The Effect of Light Availability on Photosynthetic Responses of Four Aglaonema commutatum Cultivars with Contrasting Leaf Pigment

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A potential application of irradiance adjustment simply based on leaf colorimetric characteristics for Aglaonema commutatum wide cultivation.

Abstract

Plants can be grouped into sun- and shade-type owing to their distinct irradiance adaptation. Aglaonema commutatum is a typical shade-tolerant perennial foliage plant native to South Asia with various leaf colorations. ‘Red’ cultivars are widely used for interior decorations and as courtyard ornamental plants, owing to their unique hue and bi-adaptation of both low and high irradiance. However, the underlying source of ‘red’ pigment-dependent irradiance bi-adaptation remains largely unknown. Therefore, four A. commutatum cultivars were comparatively evaluated in terms of pigment composition and photosynthetic rate under full light (sunlight) or 50% full light (shade) to unveil the relationship between pigmentation and irradiance bi-adaptation. Our results showed that the photosynthetic rate of sun leaves generated from light response curves was significantly correlated with anthocyanin content and chlorophyll in red cultivars, suggesting the influence of anthocyanin–light interactions on the high/low irradiance bi-adaptation of red A. commutatum. This study provides a theoretical basis for further cultivating red foliage A. commutatum cultivars under diverse light conditions.

1. Introduction

Photosynthesis is a crucial physiological process whereby plants convert solar energy into chemical energy through a series of complex metabolic reactions, forming the basis for plant growth and development [1]. Light is the only energy source and the most important environmental factor driving plant photosynthesis [2]. The sunlit and shaded leaves of plants can considerably differ in their relative composition of photosynthetic pigments, electron carriers, and photosynthetic rates [3,4,5], e.g., a higher chlorophyll a/b ratio, lower level of light-harvesting LHCII, and lower stacking degree of thylakoids are adaptations for high rates of photosynthetic quantum conversion in sun-type plants; however, the reverse is true for shade-type plants [6].

The photosynthetic activities of leaves, particularly the differences in net photosynthetic rates between sun and shade leaves, have primarily been determined by constructing the photosynthesis–light response curve [7], providing parameters in terms of the plant light compensation point, light saturation point, maximum net photosynthetic rate, apparent quantum efficiency, photorespiration rate, etc. [8].

Light is the only source of energy for higher plants and is an important factor affecting chlorophyll content and photosynthesis. The difference in light intensity received by leaves will directly affect plant growth and development and structural characteristics. Chlorophyll is the main absorber of light energy, which directly affects the light energy utilization of vegetation photosynthesis [9]. Under low-light conditions, the photopigment content of leaves, especially chlorophyll b (Chlb) content, will increase. Additionally, Chlb can effectively absorb low light and enhance the ability of leaves to capture light. In general, the presence of Chlb in shaded leaves of the same plant facilitates the absorption of blue-violet and orange light from diffused light in shaded environments, and the presence of positive and shaded leaves in the same plant indicates the ecological adaptation of the leaves to different light intensity environments. The relative content of photopigment proteins in photosynthetic units in plant leaves increases under low light because Chlb is mainly found in photopigment proteins; therefore, the increase in the relative content of Chlb in chlorophyll may be related to the increase in the relative content of photopigment proteins [10].

Aglaonema commutatum is a rhizomatous, evergreen perennial species belonging to the Araceae family that produces large, attractive, and distinctly marbled foliage colored silvery-white and green or red [11,12]. It is typically referred to as Chinese evergreen, a shade-tolerant plant that is extensively cultivated as a houseplant because of its attractive, often variegated foliage in vivid colors that differ among cultivars, especially due to its ability to survive under low irradiance conditions [13,14]. In recent years, A. commutatum ‘Red’ cultivars have been widely used in interior decorations and as courtyard ornamental plants because of their unique red hue and low/high irradiance bi-adaptation [13]. This phenomenon has considerable ornamental implications for the market value of the plants. Most studies on A. commutatum have focused on tissue culture [15,16,17], stress resistance, breeding [18], and interspecies genetic relationships at the physio-biochemical level. However, relatively few studies have examined the mechanisms underlying leaf color variations. Nonetheless, limited research into the relationship between ‘red’ pigments and irradiance bi-adaptation hampers the deep understanding of pigmentation–light interactions in red A. commutatum.

