1. Introduction
Shading is widely employed during the growth of plants, particularly plants with ornamental foliage to prevent damage triggered by high light intensity [
1,
2]. When light-harvesting antennas absorb more light energy than their potential for photochemical and non-photochemical energy dissipation, photodamage can occur [
3]. In the most severe cases, this may result in the discoloration of leaves or necrosis. Light damage happens often as a result of continuous exposure to high light intensity levels [
4]. As a result, growers apply shades to foliage or shade plants by covering the shutter or using a whitewash on the greenhouse cover to avoid direct exposure under high light conditions.
Light is an essential energy source for plant photosynthesis, although it can cause photodamage [
5]. Light is a key environmental factor that influences the morphological and physiological performance of plants. Plants that are exposed to a specific irradiance are mostly adapted to this light environment [
6]. For instance, plants exhibit notable adaptability and plasticity to varying light conditions by modifying their photosynthetic apparatus and morphological traits [
5]. Various indoor, foliage plants are exposed to low-light environments for a long time after being sold [
1]. The plants with a high photosynthesis ability under low light or shade conditions acclimate naturally to survive by decreasing their light compensation points and increasing leaf size and chlorophyll contents [
7,
8].
The net photosynthetic rate/chloroplastic CO
2 response curve (
Pn/
Cc curve) and the
Pn/
I curve are effective measures in plant physiology. Both curves help researchers to consider the consequences of differences in one or more major elements causing photosynthesis [
9]. Photosynthesis is a key physiological trait to assess the general performance and photosynthesis ability of plants [
10]. It is also known as the assimilation rate, which is an important physiological index to determine the growth efficiency of plants. Photosynthesis in plants can be influenced by various factors, such as leaf age and position, sink effects, mutual shading, as well as environmental factors, such as light, temperature, nutrition, and water availability [
11]. Therefore, we attempt to acquire insight into the potential of leaves at various positions, developed and matured under different shade levels to conform to the complexity of their photosynthetic response.
Studies suggest that fully expanded leaves have been used to determine the net photosynthetic rate (
Pn) in plants [
12,
13]. However, few previous studies have also compared the photosynthesis between newly emerged leaves and fully developed mature leaves. For instance, in wild-type tobacco plants, newly emerged leaves have the lowest
Pn compared to that of fully developed mature leaves [
14]. Furthermore, the application of nitrogen (N) fertilization at different concentrations did not influence the net photosynthetic rate and carboxylation rate (Vc
max) of the incomplete leaves. However, under various treatments, the
Pn and Vc
max of the incomplete leaves varied significantly [
15]. In the process of continuous differentiation and development of new leaf tissues of plants, N storage plays an important role in the synthesis of photosynthetic proteins [
16] and continues to differentiate until the leaf stops expanding. Therefore, fully developed leaves are representative of the indicative photosynthetic capacity in plants [
17,
18].
The fully expanded leaves in plants can be categorized as, young, mature, and old leaves. Evidence suggests that during their development stages, the
Pn also varies. For example, under high light conditions, the
Pn of the third leaf of wheat plants reached a maximum on the seventh day after emergence and declined thereafter [
19]. Zhou et al. [
20] compared the temperature responses of photosynthesis and respiration of both the young and old leaves of
Quercus aquifolioides in an alpine oak forest, where the old leaves have shown a lower net assimilation rate relative to the young leaves. However, the fully expanded leaves of kiwifruit have exhibited lower respiration rates compared to young leaves [
21]. In addition, leaf position is also related to light interception, which may influence the CO
2 assimilation [
22]. Escalona et al. [
23] have demonstrated that Spanish grapevine leaves exhibit comparable radiation use efficiency from all locations of the canopy except for those in the central part, although other considerations, such as different leaf age might play only a minimal role. However, contradictory results have been reported for Asian pear leaves, where nodes (3 to 16) have greater saturation vapor pressure and transpiration rates. Both the apical and basal leaves have higher stomatal resistance and lower
Pn than the leaves located in an intermediate position [
24]. The aforesaid studies did not document reliable photosynthetic responses of ornamental plants owing to their leaf age, position, and expanded conditions under varying light conditions. Therefore, the complexity of these plant’s photosynthesis and their adaptability under different shade levels require further attention.
Plants from the genus
Anthurium are popular ornamental foliage plants [
25,
26].
Anthurium × ‘Red’ is widely used as an indoor ornamental plant. It has a long flowering period as well as bright green leaves and is renowned for its high aesthetic value.
Anthurium × ‘Red’can be adaptive to shade conditions and has a long life as an indoor plant. We hypothesized that different light levels would impact not only the photosynthetic activity of
Anthurium × ‘Red’ plants but also their ornamental quality and subsequent performance under interior conditions. To test this hypothesis, the current research was designed to investigate the variations in photosynthesis of
Anthurium × ‘Red’ under different light conditions, to compare their photosynthetic potential at different leaf positions and ages, and to evaluate their performance after being placed in interior conditions. It was anticipated that such a study could provide growers and interior plantscapers with science-based information on better production and indoor use of
Anthurium, and foliage plants in general.
