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Chlorophyll a (Chl a) has an asymmetrical molecular organization, which dictates its orientation and the location of the pigment in the mature photosynthetic apparatus. Although Chl a fluorescence (ChlF) is widely accepted as a proxy for plant photosynthetic performance under countless stress conditions and across species, a mechanistic understanding of this causality is missing. Since water plays a much greater role than solvent for the photosynthetic machinery, elucidating its influence on Chl a may explain the reliable reflection of plant stress response in the ChlF signal. We examine the effect of hydration from well-watered to lethal drought on ChlF imagery results across morphologically diverse species to begin testing the impact of molecular scale hydration of Chl a on ChlF. Our results support a conceptual model where water is an integral part of the photosystems' structure and directly influences Chl a behavior leading to changes in the energy partitioning and ultimately in ChlF.

Crop improvement is crucial to ensuring global food security under climate change, and hence there is a pressing need for phenotypic observations that are both high throughput and improve mechanistic understanding of plant responses to environmental cues and limitations. In this study, chlorophyll a fluorescence light response curves and gas-exchange observations are combined to test the photosynthetic response to moderate drought in four genotypes of Brassica rapa. The quantum yield of PSII (ϕ(PSII)) is here analyzed as an exponential decline under changing light intensity and soil moisture. Both the maximum ϕ(PSII) and the rate of ϕ(PSII) decline across a large range of light intensities (0–1,000 μmol photons m(−2) s(−1); β(PSII)) are negatively affected by drought. We introduce an alternative photosynthesis model (β(PSII) model) incorporating parameters from rapid fluorescence response curves. Specifically, the model uses β(PSII) as an input for estimating the photosynthetic electron transport rate, which agrees well with two existing photosynthesis models (Farquhar-von Caemmerer-Berry and Yin). The β(PSII) model represents a major improvement in photosynthesis modeling through the integration of high-throughput fluorescence phenotyping data, resulting in gained parameters of high mechanistic value.

Phaseolus vulgaris L. plants were irradiated with different doses (0.3, 10, 50 and 100 Gy) of X-rays in order to obtain a reference curve of response to ionizing radiations for this species. Growth analysis, gas exchange and chlorophyll a fluorescence measurements were performed to estimate the radio-resistance of bean plants. Specifically, there was a negative influence of X-rays on the net photosynthesis rate at 50 and 100 Gy, already on the day of irradiation. Experimental data showed a recovery over time in the gas exchange while the theoretical maximum photochemical efficiency of the photosystem II (Fv/Fm) was fairly constant throughout the period of measurements (20 days) and for all the experimental conditions. On the other hand, the quantum yield of PSII linear electron transport (ΦPSII) and non-photochemical quenching (NPQ) were deeply influenced over time by X-ray dose, suggesting a decrease in the functionality of the photosynthetic apparatus at the highest radiation doses. The growth was affected only at the highest doses of radiation with a significant and severe reduction of leaf expansion and number of leaves per plant. Despite the arrest in growth, X-ray exposure seems to trigger an increased photochemical activity probably signifying that P. vulgaris plants have a fairly elevated resistance to this kind of ionizing radiation. Our current results will provide a complete analysis of the photosystem II (PSII) response of P. vulgaris to different doses (0.3, 10, 50 and 100 Gy) of X-rays, providing sound references for both space-oriented and radioecology questions.

Significance Photoprotection is an indispensable component of abiotic stress tolerance in all photosynthetic organisms. In contrast to land plants, the transcript abundance of photoprotection genes is strongly controlled by light in the model green alga Chlamydomonas reinhardtii. Here we show that the repression of photoprotection genes in low light depends on 2 putative components of a conserved E3 ubiquitin ligase complex. We also show that this regulation of gene expression depends on a transcription factor that is conserved among plants and algae and that acts downstream of the putative E3 ubiquitin ligase complex. Our work provides mechanistic insight into how a deeply conserved light signaling pathway is wired differently in C. reinhardtii, giving rise to light control of photoprotection gene expression., Photosynthetic organisms use nonphotochemical quenching (NPQ) mechanisms to dissipate excess absorbed light energy and protect themselves from photooxidation. In the model green alga Chlamydomonas reinhardtii, the capacity for rapidly reversible NPQ (qE) is induced by high light, blue light, and UV light via increased expression of LHCSR and PSBS genes that are necessary for qE. Here, we used a forward genetics approach to identify SPA1 and CUL4, components of a putative green algal E3 ubiquitin ligase complex, as critical factors in a signaling pathway that controls light-regulated expression of the LHCSR and PSBS genes in C. reinhardtii. The spa1 and cul4 mutants accumulate increased levels of LHCSR1 and PSBS proteins in high light, and unlike the wild type, they express LHCSR1 and exhibit qE capacity even when grown in low light. The spa1-1 mutation resulted in constitutively high expression of LHCSR and PSBS RNAs in both low light and high light. The qE and gene expression phenotypes of spa1-1 are blocked by mutation of CrCO, a B-box Zn-finger transcription factor that is a homolog of CONSTANS, which controls flowering time in plants. CONSTANS-like cis-regulatory sequences were identified proximal to the qE genes, consistent with CrCO acting as a direct activator of qE gene expression. We conclude that SPA1 and CUL4 are components of a conserved E3 ubiquitin ligase that acts upstream of CrCO, whose regulatory function is wired differently in C. reinhardtii to control qE capacity via cis-regulatory CrCO-binding sites at key photoprotection genes.

