample at packaging and at each sampling time.

Leaf color was assessed on the upper surface of ten leaves randomly selected from each sample. A Chroma-meter CR-400 (Minolta Corp., Ltd., Osaka, Japan) was used to measure the CIELAB color components L (lightness), a* (red-green axis), and b* (yellow-blue axis). These values were then used to calculate chroma (C*), a measure of color saturation, and hue angle (h°), a measure of color hue according to the following equations: C* = (a*² + b*²)^1/2 and h° = 180° + arctan(b*/a*). Moreover, total color difference (ΔE) was also determined at each sampling date as ΔE = [(L*−L*₀) + (a*−a*₀) + (b*−b*₀)]^1/2, where L*₀, a*₀, and b*₀ were the control values at the end of processing (T₀).

An informal panel of ten individuals (six men and four women, aged 28–59) assessed the overall quality (OQ) of the samples. Panelists assigned scores from 1 to 5, with 1 representing poor/unmarketable quality (off odors, extensive color changes, major defects, or decay), 3 representing the limit of marketability, and 5 representing excellent/freshly harvested quality (free from off odors, defects, and decay) [20].

2.5. Statistics and Principal Component Analyses

The experimental design consisted of four replicated samples of 30 plants each for every combination of plant density and nutrient solution concentration, randomly assigned in four blocks. To determine the effect of plant density and nutrient solution concentration on Tetragonia plants, a two-way analysis of variance (ANOVA) was carried out. The differences between treatments and the interactions between factors were assessed with the least significant difference (LSD) test at p = 0.05.

A completely randomized design with three replicates per treatment was performed for minimal processing trials. To determine the effect of storage time, plant density, and nutrient solution concentration, a three-way ANOVA was carried out. Mean values were compared by the LSD test at p = 0.05 to identify significant differences among treatments and significant interactions between factors.

Principal component analysis (PCA) was performed on the morpho-physiological parameters, yield, and mineral content of Tetragonia baby plants to identify the key parameters discriminating among plant density and nutrient solution concentration across two cultivation seasons. The input matrix for the PCA included the following: plant height; stem diameter; total, root, stem, and leaf fresh weight (FW); shoot/root FW ratio; total, root, stem, and leaf dry weight (DW); shoot/root DW ratio; shoot dry matter percentage; yield; water use efficiency (WUE); nutrient use efficiency (NUE); leaf number; leaf width; plant area; leaf area; specific leaf area (SLA); leaf color components L*, chroma and hue; and mineral content. Only factors with eigenvalues greater than 1.0 were retained to determine the optimal number of principal components (PCs). The relationships between input variables were further investigated by examining the PC plots, with correlated variables and examined variables reported within the subspace defined by the first and second PCs. PCA was conducted using SPSS version 13.0. (SPSS Inc., Chicago, IL, USA).

3. Results

3.1. Morphological and Yield Characteristics of Tetragonia Tetragonioides Baby Plants

The suitability of T. tetragonioides to grow hydroponically for baby plant production was investigated during two cultivation cycles. The plants were ready for harvest after 53 days from sowing in the first period (from November 2022 to January 2023) and after 31 days from sowing in the second period (from March to April 2024).

3.1.1. First Growing Period (Autumn–Winter)

During the first growing cycle, the seedling height was affected only by the nutrient solution (NS) concentration; it increased with full-strength (FS) NS concentration by 37% compared to half-strength (HS) and control (0) NS (2.2 cm on average) (

Table 1. Effects of plant density (365, 497, and 615 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%, half strength—HS; and 100%, full strength—FS) on morphological parameters of Tetragonia tetragonioides baby plants grown during autumn–winter.

Source of Variance		Plant Height (cm)		Stem Diameter (mm)		Plant Fresh Weight (g FW)										Plant Dry Weight (mg DW)										Shoot Dry Matter (%)
						Total		Roots		Stem		Leaves		S/R		Total		Roots		Stem		Leaves		S/R
Plant Density
365		z 2.7		1.9	a	1.3	a	0.3		0.3		0.8	a	3.7		97.3	a	24.7	a	18.7		54.0		2.9		7.2	b
497		2.4		1.7	ab	1.2	b	0.3		0.2		0.7	b	3.4		93.9	a	23.1	a	17.9		52.9		3.1		7.8	a
615		2.4		1.8	b	1.1	c	0.2		0.2		0.6	c	3.6		84.0	b	19.6	b	17.0		47.5		3.3		7.8	a
Nutrient Solution (NS)
0		2.1	b	1.4	c	0.5	c	0.1		0.2		0.2	c	3.2		43.5	c	10.9	c	11.7		20.9		3.1		8.0	a
HS		2.3	b	1.8	b	1.1	b	0.3		0.2		0.6	b	3.1		88.4	b	24.2	b	15.4		48.8		2.7		7.8	a
FS		3.1	a	2.1	a	1.9	a	0.4		0.4		1.2	a	4.3		143.4	a	32.2	a	26.5		84.7		3.5		7.1	b
Density × NS
365	0	2.2		1.5		0.58		0.14	c	0.19	bc	0.25		3.21	b	46.3		12.7		13.0	b	20.6	d	2.7	b	7.5
	HS	2.7		2.0		1.25		0.30	bc	0.24	b	0.71		3.19	b	100.8		27.3		16.6	b	56.9	b	2.7	b	7.7
	FS	3.3		2.2		2.09		0.37	ab	0.40	a	1.32		4.59	a	144.9		34.0		26.4	a	84.6	a	3.3	ab	6.5
497	0	2.3		1.4		0.58		0.15	d	0.17	c	0.25		2.82	b	47.1		12.0		12.2	bc	22.9	d	3.0	b	8.2
	HS	2.2		1.7		1.06		0.24	c	0.22	bc	0.6		3.41	b	87.6		22.7		16.3	b	48.7	bc	2.9	b	7.9
	FS	2.8		2.1		1.92		0.39	a	0.34	a	1.19		3.92	ab	146.9		34.7		25.2	a	87.1	a	3.3	ab	7.3
615	0	2.0		1.4		0.45		0.10	c	0.13	c	0.21		3.46	b	37.0		8.0		9.7	c	19.3	d	3.6	ab	8.4
	HS	2.1		1.8		0.96		0.26	c	0.19	bc	0.51		2.73	b	76.7		22.7		13.1	b	40.9	c	2.4	b	7.7
	FS	3.1		2.2		1.83		0.33	b	0.39	a	1.11		4.50	a	138.4		28.0		28.0	a	82.4	a	3.9	a	7.4
Significance x
Density		ns		*		***		**		**		***		ns		**		**		ns		*		ns		**
NS		***		***		***		***		***		***		***		***		***		***		***		***		***
Density × NS		ns		ns		ns		*		*		ns		*		ns		ns		*		*		*		ns

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). The stem was thinner using 615 plants m⁻² or control NS (1.8 and 1.4 mm, respectively) and was slightly but significantly increased by the lowest plant density, whereas it increased by 51.4% using FS nutrient solution (Table 1).