In this study, three red A. commutatum cultivars differing in red hue coverage were selected: Redder valentine, Tianshi, and Big apple; a green cultivar, Feicui, was also included as control. The correlation between the pigment composition of sun/shaded leaves and their corresponding photosynthetic characteristics was determined, revealing the effect of ‘red’ pigments on the photosynthetic light response of A. commutatum. This study provides a theoretical basis for further utilization of red foliage A. commutatum cultivars under diverse light conditions.

2. Materials and Methods

2.1. Materials

Four different cultivars of 1.5-year-old seedlings with plant height of 25–28 cm A. commutatum (Feicui, Redder valentine, Tianshi, and Big apple) were used in this study (Figure 1A). The test material is from Guangzhou Xilin Horticulture Co., Ltd. (Guangzhou, China). The material grows in good conditions and has no pests and diseases. Such cultivars differ in shape and leaf color: Feicui is a green-leaf cultivar and Redder valentine, Tianshi, and Big apple are red-leaf cultivars. The cultivars were arranged in a randomized complete block design with five replicates for each treatment.

2.2. Experimental Design

Cultivars were planted in a garden shade greenhouse at Zhongkai University of Agriculture and Engineering, China (23°22′18″ N, 113°26′30″ E). The cultivars were arranged in a randomized complete block design with five replicates for each treatment, resulting in a total of ten pots for each cultivar in this trial. Experiments were performed to measure the response of each cultivar selected under full light (sun) or 50% full light (shade). Four cultivars of A. commutatum were treated with two layers of commercial black cloth shade and sun for 30 days, beginning on 1 April 2022. Photosynthetic photon flux densities were full light: 1083 µmol‧m⁻²·s⁻¹; semi-shade: 581 µmol‧m⁻²‧s⁻¹.

2.3. Pigment and Colorimetric Analysis

The photosynthetic plant pigments Chl and Cars were extracted with 95% ethyl alcohol and placed in the dark at room temperature for 48 h to obtain the chlorophyll extract. Shimadzu UV2600 ultraviolet spectrophotometer (Japan) was used to estimate Chl a, Chl b, and carotenoids in the same clear extract solution, using the extinction coefficients and equations redetermined by Tanan et al. [19].

Anthocyanins were determined according to a procedure reported by Zhong et al. [20] with slight modifications. The leaves were soaked in 5 mL methanol containing 1% hydrochloric acid (hydrochloric acid: methanol = 1:99 (v/v)). The test tube was sealed using a bottle stopper to prevent water evaporation. After maintaining the leaves at 4 °C for 24 h, the optical density of the extracted anthocyanin was measured using a UV2600 ultraviolet spectrophotometer (Japan) at a wavelength of 531 nm. Anthocyanin content = (27.208 A + 0.0591) × D, where A is the light absorbance value and D is (10/1000/weight).

The colorimetric difference in the middle part of the A. commutatum leaves was measured using a CR-400 (Konica Minolta) color difference meter.

2.4. Determination of Physiological and Biochemical Indexes

The physiological and biochemical indices of A. commutatum were determined from the same part of the leaves in each treatment. The reducing sugar content was determined using the 5-dinitrosalicylic acid method [21]. Soluble protein content was determined using the Coomath bright blue G-250 method [22].

The projected leaf area (LAp) of all investigated leaves was estimated using an Area Meter AM-350 (ADC BioScientific Ltd., Bristol, UK), as well as their fresh weight (FW) and dry weight (DW) after drying at 80 °C for 48 h. Subsequently, the relative water content (RWC [%]; RWC = [(FW − DW)/FW] × 100), specific leaf area (SLA) [cm² g⁻¹ DW], and specific leaf weight (SLW) [g.cm⁻²] were determined, whereby the SLA = Lap/DW and SLW = DW/Lap were calculated in agreement with Gilmore et al. [23].

2.5. Determination of Light Response Curve

On a sunny day in May 2022, the light response curves of A. commutatum were measured using an LI-6800 portable photosynthesis system (LICOR, Lincoln, NE, USA). The healthy, mature functional leaves, third from the tip of the plant, were measured from 9:00 to 11:00 a.m. The measurements were repeated three times for each treatment. In addition, leaf temperature, relative air humidity, flow setpoint, and the CO₂ concentration inside the leaf chamber were kept constant at 26–30 °C, 55–60%, 500 μmol‧s⁻¹, and 400 µmol‧mol⁻¹, respectively; the minimum wait time was 180 sec; and the maximum wait time was 300 sec. The photosynthetic photon flux density for red and blue LED was set at 1800, 1500, 1200, 900, 600, 300, 200, 150, 100, 70, 30, and 0 µmol m⁻² s⁻¹.

