Growth, phytochemical, and phytohormonal responses of basil to different light durations and intensities under constant daily light integral.

Horticulture in controlled environments has been increasingly used to tackle limitations on crop production. As a crucial environmental factor, light regulate plant growth and metabolism. In the present study, basil plants were subjected to different light durations and intensities considering constant daily light integral (DLI). The lighting environment included 200, 300, and 400 µmol m- 2 s- 1 intensities for 18, 12, and 9 h, respectively. DLI amounted to 12.96 mol m- 2 d- 1 among all light treatments (LI200 for 18 h, LI300 for 12 h, and LI400 for 9 h). Half of the plants under each light treatment were exposed to 30 µmol m- 2 s- 1 of far-red light. The results indicated the general negative impact of LI400/9 on the growth of basils. Exposure to far-red light hurt the growth of the shoot, while it enhanced stem and petiole elongation. This effect was due to higher gibberellin accumulation, which resulted in shade avoidance responses. Exposure to far-red light also reduced anthocyanin and flavonoid contents, as two important nutritional components. Soluble carbohydrates increased, while storage carbohydrates decreased by increasing lighting duration/decreasing light intensity or by far-red light inclusion. The lowest antioxidant activity was detected in LI400/9. In the LI200/18, the highest level of auxin and the lowest level of cytokinin were detected, while the LI300/12 exhibited the highest level of gibberellin hormone. Low light intensity and long photoperiod enhanced plant biomass and phytochemical production and are recommended for basil production in controlled environments.

Phytochemical compositions and antioxidant activity of green and purple basils altered by light intensity and harvesting time.

Ocimum basilicum L. is one of the most important medicinal and vegetable crops due to its essential oil, pleasant aroma and taste. In this study, we evaluated the impact of different light intensities, including 100 %, 50 %, and 30 % of natural sunlight, on the growth, phytochemical compositions, and antioxidant activity of green and purple basil cultivars at two different harvest times: early morning and noon. The height of the plant, number of leaves per plant, length of the petiole, diameter of the stem, and fresh and dry weight of the shoot were all reduced by decreasing light intensity in both basil cultivars. When the plants of both cultivars were grown under full light intensity and were sampled at noon, they showed the highest phenolic and flavonoid contents. The highest antioxidant activity was detected in purple basil cultivars grown under 30 and 50 % of sunlight in both harvests. The green basil cultivar showed the highest antioxidant activity when exposed to 30 % sunlight and harvested in the early morning. The highest essential oil content and yield in both basil cultivars were obtained under full sunlight in the early morning harvests. In summary, light intensity and harvest time influence the phytochemical yield, composition, and growth of two studied basil cultivars. Optimal results, particularly for medicinal purposes, were achieved by morning harvesting to maximize the essential oil yield of basils.

The crosstalk of far-red energy and signaling defines the regulation of photosynthesis, growth, and flowering in tomatoes.

This study investigated the effect of light intensity and signaling on the regulation of far-red (FR)-induced alteration in photosynthesis. The low (LL: 440 µmol m-2 s-1) and high (HL: 1135 µmol m-2 s-1) intensity of white light with or without FR (LLFR: 545 µmol m-2 s-1 including 115 µmol m-2 s-1; HLFR: 1254 µmol m-2 s-1 + 140 µmol m-2 s-1) was applied on the tomato cultivar (Solanum Lycopersicon cv. Moneymaker) and mutants of phytochrome A (phyA) and phytochrome B (phyB1, and phyB2). Both light intensity and FR affected plant morphological traits, leaf biomass, and flowering time. Irrespective of genotype, flowering was delayed by LLFR and accelerated by HLFR compared to the corresponding light intensity without FR. In LLFR, a reduced energy flux through the electron transfer chain along with a reduced energy dissipation per reaction center improved the maximum quantum yield of PSII, irrespective of genotype. HLFR increased net photosynthesis and gas exchange properties in a genotype-dependent manner. FR-dependent regulation of hormones was affected by light signaling. It appeared that PHYB affected the levels of abscisic acid and salicylic acid while PHYA took part in the regulation of CK in FR-exposed plants. Overall, light intensity and signaling of FR influenced plants' photosynthesis and growth by altering electron transport, gas exchange, and changes in the level of endogenous hormones.