The total plant fresh weight (FW) linearly decreased with increasing plant density and linearly increased with increasing nutrient solution concentration (Table 1). As regards the plant parts, the leaf biomass was the main part of the plant and showed a similar trend to those found for the total biomass, with no significant interaction between plant density and nutrient solution concentration, whereas the root and stem fresh weight showed different trends by increasing the nutrient solution concentration for each plant density. The root biomass was lower when using 0% NS, especially with the highest plant density (0.10 g plant⁻¹). The HS NS did not affect root weight with increasing plant density, whereas the FS solution determined the highest root biomass with 497 plants m⁻² (0.39 g plant⁻¹) (Table 1). The highest stem fresh weight was recorded in each plant density using the FS NS (0.38 g plant⁻¹, on average). The FS NS also determined the highest shoot/root FW ratio, especially with 365 and 615 plants m⁻².

The total dry biomass (Table 1) was negatively affected by the plant density only with 615 m⁻² (down to 84.0 mg plant⁻¹) but increased linearly with increasing NS concentration (up to 143.4 mg plant⁻¹). A similar trend was recorded also for the root biomass. Increasing plant density, the stem dry weight significantly decreased with the lowest NS concentration while no significant change was recorded with HS and FS NSs. The leaf dry biomass was not affected by the 0% and FS NS when varying the plant density, whereas using the HS NS, the leaf dry weight lowered with increasing plant density. The shoot dry matter percentage was significantly reduced when growing plants with the lowest plant density or supplying the FS NS (7.2 and 7.1 on average, respectively) (Table 1).

The number of leaves and the leaf width were significantly affected only by the nutrient solution rate, showing a linear increase from 5.6 (0%) to 8.1 (FS) for the leaf number and from 7.3 mm (0%) to 15.5 mm (FS) for the leaf width (

Table 2. Effects of plant density (365, 497, and 615 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%, half strength—HS; and 100%, full strength—FS) on leaf characteristics of Tetragonia tetragonioides baby plants grown during autumn–winter.

Source of Variance		Number of Leaves		Leaf Width		Leaf Area (cm2 plant−1)		Leaf Area (cm2 leaf−1)		SLA (cm2 g DW−1)		L*		Chroma		Hue°
Plant Density
365		z 7.1		11.8		9.5	a	1.2		169.0	a	48.3		39.5	a	121.4
497		6.9		11.0		7.9	b	1.1		144.4	b	47.4		36.8	b	121.2
615		7.0		11.4		8.0	b	1.1		166.7	a	48.3		38.8	ab	121.1
Nutrient Solution (NS)
0		5.6	c	7.3	c	3.1	c	0.6	c	149.2	b	52.1	a	39.8	a	117.2	c
HS		7.3	b	11.4	b	7.9	b	1.1	b	162.5	a	47.7	b	40.3	a	121.8	b
FS		8.1	a	15.5	a	14.3	a	1.8	a	168.5	a	44.1	c	34.9	b	124.8	a
Density × NS
365	0	5.4		7.2		3.1		0.6		152.2		50.8		41.0		117.9
	HS	7.8		12.0		9.7		1.2		169.7		48.3		40.8		122.3
	FS	8.2		16.1		15.7		1.9		185.1		45.8		36.6		124.1
497	0	5.4		7.2		3.0		0.6		132.0		51.4		38.6		116.9
	HS	7.0		11.1		7.3		1.0		148.8		47.9		38.8		121.3
	FS	8.3		14.7		13.3		1.6		152.4		42.9		33.1		125.4
615	0	5.9		7.7		3.1		0.5		163.2		54.1		39.8		116.7
	HS	7.1		11.0		6.9		1.0		168.9		47.0		41.5		121.8
	FS	8.0		15.6		13.9		1.7		168.0		43.6		35.0		125.0
Significance x
Density		ns		ns		*		ns		***		ns		*		ns
NS		***		***		***		***		**		***		***		***
Density × NS		ns		ns		ns		ns		ns		ns		ns		ns

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). The total leaf area lowered by increasing the plant density over 365 plants m⁻², with a reduction by 17% on average for the other plant densities. The nutrient solution concentration determined a greater effect; the total leaf area on control plants (0%) was 3.1 cm² on average and increased by 156 and 361% for HS and FS, respectively (Figure 1a). The plant density had no effect on the average leaf area, while the nutrient solution concentration increased this parameter by 93% and 213%, for HS and FS, respectively, compared with 0% (3.1 cm²) (Table 2).

The SLA of Tetragonia plants dropped significantly only with intermediate plant density (−14.6%). The addition of nutrients to the irrigation water significantly increased this parameter irrespective of NS concentration (+11.0%, on average, for HS and FS) (Table 2, Figure 1b). The color of Tetragonia leaves was influenced by plant growth conditions (Table 2). When increasing the NS concentration, the leaf lightness decreased and the hue angle linearly increased, whereas the color saturation was negatively affected by the highest NS concentration. The leaf color was slightly affected by the plant density, but only as regards chroma values.

The yield of baby plants was lowest using 0% nutrient solution and did not significantly increase with this NS with increasing plant density (193.4 g m⁻² on average). The other nutrient solutions determined yield increases with increasing plant density with the highest yield recorded with FS NS and 615 plants m⁻² (922.7 g m⁻²) (Figure 2). The baby plants obtained using 0% nutrient solution showed a stunted growth and a yellowish leaf color irrespective of plant density so that they did not meet the commercial requirements.

The lowest water use efficiency (WUE) was recorded in the plants grown at the lowest plant density supplied with 0% NS (0.5 g DW L⁻¹ H₂O). The increase in NS concentration increased the WUE linearly at each plant density, with the highest WUE efficiency in the plants supplied with FS NS and grown at the two higher plant density (2.6 g DW L⁻¹ H₂O, on average) (Figure 3a). The lowest nitrogen use efficiency (NUE) was recorded in the plants grown at the lowest plant density, even with increasing NS concentration (6.8 g DW g⁻¹ N on average). The NUE dropped when using HS NS with 497 and 615 plants m⁻²; the highest NUE was recorded with 497 plants m⁻² and zero NS (13.3 g DW g⁻¹ N) followed by the plants irrigated with FS NS at the two higher plant densities (12.0 g DW g⁻¹ N, on average, for 497 and 615 plants m⁻²) (Figure 3b).

3.1.2. Second Growing Period (Spring)

Based on the results of the first experiment, the second growing season tested only the highest plant densities of the first cultivation cycle (615 plants m⁻²) and an even higher plant density (916 plants m⁻²).

The stems of Tetragonia baby plants were a little shorter but thicker than the first cycle (

Table 3. Effects of plant density (615 and 497 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%, half strength—HS; and 100%, full strength—FS) on morphological parameters of Tetragonia tetragonioides baby plants grown during spring.

