Genome sequencing and analysis uncover the regulatory elements involved in the development and oil biosynthesis of Pongamia pinnata (L.) - A potential biodiesel feedstock.

Due to rapid industrialization, the consumption of petro-products has increased, while fossil fuel resources have been gradually depleted. There has been a resurgence of interest in plant-derived biofuels as a sustainable alternative to fossil fuels for the purpose of reducing greenhouse gas emissions. Pongamia pinnata L., which is also known as Millettia pinnata is an oil-yielding, leguminous tree with a large and complex genome. Despite its multiple industrial applications, this orphan tree species has inconsistent yields and a limited understanding of its functional genomics. We assessed physiological and morphological characteristics of five high-yielding pongamia accessions and deduced important yield descriptors. Furthermore, we sequenced the genome of this potential biofuel feedstock using Illumina HiSeq, NextSeq, and MiSeq platforms to generate paired-end reads. Around 173 million processed reads amounting to 65.2 Gb were assembled into a 685 Mb genome, with a gap rate of 0.02%. The sequenced scaffolds were used to identify 30,000 gene models, 406,385 Simple-Sequence-Repeat (SSR) markers, and 43.6% of repetitive sequences. We further analyzed the structural information of genes belonging to certain key metabolic pathways, including lipid metabolism, photosynthesis, circadian rhythms, plant-pathogen interactions, and karanjin biosynthesis, all of which are commercially significant for pongamia. A total of 2,219 scaffolds corresponding to 29 transcription factor families provided valuable information about gene regulation in pongamia. Similarity studies and phylogenetic analysis revealed a monophyletic group of Fabaceae members wherein pongamia out-grouped from Glycine max and Cajanus cajan, revealing its unique ability to synthesize oil for biodiesel. This study is the first step toward completing the genome sequence of this imminent biofuel tree species. Further attempts at re-sequencing with different read chemistry will certainly improve the genetic resources at the chromosome level and accelerate the molecular breeding programs.

Activation of CDC48 and acetyltransferase encoding genes contributes to enhanced abiotic stress tolerance and improved productivity traits in rice.

World-wide crop productivity is highly impacted by various extreme environmental conditions. In the present investigation, activation tagged (AT) line A10-Ds-RFP6 of rice endowed with improved agronomic attributes was tested for its tolerance ability against drought and salinity stress conditions as well as identification of genes associated with these traits. Under both drought and salinity stress conditions, A10-Ds-RFP6 line exhibited increased seed germination rates and improved plant growth characteristics at seedling, vegetative and reproductive stages as compared to wild-type (WT) plants. Moreover, A10-Ds-RFP6 revealed effective antioxidant systems resulting in decreased accumulation of reactive oxygen species and delayed stress symptoms compared to WT plants. Reduced accumulation of malondialdehyde with concomitant increase in proline and soluble sugars in A10-Ds-RFP6 line further endorse its improved stress tolerance levels. Furthermore, A10-Ds-RFP6 disclosed enhanced plant water content, photosynthetic efficiency, stomatal conductance, water use efficiency and maximum quantum yield compared to WT plants. TAIL and qRT-PCR analyses of AT rice line revealed the integration site of Ds element in the genome and increased expression levels of CDC48 and acetyltransferase genes involved in various aspects of plant development and stress tolerance. As such, the promising AT line plausibly serve as a rare genetic resource for fortifying stress tolerance and productivity traits of elite rice cultivars.

Evaluation of high salinity tolerance in Pongamia pinnata (L.) Pierre by a systematic analysis of hormone-metabolic network.

Salinity stress results in significant losses in plant productivity and loss of cultivable lands. Although Pongamia pinnata is reported to be a salt-tolerant semiarid biofuel tree, the adaptive mechanisms to saline environments are elusive. Despite a reduction in carbon exchange rate (CER), the unchanged relative water content provides no visible salinity induced symptoms in leaves of hydroponic cultivated Pongamia seedlings for 8 days. Our Na+ -specific fluorescence results demonstrated that there was an effective apoplastic sodium sequestration in the roots. Salinity stress significantly increased zeatin (~5.5-fold), and jasmonic acid (~3.8-fold) levels in leaves while zeatin (~2.5-fold) content increased in leaves as well as in roots of salt-treated plants. Metabolite analysis suggested that osmolytes such as myo-inositol and mannitol were enhanced by ~12-fold in leaves and roots of salt-treated plants. Additionally, leaves of Pongamia showed a significant enhancement in carbohydrate content, while fatty acids were accumulated in roots under salt stress condition. At the molecular level, salt stress enhanced the expression of genes related to transporters, including the Salt Overly Sensitive 2 gene (SOS2), SOS3, vacuolar-cation/proton exchanger, and vacuolar-proton/ATPase exclusively in leaves, whereas the Sodium Proton Exchanger1 (NHX1), Cation Calcium Exchanger (CCX), and Cyclic Nucleotide Gated Channel 5 (CNGC5) were up-regulated in roots. Antioxidant gene expression analysis clearly demonstrated that peroxidase levels were significantly enhanced by ~10-fold in leaves, while Catalase and Fe-superoxide Dismutase (Fe-SOD) genes were increased in roots under salt stress. The correlation interaction studies between phytohormones and metabolites revealed new insights into the molecular and metabolic adaptations that confer salinity tolerance to Pongamia.