The total solar eclipse of August 21, 2017 created a path of totality ~115 km in width across the United States. While eclipse observations have shown distinct responses in animal behavior often emulating nocturnal behavior, the influence of eclipses on plant physiology are less understood. We investigated physiological perturbations due to rapid changes of sunlight and air temperature in big sagebrush (Artemisia tridentata ssp. vaseyana), a desert shrub common within the path of eclipse totality. Leaf gas exchange, water potential, and chlorophyll a fluorescence were monitored during the eclipse and compared to responses obtained the day before in absence of the eclipse. On the day of the eclipse, air temperature decreased by 6.4 °C, coupled with a 1.0 kPa drop in vapor pressure deficit having a 9-minute lag following totality. Using chlorophyll a fluorescence measurements, we found photosynthetic efficiency of photosystem II (Fv’/Fm’) recovered to near dark acclimated state (i.e., 87%), but the short duration of darkness did not allow for complete recovery. Gas exchange data and a simple light response model were used to estimate a 14% reduction in carbon assimilation for one day over sagebrush dominated areas within the path of totality for the Western United States.

The dynamics of local climates make development of agricultural strategies challenging. Yield improvement has progressed slowly, especially in drought-prone regions where annual crop production suffers from episodic aridity. Underlying drought responses are circadian and diel control of gene expression that regulate daily variations in metabolic and physiological pathways. To identify transcriptomic changes that occur in the crop Brassica rapa during initial perception of drought, we applied a co-expression network approach to associate rhythmic gene expression changes with physiological responses. Coupled analysis of transcriptome and physiological parameters over a two-day time course in control and drought-stressed plants provided temporal resolution necessary for correlation of network modules with dynamic changes in stomatal conductance, photosynthetic rate, and photosystem II efficiency. This approach enabled the identification of drought-responsive genes based on their differential rhythmic expression profiles in well-watered versus droughted networks and provided new insights into the dynamic physiological changes that occur during drought., eLife digest Around 60% of the food produced worldwide relies entirely on rain for its water supply. However, in the decades ahead global climate change is predicted to cause droughts to happen more often and become more severe in many regions. Therefore, in order to sustain our food supply we need to better understand how plants respond to drought and then use that knowledge to improve the ability of crops to cope with it. Unlike animals, plants cannot move away from drought or other stressful situations so they must face these difficulties ‘head on’. For example, when water is in short supply, plants close pores known as stomata on the surface of their leaves to reduce water loss. However, these pores need to be open to allow carbon dioxide gas, which plants use to make sugars in a process called photosynthesis, to enter the plant. Their response to drought must therefore be carefully controlled to make sure that the plant is still capable of performing photosynthesis. Turnip, napa cabbage, bok choy and field mustard are all varieties of a crop species known as Brassica rapa. These crops are grown in relatively dry regions such as the Canadian prairies and northern China, making drought stress a major threat to production. Previous studies had shown that drought stress causes changes in the activities of genes at certain times of day. To investigate this further, Greenham, Guadagno et al. studied how young B. rapa plants grown in a controlled environment with a steady supply of water responded when watering stopped. The experiments show that, even before the plants show obvious signs of drought stress such as wilting, there are extensive changes in the activity of many genes and processes inside plant cells that vary according to the time of day. Greenham, Guadagno et al. used an analysis technique to bring together all of the data into a network based on similar patterns of changes over time. This identified groups of genes whose changes in activity match the timing of the observed changes in the opening and closing of stomata, photosynthesis and other processes. These represent very early responses to drought stress in the plant. This work emphasizes the importance of time of day on plant stress responses. Changes that occurred only in the morning could not have been detected by measurements taken in the afternoon, and vice versa. The next step is to find out which of the changes observed in this work are most important in making plants resistant to drought. In the future, these findings may help researchers to develop strategies that would improve drought resistance in crop plants.