Source of Variance		Plant Height (cm)		Stem Diameter (mm)		Plant Fresh Weight (g FW)										Plant Dry Weight (mg DW)										Shoot Dry Matter (%)
						Total		Roots		Stem		Leaves		S/R		Total		Roots		Stem		Leaves		S/R
Plant Density
615		z 17.2	a	2.8	a	1.67		0.20	a	0.31	a	1.16		6.7	b	119.1		16.5	a	21.4		80.8		5.8		7.8
947		15.0	b	2.4	b	1.33		0.14	b	0.28	b	0.91		7.8	a	89.2		12.6	b	17.5		59.2		6.0		7.6
Nutrient Solution (NS)
0		13.7	b	2.2	b	0.50		0.12	b	0.16	c	0.22		3.5	c	48.0		9.6	b	12.8		25.7		4.3		10.1
HS		15.7	b	2.7	a	1.75		0.20	a	0.32	b	1.23		7.7	b	122.7		17.1	a	21.2		84.3		6.1		6.8
FS		19.0	a	2.9	a	2.25		0.20	a	0.41	a	1.65		10.5	a	141.9		17.0	a	24.5		100.1		7.2		6.0
Density × NS
615	0	14.6		2.3		0.54	d	0.14		0.16		0.25	d	2.8		51.5	d	11.7		13.0	d	26.9	d	3.5	c	9.9	a
	HS	17.5		3.0		2.07	b	0.24		0.35		1.48	b	7.6		145.1	a	19.2		24.7	ab	101.0	ab	6.5	ab	6.9	b
	FS	19.6		3.1		2.40	a	0.23		0.41		1.77	a	9.6		160.8	a	18.8		26.7	a	114.7	a	7.3	a	6.5	b
947	0	12.8		2.0		0.45	d	0.09		0.15		0.20	d	4.2		44.5	d	7.5		12.5	d	24.5	d	5.1	b	10.3	a
	HS	13.8		2.5		1.43	c	0.16		0.28		0.99	c	7.8		100.2	c	15.0		17.8	c	67.5	c	5.7	b	6.7	b
	FS	18.4		2.7		2.10	ab	0.17		0.40		1.53	ab	11.3		123.0	b	15.2		22.3	b	85.5	b	7.2	a	5.6	c
Significance x
Density		*		***		***		***		*		***		**		***		***		***		***		ns		ns
NS		***		***		***		***		***		***		***		***		***		***		***		***		***
Density × NS		ns		ns		**		ns		ns		**		ns		**		ns		*		***		*		***

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). Their height was significantly higher using FS NS (+29% compared to 0% and HS NS, on average) and lower when growing plants at 947 plants m⁻² compared to 615 plants m⁻². The stem diameter was negatively affected by the lowest spacing between plants but increased when adding nutrients to the irrigation water (HS and FS NSs) (Table 3).

The total plant fresh weight (FW) (Table 3) linearly increased with increasing nutrient solution concentration in the plants grown at 947 plants m⁻² ranging from 0.45 g plant⁻¹ (0 NS) to 2.10 g plant⁻¹ (FS NS), whereas at 615 plants m⁻², the plant fresh weight overcame 2 g plant⁻¹ with HS NS and was significantly higher with FS NS (2.40 g plant⁻¹). The same trend was found for the leaf and stem fresh weight, while root fresh biomass was lower when growing the plants at the highest plant density and for both densities when supplementing only water to the plants (0 NS).

The total dry weight (Table 3) was lowest at both plant densities in the plants supplemented only with water (48.0 mg plant⁻¹, on average) and increased up to the highest level when supplementing the plants grown at 615 plants m⁻² with HS or FS NS (152.9 mg plant⁻¹, on average). The plants grown at 947 plants m⁻² significantly increased dry biomass when fed with HS NS (100.2 mg plant⁻¹) and even more with FS NS (123.0 mg plant⁻¹) but did not reach the dry biomass accumulation recorded with the lower plant density. This trend was superimposable with that of shoot parts, and as found for fresh weight, the root dry biomass was higher in the plants grown at 615 plants m⁻² and in those supplemented with HS or FS NS.

The shoot dry matter percentage was significantly reduced when increasing NS concentration, but the reduction was greater at 947 plants m⁻² (−46.1%) compared to 615 plants m⁻² (−34.4%) (Table 3).

During spring, the plants had less leaves than in the first growing cycle (5.1 leaves plant⁻¹, on average). The leaf number significantly increased up to 5.7 on average at both plant densities when the plants were supplemented with HS or FS NS (

Table 4. Effects of plant density (365, 497, and 615 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%, half strength—HS; and 100%, full strength—FS) on leaf characteristics of Tetragonia tetragonioides baby plants grown during spring.

Source of Variance		Number of Leaves		Leaf Width		Leaf Area		Leaf Area (cm2 Leaf−1)		SLA (cm2 g DW−1)		L*		Chroma		Hue °
						(cm2 plant−1)
Plant Density
615		z 5.2		22.2		18.0		3.2		201.7		45.3	b	31.8	b	125.3	a
947		5.0		19.5		14.8		2.7		221.3		46.3	a	33.6	a	124.2	b
Nutrient Solution (NS)
0		3.9	b	9.0		3.4		0.9		133.6		52.4	a	42.2	a	118.4	b
HS		5.6	a	24.3		19.6		3.5		233.6		42.3	b	27.9	b	127.6	a
FS		5.9	a	29.1		26.3		4.4		267.3		42.8	b	28.0	b	128.2	a
Density × NS
615	0	3.9		9.7	d	3.8	d	1.0	d	139.5	c	52.0		42.0		119.3
	HS	5.8		27.1	b	23.0	b	4.0	b	227.7	b	42.1		27.1		127.9
	FS	6.0		29.8	a	27.3	a	4.5	a	237.8	b	41.9		26.2		128.6
947	0	3.9		8.4	d	3.1	d	0.8	d	127.7	c	52.8		42.4		117.5
	HS	5.3		21.6	c	16.1	c	3.0	c	239.4	b	42.4		28.7		127.4
	FS	5.9		28.5	ab	25.3	ab	4.3	ab	296.9	a	43.8		29.8		127.7
Significance x
Density		ns		***		***		***		***		**		**		**
NS		***		***		***		***		***		***		***		***
Density × NS		ns		***		**		**		***		ns		ns		ns

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; ** significant at p < 0.01; and *** significant at p < 0.001.

). The nutrient solution also influenced the leaf width, with higher increases in the plants grown at 615 plants m⁻² compared to 947 plants m⁻² (Table 4). Leaf area was strongly reduced in the plants fed only with water and had the highest expansion with FS NS at 615 plants m⁻² (27.3 cm² plant⁻¹; 4.5 cm² leaf⁻¹). The highest plant density recorded a lower plant area, especially with HS NS (Table 4 and Figure 4a).

The two plant densities tested also showed a different trend for specific leaf area, which increased from 139.5 cm² g DW⁻¹ (0 NS) to 232.8 cm² g DW⁻¹ (on average for HS and FS NS) in the plants grown at 615 plants m⁻² and reached a significantly higher SLA value using FS NS at 947 plants m⁻² (296.9 cm² g DW⁻¹) (Table 4 and Figure 4b).

During spring, the leaf color was significantly affected by plant density with a lighter, more saturated, and yellowish color with 947 plants m⁻². Leaf color showed changes due to the nutrient solution concentration only when supplementing only water to the plants (Table 4).