2.6. Data Analysis

Photosynthetic light response functions were formulated based on a modified rectangular hyperbolic model [24] and calculations were performed using the Photosynthesis Model Simulation Software (PMSS, YE, Zi-Piao. Jinggangshan University, Ji’an, Jiangxi, China, free of charge):

$$A\left(I\right)=\alpha \left[\frac{1-\beta I}{1+\gamma I}\right]I-Rd$$

where A is the net photosynthetic rate, α is the initial slope of the light response curve, I is the photosynthetic photon flux density (PPFD), Rd is the dark respiration rate, β is the coefficient of photoinhibition, and γ is the coefficient of satiety (the unit m².s.μmol⁻¹).

Duncan’s test (belonging to the ANOVA group of tests) was used to evaluate the statistically significant differences between the sun and shade groups. Differences were assessed at a probability level of p < 0.05. All statistical tests were performed using SPSS version 26.0 (SPSS Inc., Chicago, IL, USA).

3. Results

3.1. General Leaf Colorimetric Characteristics and Pigmentation

The red coloration in the leaves is a special trait of A. commutatum rendering it fit for ornamental applications. Here, we selected four cultivars: Feicui (green), Redder valentine (red), Tianshi (red), and Big apple (red), varying in ‘red’ hue coverage (Figure 1A). To plot absolute color coordinates and differences among cultivars, we applied the CIE Lab color space consisting of three metrics: L*: lightness, a*: redness–greenness, and b*: yellowness–blueness. As shown in Figure 1B, we found separated values between Feicui and the other three red cultivars, especially for a* value. A shade-induced a* value increase was noted in Redder valentine and Tianshi (Figure 1B and Supplementary Table S1).

Contrary to expectations, sun treatment resulted in significantly lower pigment content per leaf mass in all four cultivars, except for ACN content in Feicui. Nevertheless, the sunlit leaves of Redder valentine and Tianshi still retained a comparatively higher level of ACNs among cultivars to contribute to their ‘red’ hue (Figure 1C). The typical differences between sun and shade leaves in the pigment composition of Chls, Cars, and ACNs were found to be consistent with their colorimetric properties, while elevated values for ratios of Chls/Cars and Chls/ACNs were also found in the shade leaves of all four cultivars (Figure 1C).

3.2. Physiochemical Variations among Cultivars × Irradiance

For all cultivars, no significant differences were found between sun and shade leaves in terms of SLA, SLW, leaf water content (RWC), and leaf DW, which was not in accordance with other observations showing higher SLA and RWC in shaded leaves than in sun leaves [5]. Nonetheless, Tianshi stood out among cultivars as having the highest SLA and RWC on both sun and shade treatments, which might be related to high shade tolerance (Figure 2A).

Plants tended to reduce tissue soluble protein but accumulate reducing sugar in response to adversity such as high light intensity, drought, and low temperature, rendering them able to resist stress [25,26]. Here, we also determined the following two parameters (Figure 2B): A slight increase in reducing sugar content was only found in the shaded leaves of Redder valentine, indicating its potential stress resistance under high irradiance, particularly in the context of oxidation [27]. Meanwhile, the soluble protein content was significantly downregulated in all cultivars under sun treatment, indicating a potential occurrence of photoinhibition in plants. In spite of the sun-induced loss in soluble protein, Tianshi and Redder valentine retained comparatively higher soluble protein levels than Feicui and Big apple by ~4.7 and ~2.9 times, respectively, echoing their ACN contents.

3.3. The Comparison of Photosynthetic Light Response

To determine the photosynthetic light response in leaves, the net photosynthetic rate (A) was measured in situ at incremental PPFD. The net photosynthetic rate (A) of all cultivars in both light treatments increased rapidly as PPFD increased from 0 to 250 μmol‧m⁻²‧s⁻¹. In addition, the modified rectangular hyperbolic model was well fit into A/PPFD curves (see Materials and Methods). Pairwise comparisons of A/PPFD curves between sun and shade were significant in Feicui, while Redder valentine and Tianshi exhibited mild shifts in A/PPFD curves (Figure 3).