Interplay among photoreceptors determines the strategy of coping with excess light in tomato.

This study investigates photoreceptor's role in the adaption of photosynthetic apparatus to high light (HL) intensity by examining the response of tomato wild type (WT) (Solanum lycopersicum L. cv. Moneymaker) and tomato mutants (phyA, phyB1, phyB2, cry1) plants to HL. Our results showed a photoreceptor-dependent effect of HL on the maximum quantum yield of photosystem II (Fv/Fm) with phyB1 exhibiting a decrease, while phyB2 exhibiting an increase in Fv/Fm. HL resulted in an increase in the efficient quantum yield of photosystem II (ΦPSII) and a decrease in the non-photochemical quantum yields (ΦNPQ and ΦN0) solely in phyA. Under HL, phyA showed a significant decrease in the energy-dependent quenching component of NPQ (qE), while phyB2 mutants showed an increase in the state transition (qT) component. Furthermore, ΔΔFv/Fm revealed that PHYB1 compensates for the deficit of PHYA in phyA mutants. PHYA signaling likely emerges as the dominant effector of PHYB1 and PHYB2 signaling within the HL-induced signaling network. In addition, PHYB1 compensates for the role of CRY1 in regulating Fv/Fm in cry1 mutants. Overall, the results of this research provide valuable insights into the unique role of each photoreceptor and their interplay in balancing photon energy and photoprotection under HL condition.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits.

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Growth, photosynthetic function, and stomatal characteristics of Persian walnut explants in vitro under different light spectra.

Light plays a crucial role in photosynthesis, which is an essential process for plantlets produced during in vitro tissue culture practices and ex vitro acclimatization. LED lights are an appropriate technology for in vitro lighting but their effect on propagation and photosynthesis under in vitro condition is not well understood. This study aimed to investigate the impact of different light spectra on growth, photosynthetic functionality, and stomatal characteristics of micropropagated shoots of Persian walnut (cv. Chandler). Tissue-cultured walnut nodal shoots were grown under different light qualities including white, blue, red, far-red, green, combination of red and blue (70:30), combination of red and far-red (70:30), and fluorescent light as the control. Results showed that the best growth and vegetative characteristics of in vitro explants of Persian walnut were achieved under combination of red and blue light. The biggest size of stomata was detected under white and blue lights. Red light stimulated stomatal closure, while stomatal opening was induced under blue and white lights. Although the red and far-red light spectra resulted in the formation of elongated explants with more lateral shoots and anthocyanin content, they significantly reduced the photosynthetic functionality. Highest soluble carbohydrate content and maximum quantum yield of photosystem II were detected in explants grown under blue and white light spectra. In conclusion, growing walnut explants under combination of red and blue lights leads to better growth, photosynthesis functionality, and the emergence of functional stomata in in vitro explants of Persian walnuts.

Optimizing supplemental light spectrum improves growth and yield of cut roses.

During the seasons with limited light intensity, reductions in growth, yield, and quality are challenging for commercial cut rose production in greenhouses. Using artificial supplemental light is recommended for maintaining commercial production in regions with limited light intensity. Nowadays, replacing traditional lighting sources with LEDs attracted lots of attention. Since red (R) and blue (B) light spectra present the important wavelengths for photosynthesis and growth, in the present study, different ratios of supplemental R and B lights, including 90% R: B 10% (R90B10), 80% R: 20% B (R80B20), 70% R: 30% B (R70B30) with an intensity of 150 µmol m-2 s-1 together with natural light and without supplemental light (control) were applied on two commercial rose cultivars. According to the obtained results, supplemental light improved growth, carbohydrate levels, photosynthesis capacity, and yield compared to the control. R90B10 in both cultivars reduced the time required for flowering compared to the control treatment. R90B10 and R80B20 obtained the highest number of harvested flower stems in both cultivars. Chlorophyll and carotenoid levels were the highest under control. They had a higher ratio of B light, while carbohydrate and anthocyanin contents increased by having a high ratio of R light in the supplemental light. Analysis of chlorophyll fluorescence was indicative of better photosynthetic performance under a high ratio of R light in the supplemental light. In conclusion, the R90B10 light regime is recommended as a suitable supplemental light recipe to improve growth and photosynthesis, accelerate flowering, and improve the yield and quality of cut roses.