The yield of baby plants was the lowest using 0% nutrient solution and did not significantly change with this NS as a function of plant density (292.8 g m⁻² on average). These baby plants were undersized and with yellowish leaves and were not marketable as a result. The HS nutrient solutions determined a yield increase up to 1162.0 g m⁻² on average for both plant densities, while FS NS further increased the yield by 19% at 615 plants m⁻² (1340.3 m⁻²) and by 52.7% at 947 (1829.5 g m⁻²) (Figure 5).

Water use efficiency (WUE) was mainly affected by the nutrient solution with significant increases at each increase in NS concentration (1.9 g DW L⁻¹ H₂O, on average, with FS NS) (Figure 6a). The lowest nitrogen use efficiency (NUE) was recorded in the plants grown at both plant densities and supplemented with FS NS (10.8 g DW g⁻¹ N on average). The NUE increased when using HS NS with 947 and 615 plants m⁻² or 0 NS with 947 plants m⁻² (15.2 g DW g⁻¹ N, on average) but the highest NUE was recorded with 947 plants m⁻² and zero NS (21.4 g DW g⁻¹ N) (Figure 6b).

3.2. Biochemical Characteristics of Tetragonia Tetragonioides Baby Plants

3.2.1. First Growing Period (Autumn–Winter)

At the end of the first cycle, the Tetragonia baby plants harvested were analyzed to determine their contents of proteins, fiber, total sugars, and minerals. The HS and FS NSs significantly boosted the protein content only when applied to 497 and 615 plants m⁻² (+77% on average compared to 365 plants m⁻² using HS and FS NS) (Figure 7). The fiber content increased with increasing plant density or NS concentration (Figure 7). The total sugar content slightly but not significantly changed when increasing the NS concentration with 365 and 615 plants m⁻², while with the intermediate plant density the HS NS determined the highest sugar content (0.43 g 100 g⁻¹), which did not statistically differ from the values recorded with HS and FS NSs at 615 plants m⁻² (0.39 g 100 g⁻¹ on average) (Figure 7).

The content of K, Ca, and Mg was higher when increasing plant density or NS concentration. The Na content increased with the highest NS concentration at the lowest plant density and with both HS and FS NSs when using the other plant densities (

Table 5. Effects of plant density (365, 497, and 615 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%—HS; and 100%—FS) on mineral element and oxalic acid content of Tetragonia tetragonioides baby plants grown during autumn–winter.

Source of Variance		K (mg 100 g−1)		Na (mg 100 g−1)		Ca (mg 100 g−1)		Mg (mg 100 g−1)		P (mg 100 g−1)		Fe (mg 100 g−1)		Cu (mg 100 g−1)		Mn (mg 100 g−1)		Zn (mg 100 g−1)		Oxalic Acid (mg 100 g−1)
Plant Density
365		z 98.9	b	89.9		41.8	b	29.6	b	21.2		2.9		0.15	b	0.31		1.8		844.7	b
497		111.5	a	99.0		50.6	ab	38.8	a	28.4		2.8		0.16	ab	0.41		1.6		933.0	a
615		117.3	a	99.6		60.5	a	45.9	a	29.1		2.7		0.24	a	0.38		2.1		958.2	a
NS
0		94.6	c	88.7		41.3	b	29.5	b	20.7		2.8		0.14	b	0.29		1.6		963.8	a
HS		109.2	b	98.1		50.4	ab	40.2	a	24.1		2.8		0.17	ab	0.41		1.6		918.8	ab
FS		123.8	a	101.7		61.3	a	44.6	a	33.8		2.8		0.24	a	0.40		2.2		853.3	b
Density × NS
365	0	89.1		85.2	c	39.2		28.7		19.7	b	2.6		0.10		0.19		1.3	b	872.6
	HS	93.3		87.0	c	40.5		29.4		20.6	b	2.8		0.13		0.33		1.4	b	888.0
	FS	114.2		97.4	b	45.8		30.7		23.2	b	3.1		0.22		0.40		2.8	a	773.4
497	0	93.3		87.0	c	40.5		29.4		20.6	b	2.8		0.13		0.33		1.4	b	980.4
	HS	113.0		101.7	a	52.7		41.3		26.3	b	2.9		0.19		0.50		1.7	b	940.1
	FS	128.3		108.3	a	58.7		45.7		38.3	a	2.7		0.17		0.41		1.8	ab	878.5
615	0	101.5		93.8	bc	44.2		30.5		21.9	b	3.1		0.21		0.34		2.2	ab	1038.5
	HS	121.3		105.7	a	58.0		50.0		25.3	b	2.5		0.18		0.41		1.8	ab	928.2
	FS	129.0		99.3	a	79.3		57.3		40.0	a	2.7		0.34		0.39		2.2	ab	907.9
Significance x
Density		**		***		**		***		***		ns		*		ns		ns		**
NS		***		***		**		**		***		ns		*		ns		**		**
Density × NS		ns		**		ns		ns		*		ns		ns		ns		*		ns

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). The P content was significantly higher using FS NS with the two higher densities (39.2 mg 100 g⁻¹ on average), while the Zn content was highest in the plants grown at 365 plants m⁻² with FS NS (2.8 mg 100 g⁻¹) and reached intermediate values with FS NS at 497 plants m⁻² and all of the NSs at 615 plants m⁻² (Table 5).

The vitamin A content was significantly affected by HS NS with 497 plants m⁻² (2368 µg 100 g⁻¹) and increased with 615 plants m⁻² as the NS concentration increased (2373.5 µg 100 g⁻¹ on average) (Figure 8a). The vitamin C content was significantly increased using the HS NS with 497 plants m⁻² (22 mg 100 g⁻¹) (Figure 8a). The content of vitamins B1, B2, and B3 was slightly higher when increasing plant density and NS concentrations (Figure 8b). A significantly higher content of vitamin E was recorded in the plants grown at 615 plants m⁻² supplied with HS and FS nutrient solutions or at 497 plants m⁻² using only FS NS (1.2 mg 100 g⁻¹ on average) (Figure 8c).

The oxalic acid content (Table 5) was significantly reduced by the lowest plant density or increasing NS concentration. The lowest plant density showed a reduction by 10.7% on average compared to 497 and 615 plants m⁻² (945.6 mg 100 g⁻¹, on average); the plants grown with 0 NS had an oxalic acid content of 963.8 mg 100 g⁻¹, which significantly dropped by 11.5% using FS NS (853.3 mg 100 g⁻¹).

3.2.2. Second Growing Period (Spring)

After the second growing period the content of K, Na, Ca, Mg, Fe, and Cu was lowered by the increase in plant density up to 947 plants m⁻² but was highest with the highest NS concentration (

Table 6. Effects of plant density (365, 497, and 615 plants m⁻²) and nutrient solution (NS) concentration (0%, only water—0; 50%—HS; and 100%—FS) on mineral element and oxalic acid content of Tetragonia tetragonioides baby plants grown during spring.