As shown in

Table 1. Photosynthetic light response parameters of leaves of Feicui, Redder valentine, Tianshi, and Big apple grown in sun or shade. Data represent the mean ± SD (n = 3). Different lowercase letters in the same column indicate a significant difference (p < 0.05, Duncan’s multiple range test).

		Rd	Asat	Π	Γ	Φ
Feicui	shade	0.0663 ± 0.007 b	1.0092 ± 0.020 a	760.1623 ± 39.382 a	4.2592 ± 0.400 b	0.0049 ± 0.000 a
	sun	0.0879 ± 0.020 a	0.6852 ± 0.016 c	657.819 ± 60.751 b	6.4036 ± 0.445 a	0.0035 ± 0.000 c
Redder valentine	shade	0.0558 ± 0.006 b	0.8096 ± 0.030 b	679.2483 ± 11.734 b	2.9539 ± 0.409 c	0.0037 ± 0.000 c
	sun	0.0575 ± 0.004 b	0.9554 ± 0.043 a	639.518 ± 13.489 b	3.5211 ± 0.541 b	0.0047 ± 0.001 b
Tianshi	shade	0.0527 ± 0.006 b	0.8571 ± 0.031 b	686.6397 ± 35.981 b	3.2824 ± 0.334 b	0.0044 ± 0.000 b
	sun	0.0596 ± 0.008 b	0.9765 ± 0.042 a	636.5493 ± 9.017 b	3.7621 ± 0.491 b	0.0047 ± 0.000 b
Big apple	shade	0.0655 ± 0.003 b	0.6796 ± 0.009 c	652.6023 ± 33.848 b	3.9662 ± 0.676 b	0.0035 ± 0.000 c
	sun	0.0732 ± 0.010 a	0.6736 ± 0.034 c	548.8977 ± 9.518 c	4.9341 ± 0.604 a	0.0043 ± 0.000 b

, the highest light-saturated A (A_sat) generated from the A/PPFD curve fitting occurred in the shade plants of Feicui, whereas Redder valentine and Tianshi had notably higher A_sat among cultivars in the sun treatment, accompanied by a lower light compensation point (Γ) in both treatments. Generally, the respiration rate (R_d) was lower in shade plants than in sun plants for all cultivars, especially Tianshi. The effect of light on the apparent photosynthetic quantum yield (Φ) also reached its maximum value in Redder valentine and Tianshi. All these results implied that plasticity in irradiance adaptation was particularly well developed in Redder valentine and Tianshi.

3.4. Correlation between Pigment Composition and Photosynthetic Light Response

To explore whether the pigmentation of leaves is related to the photosynthetic light response of plants, we performed Pearson’s correlation analysis based on covariance (cf. Materials and Methods). As shown in Figure 4A, under the shade condition, contents of Chl and Car were significantly associated with two photosynthetic light response parameters, i.e., light-saturated A (A_sat) and light-saturated point (Π) levels, indicating that Chl and Car were sufficient to maintain stable photosynthesis in low irradiance like other shade-tolerant plants. However, in shade-treated leaves, it was the ACN contents that were highly correlated with the highest light-saturated A (A_sat), light-saturated point (Π), and light compensation point (Γ), apart from ‘red’ hue formation and stress resistance (Pearson’s ǀrǀ ≥ 0.7; p-value ≤ 0.05, Figure 4B). Higher ACN levels, for example in the sunlit leaves of Redder valentine, corresponded to greater plasticity in irradiance adaptation for photosynthesis, implying that ACNs would also be involved in the photosynthetic light response in the context of high irradiance adaptation.

4. Discussion

Insufficient light can be a limiting factor for photosynthesis, while too much light can also have a negative impact on photosynthesis. When the leaves absorb too much light energy and cannot use it or dissipate it in time, the plant will suffer from strong light stress. This causes a reduction in photosynthetic capacity and photoinhibition. The sensitivity of plants to photoinhibitory conditions is influenced by genetic factors and environmental factors. In the absence of environmental factors other than light stress, C3 plants such as vegetables are susceptible to photoinhibition under strong light at noon [28]. Plants chronically exposed to adverse light intensities are adapted to low and high light.