Improved photosynthetic characteristics correlated with enhanced biomass in a heterotic F1 hybrid of maize (Zea mays L.).

Heterosis is a phenomenon wherein F1 hybrid often displays phenotypic superiority and surpasses its parents in terms of growth and agronomic traits. Investigations on the physiological and biochemical properties of the heterotic F1 hybrid are important to uncover the mechanisms underlying heterosis in plants. In the present study, the photosynthetic capacity of a heterotic F1 hybrid of Zea mays L. (DHM 117) that exhibited a higher growth rate and increased biomass was compared with its parental inbreds at vegetative and reproductive stages in the field during 2017 and 2018. The net photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (E) as well as foliar carbohydrates were higher in F1 hybrid than parental inbreds at vegetative and reproductive stages. An increase in total chlorophyll content along with better chlorophyll a fluorescence characteristics including effective quantum yield of photosystem II (ΔF/Fm'), maximum quantum yield of PSII (Fv/Fm), photochemical quenching (qp) and decreased non-photochemical quenching (NPQ) was observed in F1 hybrid than the parental inbreds. Further, the expression of potential genes related to C4 photosynthesis was considerably upregulated in F1 hybrid than the parental inbreds during vegetative and reproductive stages. Moreover, the F1 hybrid exhibited distinct heterosis in yield with 63% and 62% increase relative to parental inbreds during 2017 and 2018. We conclude that improved photosynthetic efficiency associated with increased foliar carbohydrates could have contributed to higher growth rate, biomass and yield in the F1 hybrid.

Physiological, Biochemical, and Root Proteome Networks Revealed New Insights Into Salt Tolerance Mechanisms in Pongamia pinnata (L.) Pierre.

Cultivation of potential biofuel tree species such as Pongamia pinnata would rehabilitate saline marginal lands toward economic gains. We carried out a physiological, biochemical, and proteomic analysis to identify key regulatory responses which are associated with salt tolerance mechanisms at the shoot and root levels. Pongamia seedlings were grown at 300 and 500 mM NaCl (∼3% NaCl; sea saline equivalent) concentrations for 15 and 30 days, gas exchange measurements including leaf net photosynthetic rate (A sat ), stomatal conductance (g s ), and transpiration rate (E), and varying chlorophyll a fluorescence kinetics were recorded. The whole root proteome was quantified using the free-labeled nanoLC-MS/MS technique to investigate crucial proteins involved in signaling pathways associated with salt tolerance. Pongamia showed no visible salt-induced morphological symptoms. However, Pongamia showed about 50% decline in gas exchange parameters including A sat , E, and g s 15 and 30 days after salt treatment (DAS). The maximum potential quantum efficiency of photosystem (PS) II (Fv/Fm) was maintained at approximately 0.8 in salt-treated plants. The thermal component of PSII (DIo) was increased by 1.6-fold in the salt-treated plants. A total of 1,062 protein species were identified with 130 commonly abundant protein species. Our results also elucidate high abundance of protein species related to flavonoid biosynthesis, seed storage protein species, and carbohydrate metabolism under salt stress. Overall, these analyses suggest that Pongamia exhibited sustained leaf morphology by lowering net photosynthetic rates and emitting most of its light energy as heat. Our root proteomic results indicated that these protein species were most likely recruited from secondary and anaerobic metabolism, which could provide defense for roots against Na+ toxicity under salt stress conditions.

Amelioration of plant responses to drought under elevated CO2 by rejuvenating photosynthesis and nitrogen use efficiency: implications for future climate-resilient crops.