Genotype-dependent Strategies to "Overcome" Excessive Light: Insights into Non-Photochemical Quenching under High Light Intensity.

High light (HL) intensities have a significant impact on energy flux and distribution within photosynthetic apparatus. To understand the effect of high light intensity (HL) on the HL tolerance mechanisms in tomatoes, we examined the response of the photosynthesis apparatus of 12 tomato genotypes to HL. A reduced electron transfer per reaction center (ET0 /RC), an increased energy dissipation (DI0 /RC) and non-photochemical quenching (NPQ), along with a reduced maximum quantum yield of photosystem II (FV /FM ), and performance index per absorbed photon (PIABS ) were common HL-induced responses among genotypes; however, the magnitude of those responses was highly genotype-dependent. Tolerant and sensitive genotypes were distinguished based on chlorophyll fluorescence and energy-quenching responses to HL. Tolerant genotypes alleviated excess light through energy-dependent quenching (qE ), resulting in smaller photoinhibitory quenching (qI ) compared to sensitive genotypes. Quantum yield components also shifted under HL, favoring the quantum yield of NPQ (ՓNPQ ) and the quantum yield of basal energy loss (ՓN0 ), while reducing the efficient quantum yield of PSII (ՓPSII ). The impact of HL on tolerant genotypes was less pronounced. While the energy partitioning ratio did not differ significantly between sensitive and tolerant genotypes, the ratio of NPQ components, especially qI , affected plant resilience against HL. These findings provide insights into different patterns of HL-induced NPQ components in tolerant and sensitive genotypes, aiding the development of resilient crops for heterogeneous light conditions.

Response to Cadmium Toxicity: Orchestration of Polyamines and microRNAs in Maize Plant.

Cadmium (Cd) is a heavy metal that is widely contaminating the environment due to its uses in industries as corrosive reagents, paints, batteries, etc. Cd can easily be absorbed through plant roots and may have serious negative impacts on plant growth. To investigate the mechanisms utilized by plants to cope with Cd toxicity, an experiment was conducted on maize seedlings. We observed that the plant growth and photosynthetic mechanism were negatively influenced during 20 days of Cd stress. The expression levels of ornithine decarboxylase (ORDC) increased in the six seedlings under Cd exposure compared to the control. However, Cd toxicity led to an increase in putrescine (Put) content only on day 15 when compared to the control plants. In fact, with the exception of day 15, the increases in the ORDC transcript levels did not show a direct correlation with the observed increases in Put content. Spermidine and Spermine levels were reduced on day 6 by Cd application, which was parallel with suppressed Spermidine synthase gene. However, an increase in Spermidine and Spermine levels was observed on day 12 along with a significant elevation in Spermidine synthase expression. On day 6, Cd was observed to start accumulating in the root with an increase in the expression of microRNA 528; while on day 15, Cd started to be observed in the shoot part with an increase in microRNA 390 and microRNA 168. These results imply that different miRNAs may regulate polyamines (PAs) in maize under Cd toxicity, suggesting a plant-derived strategy to commit a PAs/miRNA-regulated mechanism/s in different developmental stages (time points) in response to Cd exposure.

Artificial neural network (ANN)-based algorithms for high light stress phenotyping of tomato genotypes using chlorophyll fluorescence features.