Source of Variance		K (mg 100 g−1)		Na (mg 100 g−1)		Ca (mg 100 g−1)		Mg (mg 100 g−1)		P (mg 100 g−1)		Fe (mg 100 g−1)		Cu (mg 100 g−1)		Mn (mg 100 g−1)		Zn (mg 100 g−1)		Oxalic Acid (mg 100 g−1)
Plant Density
616		z 130.5	a	111.3	a	65.1		40.4	a	33.7		4.9	a	0.26	a	0.42		1.6		1008.5	b
947		115.7	b	101.8	b	58.7		34.1	b	31.3		3.8	b	0.20	b	0.41		1.4		894.5	a
NS
0		120.3	b	98.6	b	41.3	b	31.8	b	25.6		3.7	b	0.2	b	0.3	b	1.3		1011.5	a
HS		119.4	b	104.6	b	50.0	ab	31.3	b	29.0		4.4	ab	0.2	ab	0.4	a	1.3		957.1	ab
FS		129.6	a	116.5	a	94.5	a	48.8	a	43.0		4.9	a	0.3	a	0.5	a	1.9		885.9	b
Density × NS
615	0	128.9		104.9		46.4		35.5		27.2	bc	4.5		0.20		0.34		1.49	b	1086.5
	HS	128.4		108.0		51.5		34.5		32.1	b	4.9		0.26		0.46		1.50	b	997.2
	FS	134.2		120.9		97.5		51.3		41.8	a	5.3		0.32		0.47		1.90	a	941.9
947	0	111.7		92.2		36.1		28.2		23.9	c	2.8		0.14		0.31		1.14	c	936.5
	HS	110.3		101.1		48.5		28.0		25.9	c	3.9		0.19		0.43		1.17	c	917.1
	FS	125.1		112.0		91.5		46.2		44.2	a	4.6		0.25		0.49		1.95	a	829.9
Significance x
Density		***		**		ns		***		*		*		*		ns		ns		**
NS		*		***		***		***		***		*		**		***		**		**
Density × NS		ns		ns		ns		ns		**		ns		ns		ns		*		ns

^z Each value is the mean of four replicated samples of 30 plants each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). The content of P and Zn was significantly higher using FS NS with both plant densities (43.0 mg 100 g⁻¹ and 1.92 mg 100 g⁻¹, on average, respectively), but using 947 plants m⁻², it was significantly lower with 0 and HS NS than 615 plants m⁻². Mn content was affected only by the nutrient solution concentration and recorded the lowest value in the plants supplemented with 0 NS.

The oxalic acid content (Table 6) dropped significantly by increasing plant density up to 947 plants m⁻² (894.5 mg 100 g⁻¹, on average) or increasing NS rate (885.9 mg 100 g⁻¹, on average with FS NS).

3.3. Storage of Minimally Processed Baby Plants

3.3.1. First Growing Period (Autumn–Winter)

After harvesting, the minimally processed baby plants fed with FS NS were stored for 21 days at 4 °C to assess their shelf life and post-harvest quality changes (

Table 7. Effects of plant density (365, 497, and 615 plants m⁻²) and storage (21 days at 4 °C) on minimally processed Tetragonia tetragonioides baby plants grown during autumn–winter.

Plant Density (Plant m−2)	Storage (d at 4 °C)	Weight Loss (g 100 g−1 FW)		L*		Chroma		Hue		∆E
365		2.86	a	44.49		34.77		123.79		5.70
497		2.23	b	45.40		35.68		124.27		5.25
615		3.59	b	46.26		37.76		123.14		6.80
	0			44.10		34.91		124.82	a
	7	1.66	b	44.68		35.91		124.36	a	4.64	b
	14	3.02	ab	44.81		35.22		124.06	a	4.83	b
	21	3.99	a	47.93		38.23		121.70	b	8.27	a
365	0			42.87	c	33.05	b	124.08
	7	1.69		44.65	bc	36.15	b	125.12		5.22
	14	3.14		44.74	bc	34.75	b	124.55		4.96
	21	3.74		45.69	bc	35.14	b	121.41		6.91
497	0			45.79	bc	36.64	ab	125.39
	10	1.40		43.48	c	33.60	b	124.22		4.41
	7	1.97		44.22	bc	33.75	b	124.13		4.09
	21	3.31		48.10	ab	38.72	ab	123.36		7.25
615	0			43.65	c	35.04	b	125.01
	7	1.88		45.92	b	37.98	ab	123.74		4.29
	14	3.96		45.46	bc	37.17	ab	123.50		5.44
	21	4.92		50.01	a	40.85	a	120.33		10.66
Plant Density		*		*		***		ns		ns
Storage		***		***		**		***		***
Plant Density × Storage		ns		*		*		ns		ns

Each value is the mean of three replicated samples of 50 g each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

).

The weight loss linearly increased during cold storage and was 3.99%, on average, after 21 days. Plant density also affected this parameter with an increase in weight loss with increasing plant density (from 2.23% to 3.59%, on average, for 365 and 615 plants m⁻², respectively). Leaf color was variously affected by plant density and storage time. The baby plants grown at 365 plants m⁻² recorded no significant change in color lightness (L*), whereas with increasing plant density, baby plants showed an increase in leaf lightness especially at the end of storage. The color saturation (chroma) recorded a significant increase at the end of the cold storage only in the baby plants grown at the highest plant density of 615 plants m⁻². The hue angle tended to drop after 21 days of cold storage. The color changes were also evaluated by calculating the color difference (∆E), which was influenced mainly by the storage time. The changes in color recorded after 7 days (4.64) increased significantly only at the end of storage (8.27).

The scores for overall quality assessed by an informal panel dropped continuously during storage. The quality loss was more severe with increasing plant density, resulting in the loss of marketability between 14 and 21 days of cold storage (Figure 9).

3.3.2. Second Growing Period (Spring)

After the second growing period, even the baby plants fed with HS NS were minimally processed and stored for 21 days at 4 °C.

The weight loss increased throughout the storage up to 3.78% on average after 21 days (

Table 8. Effects of plant density (615 and 947 plants m⁻²), nutrient solution (NS) concentration (50%—HS and 100%—FS), and storage (21 days at 4 °C) on minimally processed Tetragonia tetragonioides baby plants grown during spring.