As a typical shade-tolerant foliar plant, A. commutatum might be physiochemically disturbed when exposed to high irradiance above its optimal intensity. All four cultivars of A. commutatum in this study exhibited major differences between sun and shade leaves, with respect to pigment composition and stress resistance indices, which influenced their ornamental application (Figure 1 and Figure 2B). These responses are consistent with those found in other plants [29]. There was no significant reduction in RWC and SLA in the sun leaves of all A. commutatum cultivars, which was not the case in some shade-sensitive plants, such as Acer pseudoplatanus, Fagus sylvatica, and Tilia cordata (Figure 2A) [5].

It is well known that shade-tolerant plants have evolved shade-type chloroplasts with higher and broader grana thylakoid stacks and have primarily invested in the Chl antenna to maintain a moderate photosynthetic rate at low irradiance [30,31]. Therefore, photodamage is more severe in shade-tolerant plants, which limits photosynthesis by photoinhibition, resulting in reduced photosynthetic rate and carbon gain, which is called photodamage [32,33,34]. In this study, we confirmed a dramatic decrease in the photosynthetic rate in the sun leaves of Feicui (a larger light-induced A/PPFD shift, higher light-saturated point, lower A_sat, and light compensation point), regardless of its reduced Chls/Cars ratio, a typical photoinhibition phenomenon that could not be mitigated by losing excitation energy through the xanthophyll cycle for carotenoids (Figure 1C and Figure 3; Table 1) [35,36,37]. On the contrary, three ‘red foliar’ cultivars displayed improved ACN percentage of pigments, exhibiting stable (Big apple) or even enhanced (Redder valentine and Tianshi) photosynthetic rates in sunlit leaves compared with those in shaded leaves (Figure 3 and Table 1). This phenomenon could be explained by the photoprotective and antioxidant effects of ACNs; those in the epidermis and/or hypodermis screen excessive light, for example UV-B [38]; those in the mesophyll protect chloroplasts from photoinhibition, for instance reactive oxygen species (ROS) scavenging; those in the lower tissues trigger internal reflection capture. When plants were grown under sunlight exposure, the ACN levels of sun leaves correlated well with the photosynthetic light response apart from light-induced stress tolerance (Figure 4); however, shade leaves of ‘red foliar’ cultivars did not benefit from even higher ACN content, probably due to the competition between light-harvesting shade-type chloroplasts (Chls and Cars) and vacuoles (ACNs). Moreover, one reason for the light-induced pigment degradation observed in this study (Figure 1C) might be a shade-adaptive strategy via shade-type biochemical organization and arrangement of pigments to ensure normal photosynthesis under low irradiance [5].

Taken together, whether leaves are shaded or sunlit when the ACN content is maintained at a certain level, the plant’s biological processes stabilize photosynthetic robustness. Therefore, higher-than-optimal irradiance does not negate the beneficial effects of ACNs in A. commutatum.

5. Conclusions and Further Work

In this study, four A. commutatum cultivars differing in leaf pigmentation were investigated for photosynthetic light responses: 1.5-year-old seedlings grown in full light (sun) or 50% of full light (shade) were comparatively determined in terms of pigment compositions and photosynthetic rates aiming to guide the relationship between pigmentation and irradiance bi-adaptation in red A. commutatum cultivars. Our results showed that the photosynthetic rate of sun leaves significantly correlated with anthocyanin content in addition to chlorophyll in red cultivars, suggesting the notable effect of anthocyanin–light interactions on high/low irradiance bi-adaptation of red A. commutatum. It would provide a theoretical basis for the further cultivation of red foliage A. commutatum cultivars under diverse light condition.

Given the encouraging results in this study, we plan to expand this work to construct a precise correlation between leaf colorimetric characteristics and optimal light conditions, so that we can adjust irradiance for the successful cultivation of different A. commutatum simply based on the leaf colorimetric characteristics.

Furthermore, in view of the stable photosynthetic rate of red A. commutatum converting solar energy into chemical energy for storage in terms of substantial biomass, it is a fair candidate for materials applied in bioreactors that mimic some or all the reactions of photosynthesis, which is so-called artificial photosynthesis. Artificial photosynthesis can overcome the shortcomings of natural photosynthetic mechanisms, free from the limitations of the natural environment, achieve more efficient photosynthesis, reduce the demand for land and water resources for traditional agricultural production, curb the increase in atmospheric CO₂ concentration, mitigate the warming trend, and help overcome the energy crisis [39].