Light quality affects light harvesting and carbon sequestration during the diel cycle of crassulacean acid metabolism in Phalaenopsis

Simultaneous Application of Red and Blue Light Regulate Carbon and Nitrogen Metabolism, Induces Antioxidant Defense System and Promote Growth in Rice Seedlings under Low Light Stress

The purpose of this study is to determine the effect of light quality on growth, carbon and nitrogen metabolism, and antioxidant defense system of rice seedlings. Six light conditions were employed, including white (W), red (R), blue (B), combined LED of R and B at 3:1 (R3B1), combined LED of R and B at 1:1 (R1B1), as well as combined LED of R and B at 1:3 (R1B3). Combined application of red light and blue light could promote the growth of rice seedling leaves and roots under low light stress to varying degrees, increase the photosynthetic area by increasing the leaf area, improve the root characteristics by increasing the root volume, and increase the dry matter accumulation of rice seedlings. In addition, the combination of red light and blue light could increase carbon and nitrogen metabolites in rice seedling leaves, regulate the expression of genes related to carbon and nitrogen metabolism and enzyme activity, and enhance the antioxidant enzyme activity of rice seedlings. These results indicate that red light and blue light directly have synergistic effects which can regulate the carbon and nitrogen metabolism of rice seedlings, promote the morphogenesis of rice seedlings under low light stress, and promote growth, which has never been reported in previous studies. This study is a new discovery in the application of light quality in crop production and provides new avenues to enhance crop stress resistance. However, further study is needed to explore the physio-biochemical and molecular mechanisms of light quality in crop production.

Changes in crassulacean acid metabolism expression, chloroplast ultrastructure, photochemical and antioxidant activity in the Aloe vera during acclimation to combined drought and salt stress

We determined time course changes of photochemical and antioxidant activity during the induction of strong crassulacean acid metabolism (CAM) in Aloe vera L. plants grown under salt and drought stress. We found that the strong CAM was induced during 25-30days of drought alone treatment. After 25-30days, we showed the withdrawal of strong CAM back to constitutive CAM background under the combination of simultaneous drought and salt stress, which coincided with the accumulation of malondialdehyde, and the decrease in the contents of endogenous nitric oxide (NO) and non-enzymatic antioxidants. At the same time, the chloroplast ultrastructure was damaged with a parallel accumulation of reactive oxygen species, and the whole photosynthetic electron transport flux was impaired by combined stress treatment. In conclusion, the changes in CAM expression parameters was attended by a similar pattern of antioxidant and photochemical change in Aloe plants subjected to only drought or combined stress.

Genetic architecture of photosynthesis energy partitioning as revealed by a genome-wide association approach

The photosynthesis process is determined by the intensity level and spectral quality of the light; therefore, leaves need to adapt to a changing environment. The incident energy absorbed can exceed the sink capability of the photosystems, and, in this context, photoinhibition may occur in both photosystem II (PSII) and photosystem I (PSI). Quantum yield parameters analyses reveal how the energy is managed. These parameters are genotype-dependent, and this genotypic variability is a good opportunity to apply mapping association strategies to identify genomic regions associated with photosynthesis energy partitioning. An experimental and mathematical approach is proposed for the determination of an index which estimates the energy per photon flux for each spectral bandwidth (Δ_λ) of the light incident (QI index). Based on the QI, the spectral quality of the plant growth, environmental lighting, and the actinic light of PAM were quantitatively very similar which allowed an accurate phenotyping strategy of a rice population. A total of 143 genomic single regions associated with at least one trait of chlorophyll fluorescence were identified. Moreover, chromosome 5 gathers most of these regions indicating the importance of this chromosome in the genetic regulation of the photochemistry process. Through a GWAS strategy, 32 genes of rice genome associated with the main parameters of the photochemistry process of photosynthesis in rice were identified. Association between light-harvesting complexes and the potential quantum yield of PSII, as well as the relationship between coding regions for PSI-linked proteins in energy distribution during the photochemical process of photosynthesis is analyzed.

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO₂ uptake, via phosphoenolpyruvate carboxylase (PEPC, C₄ carboxylation), from the diurnal refixation by Rubisco (C₃ carboxylation). At the end of the day (CAM-Phase IV), when nocturnally stored CO₂ has depleted, stomata reopen and allow additional CO₂ uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO₂ uptake via C₃ and C₄ carboxylation in phase IV in the CAM species Phalaenopsis "Sacramento" and Kalanchoe blossfeldiana "Saja." Short blackout periods during phase IV caused a sharp drop in CO₂ uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O₂ indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.