Source of Variance			Weight Loss (%)		L*		Chroma		Hue		∆E
Days at 4 °C
0					42.55		27.96		127.91	a
7			z 0.75	c	42.96		28.25		127.11	b	2.27	b
14			2.38	b	42.64		28.89		126.65	c	2.96	a
21			3.78	a	42.96		28.14		126.65	c	3.07	a
Plant Density
615			2.14	b	42.14	b	27.25	b	127.37	a	2.63
947			2.46	a	43.41	a	29.37	a	126.79	b	2.91
Nutrient Solution (NS)
HS			2.12	b	42.40	b	28.26		126.78	b	2.66
FS			2.48	a	43.15	a	28.35		127.37	a	2.88
Day × Density × NS
0	615	HS			42.1		27.1		127.9
		FS			41.9		26.2		128.6
	947	HS			42.4		28.7		127.4
		FS			43.8		29.8		127.7
7	615	HS	0.7		42.0		27.2		127.0		2.2
		FS	0.9		42.6		27.0		127.7		2.2
	947	HS	0.7		43.4		29.4		126.5		2.5
		FS	0.8		43.9		29.4		127.3		2.1
14	615	HS	2.0		41.1		27.2		126.7		2.5
		FS	2.5		42.4		27.6		127.3		2.8
	947	HS	2.4		42.8		30.2		126.1		3.2
		FS	2.6		44.3		30.6		126.4		3.4
21	615	HS	3.2		42.2		27.4		126.8		2.4
		FS	3.6		43.0		28.3		127.0		3.6
	947	HS	3.8		43.3		28.9		125.9		3.1
		FS	4.6		43.4		27.9		127.0		3.1
Significance x
Days			***		ns		ns		***		*
Density			*		***		***		***		ns
NS			*		**		ns		***		ns
Day × Density			ns		ns		ns		ns		ns
Day × NS			ns		ns		ns		ns		ns
Density × NS			ns		ns		ns		ns		ns
Day × Density × NS			ns		ns		ns		ns		ns

^z Each value is the mean of three replicated samples of 50 g each. For each factor, values in a column followed by the same letter are not significantly different, according to the LSD test. ^x Significance: ns = not significant; * significant at p < 0.05; ** significant at p < 0.01; and *** significant at p < 0.001.

). A higher weight loss was recorded in the baby plants supplemented with FS NS compared with HS NS and with increasing plant density. The color parameters L* and chroma did not record changes during storage, but the leaves had a lighter color in the plants grown at 947 plants m⁻² or fed with FS NS and a more saturated color in the plants grown at the highest density (Table 8). The hue angle was higher when using the FS NS or 615 plants m⁻² but decreased during storage from 127.9 at the beginning of cold storage to 126.7 at day 14, with no further change until the end of the storage period. The color variations expressed as ΔE showed moderate changes mainly due to the time of storage with a value of about three after 14 and 21 days of storage (Table 8).

The scores for the overall quality dropped more rapidly during cold storage in the baby plants fed with HS NS and approached the limit of marketability after 2 weeks of storage, whereas the plants fed with FS NS maintained a good overall quality for the first two weeks and were near but over the limit of marketability after 21 days (Figure 10).

3.4. Principal Component Analysis

The principal component analysis identified four principal components (PCs) with eigenvalues exceeding 1.00 (

Table 9. Correlation of variables to the factors of the principal components analysis (PCA) based on the morpho-physiological parameters, yield, and mineral content of Tetragonia baby plants.

Variable	PC1	PC2	PC3	PC4
Plant height	0.583	−0.774	0.051	0.123
Stem diameter	0.868	−0.350	0.022	0.222
Total FW	0.955	0.233	−0.124	0.078
Roots FW	0.459	0.833	0.014	0.270
Stem FW	0.927	0.254	−0.121	0.028
Leaves FW	0.977	0.107	−0.140	0.048
S/R_FW	0.833	−0.453	−0.207	−0.156
Total DW	0.912	0.338	−0.056	0.193
Roots DW	0.423	0.850	−0.028	0.259
Stem DW	0.875	0.342	0.013	0.178
Leaves DW	0.961	0.168	−0.072	0.160
S/R DW	0.735	−0.634	−0.054	−0.010
Shoot DM	−0.758	−0.245	0.492	0.281
WUE	0.789	0.471	0.170	0.098
NUE	−0.035	−0.616	0.425	0.528
Leaf number	0.318	0.922	−0.091	−0.029
Leaf width	0.950	−0.257	−0.159	−0.008
Plant area	0.976	−0.095	−0.172	−0.013
Leaf area	0.926	−0.325	−0.178	−0.016
SLA	0.812	−0.313	−0.343	−0.256
L*	−0.914	−0.179	0.116	−0.199
Chroma	−0.898	0.206	0.193	−0.082
Hue	0.960	0.033	−0.124	0.148
Yield	0.919	−0.241	−0.089	−0.084
K	0.734	−0.064	0.617	0.124
Na	0.817	−0.174	0.387	−0.062
Ca	0.821	−0.021	0.280	−0.413
Mg	0.593	0.308	0.596	−0.288
P	0.853	−0.036	0.370	−0.118
Fe	0.653	−0.623	−0.018	−0.026
Cu	0.785	0.002	0.384	−0.211
Mn	0.773	0.015	0.197	0.010
Zn	0.378	0.538	0.162	−0.431

Values in bold within the same factor indicate the variable with the largest correlation.

). These four PCs accounted for 91.42% of the total variance, with individual contributions of 62.98%, 17.79%, 6.54%, and 4.12%, respectively. This allowed the initial 33 variables to be effectively summarized by the four PCs.

PC1 was primarily associated with stem diameter; total, stem, and leaf fresh weight; shoot/root fresh weight; total, stem, and leaf dry weight; shoot/root dry weight; shoot dry matter; water use efficiency (WUE); leaf width; plant and leaf area; specific leaf area (SLA); leaf length (L); chroma; hue; yield; and all mineral elements except Zn. PC2 correlated with root fresh and dry weight, nutrient use efficiency (NUE), leaf number, Fe, and Zn. PC3 was related to K and Mg (Table 9). These relationships are visualized by the projection of the original variables onto the plane of the first two PCs in the loadings plot (Figure 11a). The discrimination among plant densities, nutrient solutions, and cultivation seasons is presented in the scores plot (Figure 11b), where the scores for each treatment are clearly separated into distinct clusters. Plants supplied only with water had scores located in the third quadrant. The winter harvest showed a stronger positive relationship with PC2 than the spring harvest and increasing nutrient solution concentration enhanced both PC1 and PC2 values. During spring, the differences among treatments were less pronounced, with all of their values located in the second quadrant. However, lower plant density and the full-strength nutrient solution (FS NS) appeared more related to PC1.

4. Discussion

Existing literature on New Zealand spinach cultivation predominantly focuses on the utilization of mature leaves as the usable component. In contrast, the present research investigates for the first time the feasibility of employing whole baby plants (including stem and leaves) as raw material for ready-to-eat vegetable production. The growing interest in hydroponic systems to cultivate leafy vegetables for ready-to-eat leafy salads necessitates specific studies on agronomic techniques and nutrient management for new or underutilized vegetables such as New Zealand spinach [29].

In its zone of origin, Tetragonia plants can establish at any time of the year but they prefer warm and frost-free sites [7]. The growing conditions (temperature, humidity, and light) during our trial satisfied Tetragonia’s climatic needs well. However, during the second cultivation cycle (spring), warmer temperatures and longer daylight length led to a higher growth rate, resulting in more vigorous and faster plant growth compared to the first growing cycle (winter). The first and second growing cycles (from sowing to harvest) lasted 55 and 36 days, respectively. Cultivation conditions can affect the crop duration of leafy vegetables, and a similar timeframe was also observed for Valerianella locusta baby plant production [30]. The climatic conditions during spring increased also the yield of baby plants as found comparing the yield recorded in the two growing seasons using 615 plant m⁻². At this plant density, and using the full-strength (FS) nutrient solution, the spring yield increased by 45.3% compared to the winter yield, with an increase in both fresh and dry biomass. This could be related to the higher solar radiation resulting from the high level of natural light and long photoperiod that increased photosynthesis in the spring season with respect to the winter season [31].