4. Materials and Methods
4.1. Plant Materials and Growth Conditions
The current study was conducted in Central Florida from October 2018 to April 2019. Tissue-cultured liners of
Anthurium × ‘Red’ were transplanted to 15 cm diameter containers (height = 30 cm and diameter = 15 cm) filled with Vergo Mix A. Each container was top-dressed with 5 g of an eight-month formulation of Osmocote 17-7-12 (The Scott Co., Marysville, OH, USA) and watered once a week. Fifteen plants with a similar growth size and leaf color were selected, divided into three groups, and grown in a shaded greenhouse under three light levels. The greenhouse was covered by double layer polyethylene film, and shade cloth with three different densities was installed inside, resulting in three sections with daily maximum PPFDs of 550, 350, and 255 μmol·m
−2·s
−1 as high (H), medium (M), and low (L) levels, respectively. The experiment was arranged as a completely randomized design with five replications. After plants were established in a shaded greenhouse for six months, three leaves were selected and marked as 1, 2, and 3 for a bottom old leaf, center mature leaf, and top young expanded leaf, respectively on each plant. We marked the leaves of H group as H1, H2, and H3, M group as M1, M2, and M3, and L group as L1, L2, and L3. Thereafter, all the replicates were moved to the interior room with a light intensity of 30 μmol·m
−2·s
−1,12 h a day, provided by white fluorescent lamps following Li et al. [
2].
4.2. Leaf Greenness Estimated by SPAD
Leaf SPAD (Soil—Plant Analysis Development) readings of the marked leaves were recorded before and one-month after plants were moved to the interior rooms by using a SPAD-502 m (Konica-Minolta, Japan) as described by Wang et. al. [
44]. Five independent SPAD measurements were determined on each marked leaf of each plant, and total chlorophyll concentrations were determined following Wang et al. [
45].
4.3. Net Photosynthetic Rate Comparison of Anthurium × ‘Red’ under 30 PPFD
The Pn was measured once every two months (10/15/2018, 12/18/2019, 2/15/2019, and 4/16/2019) during greenhouse growing as well as 1, 2, 3, 6, 8, 12, 18, and 24 days after moving to the interior room, respectively. All the measurements were carried out at a photosynthetic photon flux density (PPFD) value of 30 μmol·m−2·s−1, which was the same as the interior room light condition, a CO2 concentration of 400 μmol·mol−1 on a sunny day between local time 9:00 and 12:00 a.m. by using the Li-6800 portable photosynthesis system (LI-COR, Inc., Lincoln, NE, USA). Three leaves from different positions on each plant were measured and there were five plants in each group.
4.4. Light–Response Curve Comparisons of Anthurium× ‘Red’
For each treatment, three marked leaves were measured for light—response curves before (0) and 12 days after moving to the interior rooms, respectively. A photosynthetic photon flux density (PPFD) gradient of 0, 10, 20, 30, 50, 100, 200, 400, 600, and 800 μmol·m−2·s−1 and a CO2 concentration of 400 μmol·mol−1 were used for the measurement of irradiance responses using a Li-6800 portable photosynthesis system (LI-COR, Inc., Lincoln, NE, USA). A half an hour photoinduction under 300 μmol·m−2·s−1 was carried out before each measurement. The leaf temperature was 25 ± 0.5 °C, and the relative humidity was 50 ± 1%.
The irradiance (I)–response curves of photosynthesis were fitted following the modified model of the rectangular hyperbola [
46] as follows:
where
P(I) is
Pn;
Rd is the rate of dark respiration; and α, β, and γ are the coefficients that are independent of I.
The compensation irradiance,
Ic, was calculated as follows [
47]:
The saturation irradiance,
Isat, was determined using the following formula [
47]:
The maximum photosynthetic rate,
Pn-max, was calculated as follows [
47]:
4.5. Changes in Plant Morphology before and after Moving into the Interior Rooms
The number of leaves, newly emerged leaves, flower count, flower longevity, and growth index were also determined monthly for the first six months, then, all the plants were moved to interior rooms, where these attributes were recorded weekly.
4.6. Data Analysis
SPSS software (version 19.0, IBM Corp., Armonk, NY, USA) was used for the statistical analysis of the data. All data were subjected to analysis of variance (ANOVA). If significance occurred among treatments, means were separated by Tukey HSD (honestly significant difference) test at P < 0.05 level. All the values were presented as mean ± standard errors. Additionally, the software Origin® v. 8.5 (Origin-Lab Corp., Northampton, MS, USA), Prism v. 8.0.1 (GraphPad, San Diego, CA, USA), and Microsoft Excel-2016 were used for visualization (light–response curve fitting model) and tables, respectively.