The circadian clock regulates many aspects of leaf gas supply and biochemical demand for CO2 20 , and is hypothesized to improve plant performance. Yet the extent to which the clock may regulate the efficiency of photosystem II (PSII) and photoprotective mechanisms such as heat dissipation remains largely unexplored. Based on measurements of chlorophyll a fluorescence, we estimated the maximum efficiency of photosystem II in light (Fv'/Fm') and heat dissipation by non-photochemical quenching (NPQ). We further dissected total NPQ into its main components, qE (pH-dependent quenching), qT (state-transition quenching) and qI (quenching related to photoinhibition), in clock mutant genotypes of Arabidopsis thaliana, the cognate wild-type genotypes, and a panel of recombinant inbred lines (RILs) expressing quantitative variation in clock period. Compared to mutants with altered clock function, we observed that wild-type genotypes with clock period lengths of approximately 24 hr had both higher levels of Fv'/Fm ', indicative of improved PSII function, and reduced NPQ, suggestive of lower stress on PSII light harvesting complexes. In the RILs, genetic variances were significant for Fv'/Fm' and all three components of NPQ, with qE explaining the greatest proportion of NPQ. Bivariate tests of association and structural equation models of hierarchical trait relationships showed that quantitative clock variation was empirically associated with Fv'/Fm' and NPQ, with qE mediating the relationship with gas exchange. The results demonstrate significant segregating variation for all photoprotective components, and suggest the adaptive significance of the clock may partly derive from its regulation of the light reactions of photosynthesis and of photoprotective mechanisms. Key words: Arabidopsis thaliana, circadian rhythms, chlorophyll a fluorescence, maximum efficiency of PSII, non-photochemical quenching

Non-photochemical quenching (NPQ) of chlorophyll a fluorescence and leaf gas-exchange were investigated in relation to the chlorophyll and carotenoid content, and the xanthophyll cycles in wild type tomato (Solanum lycopersicum, L. cv Red Setter (RS)) and in two transgenic lines (UO and UU) over-expressing β-carotene hydroxylase. Potted plants were grown in a glasshouse under low light (LL, 100 μmol m−2 s−1) or high light (HL, 300 μmol m−2 s−1). The maximum quantum efficiency of photosystems II (PSII) photochemistry in dark-adapted leaves (Fv/Fm) was higher than 0.82 in all treatments while photosynthetic CO2 assimilation (A) was higher than 14 μmol m−2 s−1, and stomatal conductance (gs) higher than 0.4 mol m−2 s−1 in HL plants, indicating no effects induced by the genetic modification. Chlorophyll content and composition changed little, whereas transgenic plants had up to 47% higher total carotenoid content than wild type plants. Violaxanthin was the most abundant carotenoid in transgenic plants, with more than 2-fold higher content than the average 0.586 mg g−1 found in RS plants. Transgenic plants had similar light-induced steady-state NPQ compared to wild type plants, but had slower dark relaxation because of the decreased de-epoxydation state index due to the higher violaxanthin accumulation, despite the higher zeaxanthin content.

Non-photochemical quenching (NPQ) is a complex and still unclear mechanism essential for higher plants. The intensive research on this subject has highlighted three main components of NPQ: energy-dependent process (qE); state transitions to balance the excitation of PSII and PSI (qT); and photoinhibitory processes (qI). Recently, these components have been resolved as quantum yields according to the energy partitioning approach that takes into account the rate constants of every process involved in the quenching mechanisms of excited chlorophylls. In this study a fully extended quantum yield approach and the introduction of novel equations to assess the yields of each NPQ component are presented. Furthermore, a complete analysis of the yield of NPQ in Beta vulgaris exposed to different irradiances has been carried out. In agreement with experimental results here it is shown that the previous approach may amplify the yield of qE component and flatten the quantitative results of fluorescence analysis. Moreover, the significance of taking into account the physiological variability of NPQ for a correct assessment of energy partitioning is demonstrated.