High light (HL) is a common environmental stress directly imposes photoinhibition on the photosynthesis apparatus. Breeding plants for tolerance against HL is therefore highly demanded. Chlorophyll fluorescence (ChlF) is a sensitive indicator of stress in plants and can be evaluated using OJIP transients. In this study, we compared the ChlF features of plants exposed to HL (1200 µmol m-2 s-1) with that of control plants (300 µmol m-2 s-1). To extract the most reliable ChlF features for discrimination between HL-stressed and non-stressed plants, we applied three artificial neural network (ANN)-based algorithms, namely, Boruta, Support Vector Machine (SVM), and Recursive Feature Elimination (RFE). Feature selection algorithms identified multiple features but only two features, namely the maximal quantum yield of PSII photochemistry (FV/FM) and quantum yield of energy dissipation (ɸD0), remained consistent across all genotypes in control conditions, while exhibited variation in HL. Therefore, considered reliable features for HL stress screening. The selected features were then used for screening 14 tomato genotypes for HL. Genotypes were categorized into three groups, tolerant, semi-tolerant, and sensitive genotypes. Foliar hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents were measured as independent proxies for benchmarking selected features. Tolerant genotypes were attributed with the lowest change in H2O2 and MDA contents, while the sensitive genotypes displayed the highest magnitude of increase in H2O2 and MDA by HL treatment compared to the control. Finally, a FV/FM higher than 0.77 and ɸD0 lower than 0.24 indicates a healthy electron transfer chain (ETC) when tomato plants are exposed to HL.

Natural variation in photosynthesis and water use efficiency of locally adapted Persian walnut populations under drought stress and recovery.

Persian walnut is a drought-sensitive species with considerable genetic variation in the photosynthesis and water use efficiency of its populations, which is largely unexplored. Here, we aimed to elucidate changes in the efficiency of photosynthesis and water content using a diverse panel of 60 walnut families which were submitted to a progressive drought for 24 days, followed by two weeks of re-watering. Severe water-withholding reduced leaf relative water content (RWC) by 20%, net photosynthetic rate (Pn) by 50%, stomatal conductance (gs) by 60%, intercellular CO2 concentration (Ci) by 30%, and transpiration rate (Tr) by 50%, but improved water use efficiency (WUE) by 25%. Severe water-withholding also inhibited photosystem II functionality as indicated by reduced quantum yield of intersystem electron transport (φEo) and transfer of electrons per reaction center (ET0/RC), also enhanced accumulation of QA (VJ) resulted in the reduction of the photosynthetic performance (PIABS) and maximal quantum yield of PSII (FV/FM); while elevated quantum yield of energy dissipation (φDo), energy fluxes for absorption (ABS/RC) and dissipated energy flux (DI0/RC) in walnut families. Cluster analysis classified families into three main groups (tolerant, moderately tolerant, and sensitive), with the tolerant group from dry climates exhibiting lesser alterations in assessed parameters than the other groups. Multivariate analysis of phenotypic data demonstrated that RWC and biophysical parameters related to the chlorophyll fluorescence such as FV/FM, φEo, φDo, PIABS, ABS/RC, ET0/RC, and DI0/RC represent fast, robust and non-destructive biomarkers for walnut performance under drought stress. Finally, phenotype-environment association analysis showed significant correlation of some photosynthetic traits with geoclimatic factors, suggesting a key role of climate and geography in the adaptation of walnut to its habitat conditions.

Plants exposed to titanium dioxide nanoparticles acquired contrasting photosynthetic and morphological strategies depending on the growing light intensity: a case study in radish.