In both periods, the rise of plant density determined a significant increase in baby plant yield that was highest when the highest plant density was adopted (615 plant m⁻² in autumn–winter and 947 plant m⁻² in spring). This occurred despite the negative effect that a lower plant spacing had on various yield attributes such as the plant height, stem diameter, fresh and dry weight of baby plants, and leaf area. A similar plant density range (412 and 824 plants m⁻²) was tested on other wild edible leafy plants to evaluate their domestication using a hydroponic cultivation system, yielding similar results in terms of yield and plant characteristics [19]. An increase in plant density was found to decrease plant height, stem and canopy width, leaf number, leaf area, and plant fresh and dry mass per plant in lettuce plants grown hydroponically [32]. The plants of Tetragonia showed a very satisfying yield during spring, especially at the highest plant density (1.8 kg m⁻²), that was consistent with the yield of lamb’s lettuce grown hydroponically for baby plant production [29]. Optimizing plant density is a critical factor in the hydroponic cultivation of leafy vegetables, significantly influencing both plant morphology and physiology. Achieving the ideal density maximizes yield while ensuring optimal individual plant quality and resource efficiency. Higher plant densities generally lead to a greater fresh and dry weight yield per unit area due to more plants occupying the space, as found for mustard spinach, lettuce, and basil [33,34]. However, this often comes at the expense of individual plant size, characterized by reduced fresh and dry weight per plant, shorter plant height, narrower stems, and smaller individual leaf area, as also found in our trials for Tetragonia baby plants. They showed a drop of these parameters of about 14% on average in the first trial (autumn–winter) and 20% in the second (spring). This is primarily due to intensified inter-plant competition for resources such as light and nutrients [33]. Leaf morphology can be also significantly affected by plant density. In dense plantings, plants often adapt by developing thinner, more elongated leaves and stems in search of light. This led to an increase in specific leaf area (SLA) in lettuce plants [35] as observed in our experiment. Plant density ranging from 365 to 615 plants m⁻² during the winter season did not change SLA, but when increasing it from 615 up to 947 plants m⁻² in the spring trial, SLA increased by up to 24.9% in the plants supplied with FS NS. The increase in SLA indicates a reduction in leaf thickness and potentially nutrient content per unit area, impacting overall nutritional quality.

The use of cell trays as a plant support in our ebb and flow hydroponic system resulted in a reduction in the substrate volume per plant due to increased plant density. Thus, together with limitations of space and light for the epigeal parts, plants also suffered a reduction in space for roots and a smaller stock of water and nutrients. The volume of the substrate supporting the plants in hydroponic systems plays a significant role in influencing various aspects of plant development, physiology, biomass accumulation, and leaf morphology [36]. A larger or more appropriate substrate volume can lead to greater fresh and dry biomass production. In contrast, limiting substrate volume or using unsuitable substrates can restrict root growth and, consequently, reduce overall fresh and dry plant mass as found in hydroponically grown arugula and lettuce plants [37,38]. It is important to consider that while a larger substrate volume generally offers more space for root growth, the efficiency of nutrient delivery remains paramount. Reducing substrate volume in hydroponic systems can significantly lower both operational costs and environmental impact. Studies show that optimizing irrigation can reduce the amount of substrate needed without compromising plant growth or yield. This could lead to substantial savings in fertilizer costs (up to 61%) and a marked decrease in nutrient-rich effluent that can pollute the environment [39,40]. The interaction between the growing medium and the nutrient solution, governed by factors such as porosity, water-holding capacity, and exchange capacity, is fundamental to the efficacy of the growing environment for leafy vegetables [41].

Plant growth and dry biomass accumulation is linked to both light interception and photosynthesis and nutrient availability. In hydroponic systems, nutrients are almost fully supplied by the nutrient solution. Thus, the optimization of nutrient solution concentration is a critical factor in hydroponic cultivation, directly influencing the growth, biomass accumulation, and morphological and biochemical characteristics of leafy vegetables. Hydroponic systems offer precise control over nutrient delivery, allowing for tailored approaches to maximize crop productivity and quality [42]. However, determining the optimal concentration is complex, as it is highly dependent on the specific plant species, developmental stage, and environmental conditions such as plant space and access to the light [31]. The concentration of the nutrient solution significantly impacts various growth parameters, including plant height, leaf number, and leaf area, as also observed in our experiments. While some studies suggest that within certain ranges, growth may be less affected by nutrient concentration than by environmental factors like growing season, others demonstrate clear responses. As expected, the nutrients supplied only with the substrate in the control treatment with 0% NS were insufficient to allow correct plant growth. This resulted in unmarketable baby plants (undersized and with yellowish leaf color) especially with the reduced substrate volume at higher densities. There was an evident increasing tendency in growth and yield parameters (shoot fresh and dry biomass accumulation, leaf area, leaf number, and WUE) when increasing nutrient solution concentration up to full strength. The yield increases in Tetragonia due to the increasing NS concentration fit a linear model, although the response of most leafy vegetables to increasing nutrient concentrations in the nutrient solution generally fits a quadratic polynomial model [43]. Our results indicate that the full-strength nutrient solution we used fulfilled the nutrient requirements of Tetragonia baby plants without determining nutrient imbalances or osmotic stress that could occur at higher concentrations. Various studies have proposed different optimal nitrogen ranges for hydroponically grown leafy vegetables. For instance, research on lettuce or arugula often suggests optimal N concentrations between 100 and 150 mg L⁻¹ for maximizing fresh weight [44,45]. For more robust leafy greens like kale, slightly higher concentrations, possibly up to 180–200 mg L⁻¹, might be beneficial for fresh biomass yield [46]. This agrees with our results, as we found the best growth and yield response at 215 mg L⁻¹ N with an electrical conductivity (EC) of the FS nutrient solution of about 2.5 dS·m⁻¹. The EC is a measure of total ion content in the nutrient solution. Leafy vegetables, such as arugula, generally suffer with NSs with EC higher than 1.8–2.0 dS·m⁻¹ [43]; however, a salinity tolerance threshold up to an EC of 18 dS m⁻¹ has been assessed for Tetragonia, which is a halophyte species [47].

Tailoring nutrient solution concentrations can not only optimize biomass but also influence the mineral element content and phytochemical profiles of plant tissues. Higher nutrient solution concentrations increased the levels of macronutrients like nitrogen, phosphorus, potassium, and magnesium in lettuce, but also led to higher nitrate content and a slight reduction in some nutritional parameters such as carbohydrates and proteins [48]. The higher nutrient supply determined an increase in the content of mineral elements in the baby plants of Tetragonia in both cultivation seasons without great variations due to the environmental effect. The values of mineral and vitamin content partially agree with those reported by Jaworska, and Kmiecik [49] and provided by the U.S. Department of agriculture [50]. A serving of 100 g of baby plants of Tetragonia produced in our trial with FS NS can, on average, satisfy the recommended daily values (DV) of β-carotene and can contribute to the intake of vitamin C (21% DV), iron (15.8% DV), Cu (23.6% DV), Mn (20.3% DV), and Zn (20.4% DV) [51].