Effects of sub-optimal illumination in plants. Comprehensive chlorophyll fluorescence analysis

The fluorescence signals emitted by chlorophyll molecules of plants is a promising non-destructive indicator of plant physiology due to its close link to photosynthesis. In this work, a deep photophysical study of chlorophyll fluorescence was provided, to assess the sub-optimal illumination effects on three plant species: L. sativa, A. hybridus and S. dendroideum. In all the cases, low light (LL) treatment induced an increase in pigment content. Fluorescence ratios - corrected by light reabsorption processes - remained constant, which suggested that photosystems stoichiometry was conserved. For all species and treatments, quantum yields of photophysical decay remained around 0.2, which meant that the maximum possible photosynthesis efficiency was about 0.8. L. sativa (C3) acclimated to low light illumination, displayed a strong increase in the LHC size and a net decrease in the photosynthetic efficiency. A. hybridus (C4) was not appreciably stressed by the low light availability whereas S. dendroideum (CAM), decreased its antenna and augmented the quantum yield of primary photochemistry. A novel approach to describe NPQ relaxation kinetics was also presented here and used to calculate typical deactivation times and amplitudes for NPQ components. LL acclimated L. sativa presented a much larger deactivation time for its state-transition-related quenching than the other species. Comprehensive fluorescence analysis allowed a deep study of the changes in the light-dependent reactions of photosynthesis upon low light illumination treatment.

Effects of light quality on leaf growth and photosynthetic fluorescence of Brasenia schreberi seedlings

Brasenia schreberi J. F. Gmel, a perennial floating-leaved macrophyte with high economic value as an aquatic vegetable, has been listed as first-class endangered species in China, mainly due to its habitat loss. Protected cultivation is a potential strategy to meet the demand of both plant conservation and vegetable market, whereas pre-experiments are still needed before series of parameters can be properly set for the large-scale growth of the plants indoor. Light quality is one of the major factors controlling the development of plants and consequently becomes an important factor when planting B. schreberi indoor. This experiment used three artificial light sources to investigate the response of B. schreberi seedlings to different light qualities, including the red-blue LED light (red: blue = 5:1, RB-LED), the white LED light (W-LED) and the white fluorescent (W-Fluo). Our results indicated that the responses of B. schreberi towards varied light qualities differed from those of most terrestrial plants. The total leaf number of the RB-LED treatment was the highest; the number of the submerged leaf and the rolled leaf of the RB-LED treatment was higher than that of the other two treatments, but the number of floating leaves was the lowest. Both the specific leaf weight and the pigment contents per unit leaf area were the lowest in the RB-LED treatment. Quantum yield of PSⅡ (Φ_PSⅡ), electron transport rate (ETR) and photochemical quenching (qP) measured through light induction curves followed the sequence from high to low as W-Fluo > W-LED > RB-LED, whereas the trend of non-photochemical quenching (NPQ) reversed. The maximum potential ETR (P_s) and maximum ETR (ETR_m) derived from ETR curves further verified the trends.

Performance Index and PSII Connectivity Under Drought and Contrasting Light Regimes in the CAM Orchid Phalaenopsis

Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis characterized by improved water use efficiency mediated by major nocturnal CO2 fixation. Due to its inherent metabolic plasticity CAM represents a successful physiological strategy for plant adaptation to abiotic stress. The present study reports on the impact of drought stress and different light intensities (PPFD 50 and 200 μmol m-2 s-1) on the photosynthetic performance of the obligate CAM orchid Phalaenopsis "Edessa" by integrating diel gas exchange patterns with assessments of the light reactions by analyzing fast chlorophyll a fluorescence induction. Parameters such as PIabs (performance index), different energy fluxes per active reaction centre (RC) reflecting the electron flow from photosystem II to photosystem I and the energetic communication between PSII complexes defined as connectivity were considered for the first time in a CAM plant. A higher PS II connectivity for plants grown under low light (p ∼ 0.51) compared to plants grown under high light (p ∼ 0.31) brought about similar specific energy fluxes of light absorbance, dissipation and processing through the electron transport chain, irrespective of the light treatment. With a 25% higher maximum quantum yield and comparable biomass formation, low light grown plants indeed proved to process light energy more efficiently compared to high light grown plants. The performance index was identified as a very reliable and sensitive parameter to indicate the onset and progress of drought stress. Under restricted CO2 availability (due to closed stomata) leaves showed higher energy dissipation and partial inactivation of PSII reaction centres to reduce the energy input to the electron transport chain and as such aid in avoiding overexcitation and photodamage. Especially during CAM idling there is a discrepancy between continuous input of light energy but severely reduced availability of both water and CO2, which represents the ultimate electron acceptor. Taken together, our results show a unique flexibility of CAM plants to optimize the light reactions under different environmental conditions in a dual way by either attenuating or increasing energy flux.