Due to the photocatalytic property of titanium dioxide (TiO2), its application may be dependent on the growing light environment. In this study, radish plants were cultivated under four light intensities (75, 150, 300, and 600 µmol m-2 s-1 photosynthetic photon flux density, PPFD), and were weekly sprayed (three times in total) with TiO2 nanoparticles at different concentrations (0, 50, and 100 µmol L-1). Based on the obtained results, plants used two contrasting strategies depending on the growing PPFD. In the first strategy, as a result of exposure to high PPFD, plants limited their leaf area and send the biomass towards the underground parts to limit light-absorbing surface area, which was confirmed by thicker leaves (lower specific leaf area). TiO2 further improved the allocation of biomass to the underground parts when plants were exposed to higher PPFDs. In the second strategy, plants dissipated the absorbed light energy into the heat (NPQ) to protect the photosynthetic apparatus from high energy input due to carbohydrate and carotenoid accumulation as a result of exposure to higher PPFDs or TiO2 concentrations. TiO2 nanoparticle application up-regulated photosynthetic functionality under low, while down-regulated it under high PPFD. The best light use efficiency was noted at 300 m-2 s-1 PPFD, while TiO2 nanoparticle spray stimulated light use efficiency at 75 m-2 s-1 PPFD. In conclusion, TiO2 nanoparticle spray promotes plant growth and productivity, and this response is magnified as cultivation light intensity becomes limited.

Growth, Biomass Partitioning, and Photosynthetic Performance of Chrysanthemum Cuttings in Response to Different Light Spectra.

Chrysanthemum (Chrysanthemum morifolium) is among the most popular ornamental plants, propagated mainly through stem cuttings. There is a lack of information regarding the impact of the lighting environment on the successful production of cuttings and underlying mechanisms. The light spectrum affects plant morphology, growth, and photosynthesis. In the present study, chrysanthemum, cv. 'Katinka' cuttings, were exposed to five lighting spectra, including monochromatic red (R), blue (B) lights, and multichromatic lights, including a combination of R and B (R:B), a combination of R, B, and far red (R:B:FR) and white (W), for 30 days. B light enhanced areal growth, as indicated by a higher shoot mass ratio, while R light directed the biomass towards the underground parts of the cuttings. Monochromatic R and B lights promoted the emergence of new leaves. In contrast, individual leaf area was largest under multichromatic lights. Exposing the cuttings to R light led to the accumulation of carbohydrates in the leaves. Cuttings exposed to multichromatic lights showed higher chlorophyll content than monochromatic R- and B-exposed cuttings. Conversely, carotenoid and anthocyanin contents were the highest in monochromatic R- and B-exposed plants. B-exposed cuttings showed higher photosynthetic performance, exhibited by the highest performance index on the basis of light absorption, and maximal quantum yield of PSII efficiency. Although R light increased biomass toward roots, B light improved above-ground growth, photosynthetic functionality, and the visual performance of Chrysanthemum cuttings.

Diverse Physiological Roles of Flavonoids in Plant Environmental Stress Responses and Tolerance.

Flavonoids are characterized as the low molecular weight polyphenolic compounds universally distributed in planta. They are a chemically varied group of secondary metabolites with a broad range of biological activity. The increasing amount of evidence has demonstrated the various physiological functions of flavonoids in stress response. In this paper, we provide a brief introduction to flavonoids' biochemistry and biosynthesis. Then, we review the recent findings on the alternation of flavonoid content under different stress conditions to come up with an overall picture of the mechanism of involvement of flavonoids in plants' response to various abiotic stresses. The participation of flavonoids in antioxidant systems, flavonoid-mediated response to different abiotic stresses, the involvement of flavonoids in stress signaling networks, and the physiological response of plants under stress conditions are discussed in this review. Moreover, molecular and genetic approaches to tailoring flavonoid biosynthesis and regulation under abiotic stress are addressed in this review.

Identification of intelligence-related proteins through a robust two-layer predictor.