The response of plants to hydroponic cultivation may vary greatly according to species or even to varieties, to agronomic factors like spacing and nutrient solution compositions, or to environmental factors like temperature, light intensity, and duration [13]. The effects of the experimental factors (season, plant density, and nutrient solution concentration) were shown well by the PCA that summarized the different responses of New Zealand spinach to the treatments. The PCA showed that increasing plant density and nutrient solution concentration differentially affected Tetragonia baby plants depending on the cultivation season, particularly during winter and at lower plant densities.

In this work, we hypothesized that baby plants of New Zealand spinach could be a viable ready-to-eat product. Consumers’ need to vary their vegetable consumption and the increasing demand for convenience vegetables could promote the spread of this species in the ready-to-eat (RTE) market. To achieve this goal, the leafy vegetable would require extended shelf life and quality maintenance [52,53].

Pre-harvest environmental conditions and agronomic practices can influence product characteristics and post-harvest physiology [20,54]. To investigate the effects of hydroponic agronomic management on Tetragonia baby plant shelf life, plants were minimally processed and cold-stored as a ready-to-eat (RTE) product for 21 days immediately after harvest. Minimally processed leafy vegetables are highly perishable and prone to rapid quality degradation during storage, leading to a loss of commercial value [20,55]. At harvest, vegetables are separated from the plant, severing their supply of water and nutrients. This can lead to physiological disorders, which may be exacerbated by pre-harvest environmental conditions or mineral imbalances during growth.

Deterioration during vegetable storage is often significantly influenced by weight loss, which directly affects product appearance and quality, especially in minimally processed products. This weight loss is primarily attributed to water loss through transpiration or evaporation, along with the degradation of carbohydrate reserves via respiration [56,57]. A linear increase in weight loss was observed in Tetragonia baby plants throughout the storage period. This loss was more pronounced in plants harvested during winter compared to spring. It was also shown for lettuce and baby-leaf rocket and spinach that the shelf-life attribute can vary greatly depending on the harvesting season [58]. Across both harvesting seasons, elevated plant density correlated with increased weight loss. Furthermore, in the spring trial, a higher nutrient supply (comparing half-strength and full-strength nutrient solutions) resulted in increased weight loss. Weight losses exceeding 4–6% can compromise the marketability of leafy vegetables [59]. In our trials, the baby plants did not overcome this threshold except after 21 days of cold storage in the baby plant grown at higher density and supplied with the full-strength nutrient solution. The density at which plants are cultivated in hydroponic systems can significantly affect their growth and morphology, which, in turn, may influence their post-harvest water retention. The direct influence of plant density on moisture loss in cold stored leafy vegetables is not well documented yet. Higher plant densities can lead to increased competition for light, nutrients, and space, potentially resulting in smaller, more succulent leaves with a lower thickness. Yang et al. [60] have related a lower water loss rate with the higher thickness of lettuce leaves. Leaf thickness and SLA are closely related leaf traits in leafy vegetables: as leaf thickness increases, specific leaf area decreases [61]. In our experiments, especially during the spring season, an increase in the specific leaf area (SLA) occurred when plants had lower space.

The concentration of the nutrient solution is another critical pre-harvest factor in hydroponic cultivation that can affect the post-harvest quality of leafy vegetables. Studies show that while higher nutrient concentrations can boost yield and increase certain mineral contents, they may also reduce levels of carbohydrates and proteins, and can negatively impact post-harvest storability, such as increasing weight loss and reducing the shelf life of leaf lettuce [62,63]. Additionally, the effects of nutrient concentration can vary by season, genotype, and specific quality parameters such as texture, color, and vitamin content as found in fresh-cut red and green lettuces [64]. The appearance and color of minimally processed leafy vegetables are critical determinants of consumer perception and choice, influencing overall satisfaction and perceived sensory quality. Both pre-harvest [55,65,66] and post-harvest factors [67,68] significantly influence these crucial visual characteristics and their post-harvest changes. During storage, winter baby plants experienced greater color changes (higher ΔE), irrespective of plant density. For spring baby plants, a lighter color was observed with increased plant density and nutrient solution concentration. Interestingly, these factors had an inverse effect on leaf greenness (hue values), which was higher with the full-strength solution but decreased at the highest plant density. Optimizing nutrient solution can delay senescence and help maintain leaf color, reduce physiological deterioration, and preserve visual quality [69]. Lower plant density may allow for better nutrient uptake and more robust tissue development, as water status and structural integrity of leaves are key indicators of freshness and color retention in leafy vegetables such as lettuce [69]. Despite seasonal variations, Tetragonia baby plants exhibited marketable quality for at least 14 days. Minimally processed leafy vegetables, while offering convenience, are inherently highly perishable due to the wounding stress of cutting, which accelerates physiological degradation, water loss, and microbial growth [70,71]. This rapid deterioration often results in a very short shelf life, presenting a significant challenge for both producers and consumers. While various post-harvest strategies aim to extend this period, understanding the minimum achievable shelf life is crucial for setting realistic expectations, optimizing supply chains, and ensuring product quality and safety. Generally, under optimal cold storage conditions (typically 0–5 °C and high relative humidity), the minimum shelf life for minimally processed leafy vegetables is often reported to be in the range of 4 to 7 days. Some sources indicate that even with diligent handling, certain highly perishable items, such as some types of cut lettuce, may exhibit satisfactory visual quality and freshness for as little as 5–9 days under refrigeration, with sensory changes such as browning or a loss of texture being the main limiting factors rather than microbial spoilage [72,73]. Technological interventions can extend shelf life by a few days; however, most minimally processed vegetables still maintain acceptable quality for less than two weeks [72,73]. This reinforces the excellent quality retention achieved with Tetragonia baby plants.

The results yielded important practical information. The obtained data on the response to climatic conditions and agronomic management can be used to establish the commercial production of New Zealand spinach baby plants and their processing as ready-to-eat (RTE) vegetables. Nevertheless, a finer tuning of the nutrient solution composition could help improve the nutritional quality.

5. Conclusions

Tetragonia plants demonstrated adaptability to hydroponic cultivation for baby plant production across the different growing periods of the two experiments. Nevertheless, climatic conditions, plant density, and nutrient supply significantly influenced plant growth, yield, nutritional quality, and post-harvest storage. The highest plant density combined with the full-strength nutrient solution resulted in the highest yield and favorable nutritional characteristics. Moreover, Tetragonia baby plants proved suitable for minimal processing, maintaining good quality retention for a minimum of 14 days. Further studies could be useful to assess the tolerance of this halophytic species to salt stress under hydroponic cultivation for baby plant production and the effects on their nutritional value.