In this study, we advance a robust methodology for identifying specific intelligence-related proteins across phyla. Our approach exploits a support vector machine-based classifier capable of predicting intelligence-related proteins based on a pool of meaningful protein features. For the sake of illustration of our proposed general method, we develop a novel computational two-layer predictor, Intell_Pred, to predict query sequences (proteins or transcripts) as intelligence-related or non-intelligence-related proteins or transcripts, subsequently classifying the former sequences into learning and memory-related classes. Based on a five-fold cross-validation and independent blind test, Intell_Pred obtained an average accuracy of 87.48 and 88.89, respectively. Our findings revealed that a score >0.75 (during prediction by Intell_Pred) is a well-grounded choice for predicting intelligence-related candidate proteins in most organisms across biological kingdoms. In particular, we assessed seismonastic movements and associate learning in plants and evaluated the proteins involved using Intell_Pred. Proteins related to seismonastic movement and associate learning showed high percentages of similarities with intelligence-related proteins. Our findings lead us to believe that Intell_Pred can help identify the intelligence-related proteins and their classes using a given protein/transcript sequence.

Stb6 mediates stomatal immunity, photosynthetic functionality, and the antioxidant system during the Zymoseptoria tritici-wheat interaction.

This study offers new perspectives on the biochemical and physiological changes that occur in wheat following a gene-for-gene interaction with the fungal pathogen Zymoseptoria tritici. The Z. tritici isolate IPO323, carries AvrStb6, while ΔAvrStb6#33, lacks AvrStb6. The wheat cultivar (cv.) Shafir, bears the corresponding resistance gene Stb6. Inoculation of cv. Shafir with these isolates results in two contrasted phenotypes, offering a unique opportunity to study the immune response caused by the recognition of AvrStb6 by Stb6. We employed a variety of methodologies to dissect the physiological and biochemical events altered in cv. Shafir, as a result of the AvrStb6-Stb6 interaction. Comparative analysis of stomatal conductance demonstrated that AvrStb6-Stb6 mediates transient stomatal closures to restrict the penetration of Zymoseptoria tritici. Tracking photosynthetic functionality through chlorophyll fluorescence imaging analysis demonstrated that AvrStb6-Stb6 retains the functionality of photosynthesis apparatus by promoting Non-Photochemical Quenching (NPQ). Furthermore, the PlantCV image analysis tool was used to compare the H2O2 accumulation and incidence of cell death (2, 4, 8, 12, 16, and 21 dpi), over Z. tritici infection. Finally, our research shows that the AvrStb6-Stb6 interaction coordinates the expression and activity of antioxidant enzymes, both enzymatic and non-enzymatic, to counteract oxidative stress. In conclusion, the Stb6-AvrStb6 interaction in the Z. tritici-wheat pathosystem triggers transient stomatal closure and maintains photosynthesis while regulating oxidative stress.

Genome-wide association analysis and pathway enrichment provide insights into the genetic basis of photosynthetic responses to drought stress in Persian walnut.

Uncovering the genetic basis of photosynthetic trait variation under drought stress is essential for breeding climate-resilient walnut cultivars. To this end, we examined photosynthetic capacity in a diverse panel of 150 walnut families (1500 seedlings) from various agro-climatic zones in their habitats and grown in a common garden experiment. Photosynthetic traits were measured under well-watered (WW), water-stressed (WS) and recovery (WR) conditions. We performed genome-wide association studies (GWAS) using three genomic datasets: genotyping by sequencing data (∼43 K SNPs) on both mother trees (MGBS) and progeny (PGBS) and the Axiom™ Juglans regia 700 K SNP array data (∼295 K SNPs) on mother trees (MArray). We identified 578 unique genomic regions linked with at least one trait in a specific treatment, 874 predicted genes that fell within 20 kb of a significant or suggestive SNP in at least two of the three GWAS datasets (MArray, MGBS, and PGBS), and 67 genes that fell within 20 kb of a significant SNP in all three GWAS datasets. Functional annotation identified several candidate pathways and genes that play crucial roles in photosynthesis, amino acid and carbohydrate metabolism, and signal transduction. Further network analysis identified 15 hub genes under WW, WS and WR conditions including GAPB, PSAN, CRR1, NTRC, DGD1, CYP38, and PETC which are involved in the photosynthetic responses. These findings shed light on possible strategies for improving walnut productivity under drought stress.

Elevated light intensity compensates for nitrogen deficiency during chrysanthemum growth by improving water and nitrogen use efficiency.

Identifying environmental factors that improve plant growth and development under nitrogen (N) constraint is essential for sustainable greenhouse production. In the present study, the role of light intensity and N concentrations on the biomass partitioning and physiology of chrysanthemum was investigated. Four light intensities [75, 150, 300, and 600 µmol m-2 s-1 photosynthetic photon flux density (PPFD)] and three N concentrations (5, 10, and 15 mM N L-1) were used. Vegetative and generative growth traits were improved by increase in PPFD and N concentration. High N supply reduced stomatal size and gs in plants under lowest PPFD. Under low PPFD, the share of biomass allocated to leaves and stem was higher than that of flower and roots while in plants grown under high PPFD, the share of biomass allocated to flower and root outweighed that of allocated to leaves and stem. As well, positive effects of high PPFD on chlorophyll content, photosynthesis, water use efficiency (WUE), Nitrogen use efficiency (NUE) were observed in N-deficient plants. Furthermore, photosynthetic functionality improved by raise in PPFD. In conclusion, high PPFD reduced the adverse effects of N deficiency by improving photosynthesis and stomatal functionality, NUE, WUE, and directing biomass partitioning toward the floral organs.

Regulatory Role of Circadian Clocks on ABA Production and Signaling, Stomatal Responses, and Water-Use Efficiency under Water-Deficit Conditions.

Plants deploy molecular, physiological, and anatomical adaptations to cope with long-term water-deficit exposure, and some of these processes are controlled by circadian clocks. Circadian clocks are endogenous timekeepers that autonomously modulate biological systems over the course of the day-night cycle. Plants' responses to water deficiency vary with the time of the day. Opening and closing of stomata, which control water loss from plants, have diurnal responses based on the humidity level in the rhizosphere and the air surrounding the leaves. Abscisic acid (ABA), the main phytohormone modulating the stomatal response to water availability, is regulated by circadian clocks. The molecular mechanism of the plant's circadian clock for regulating stress responses is composed not only of transcriptional but also posttranscriptional regulatory networks. Despite the importance of regulatory impact of circadian clock systems on ABA production and signaling, which is reflected in stomatal responses and as a consequence influences the drought tolerance response of the plants, the interrelationship between circadian clock, ABA homeostasis, and signaling and water-deficit responses has to date not been clearly described. In this review, we hypothesized that the circadian clock through ABA directs plants to modulate their responses and feedback mechanisms to ensure survival and to enhance their fitness under drought conditions. Different regulatory pathways and challenges in circadian-based rhythms and the possible adaptive advantage through them are also discussed.

The regulatory role of γ-aminobutyric acid in chickpea plants depends on drought tolerance and water scarcity level.

γ-Aminobutyric acid (GABA) is a non-protein amino acid with multifunctional roles in dynamic plant responses. To determine the effects of exogenous GABA application (0, 25 and 50 µM) on drought response, two chickpea cultivars with contrasting tolerance to water deficit were examined. Plants were exposed to four irrigation levels (irrigation to 100, 60, 40 and 20% field capacity). Water deficit decreased growth, chlorophyll content, and photosynthetic efficiency. It increased electrolyte leakage and lipid peroxidation owing to both higher ROS accumulation and lower antioxidant enzyme activity. These negative effects of water deficit and the alleviating role of GABA application were more prominent in the sensitive, as compared to the tolerant cultivar. Water deficit also increased proline and GABA contents more in the tolerant cultivar, whereas their content was more enhanced by GABA application in the sensitive one. This may confer an additional level of regulation that results in better alleviation of drought damage in tolerant chickpea cultivars. In conclusion, the stimulatory effect of GABA on growth and physiological modulation depends on both the water stress severity and the cultivar sensitivity to it, implying a probable unknown GABA-related mechanism established by tolerant chickpea cultivars; a lost or not gained mechanism in susceptible ones.