The contemporary global agriculture is beset with serious threats from diverse eco-environmental conditions causing decreases in crop yields by ~ 15%. These yield losses might increase further due to climate change scenarios leading to increased food prices triggering social unrest and famines. Urbanization and industrialization are often associated with rapid increases in greenhouse gases (GHGs) especially atmospheric CO2 concentration [(CO2)]. Increase in atmospheric [CO2] significantly improved crop photosynthesis and productivity initially which vary with plant species, genotype, [CO2] exposure time and biotic as well as abiotic stress factors. Numerous attempts have been made using different plant species to unravel the physiological, cellular and molecular effects of elevated [CO2] as well as drought. This review focuses on plant responses to elevated [CO2] and drought individually as well as in combination with special reference to physiology of photosynthesis including its acclimation. Furthermore, the functional role of nitrogen use efficiency (NUE) and its relation to photosynthetic acclimation and crop productivity under elevated [CO2] and drought are reviewed. In addition, we also discussed different strategies to ameliorate the limitations of ribulose-1,5-bisphosphate (RuBP) carboxylation and RuBP regeneration. Further, improved stomatal and mesophyll conductance and NUE for enhanced crop productivity under fast changing global climate conditions through biotechnological approaches are also discussed here. We conclude that multiple gene editing approaches for key events in photosynthetic processes would serve as the best strategy to generate resilient crop plants with improved productivity under fast changing climate.

Mitigation of drought-induced oxidative damage by enhanced carbon assimilation and an efficient antioxidative metabolism under high CO2 environment in pigeonpea (Cajanus cajan L.).

In the current study, pigeonpea (Cajanus cajan L.), a promising legume food crop was assessed for its photosynthetic physiology, antioxidative system as well as C and N metabolism under elevated CO2 and combined drought stress (DS). Pigeonpea was grown in open top chambers under elevated CO2 (600 µmol mol-1) and ambient CO2 (390 ± 20 µmol mol-1) concentrations, later subjected to DS by complete water withholding. The DS plants were re-watered and recovered (R) to gain normal physiological growth and assessed the recoverable capacity in both elevated and ambient CO2 concentrations. The elevated CO2 grown pigeonpea showed greater gas exchange physiology, nodule mass and total dry biomass over ambient CO2 grown plants under well-watered (WW) and DS conditions albeit a decrease in leaf relative water content (LRWC). Glucose, fructose and sucrose levels were measured to understand the role of hexose to sucrose ratios (H:S) in mediating the drought responses. Free amino acid levels as indicative of N assimilation provided insights into C and N balance under DS and CO2 interactions. The enzymatic and non-enzymatic antioxidants showed significant upregulation in elevated CO2 grown plants under DS thereby protecting the plant from oxidative damage caused by the reactive oxygen species. Our results clearly demonstrated the protective role of elevated CO2 under DS at lower LRWC and gained comparative advantage of mitigating the DS-induced damage over ambient CO2 grown pigeonpea.

Molecular insights into photosynthesis and carbohydrate metabolism in Jatropha curcas grown under elevated CO2 using transcriptome sequencing and assembly.

Jatropha curcas L. (Family - Euphorbiaceae) is a perennial tree of special interest due to its potential as a biofuel plant with high carbon sequestration. In this study, physiological investigations coupled with transcriptomics in relation to photosynthesis were evaluated in Jatropha grown under ambient (395 ppm) and elevated (550 ppm) CO2 atmosphere. Morphophysiological analysis revealed that Jatropha sustained enhanced photosynthesis during its growth under elevated CO2 for one year which might be linked to improved CO2 assimilation physiology and enhanced sink activity. We sequenced and analyzed the leaf transcriptome of Jatropha after one year of growth in both conditions using Illumina HiSeq platform. After optimized assembly, a total of 69,581 unigenes were generated. The differential gene expression (DGE) analysis revealed 3013 transcripts differentially regulated in elevated CO2 conditions. The photosynthesis regulatory genes were analysed for temporal expression patterns at four different growth phases which highlighted probable events contributing to enhanced growth and photosynthetic capacity including increased reducing power, starch synthesis and sucrose mobilization under elevated CO2. Overall, our data on physiological and transcriptomic analyses suggest an optimal resource allocation to the available and developing sink organs thereby sustaining improved photosynthetic rates during long-term growth of Jatropha under CO2 enriched environment.

Genotypic variation in tolerance to drought stress is highly coordinated with hydraulic conductivity-photosynthesis interplay and aquaporin expression in field-grown mulberry (Morus spp.).

Hydraulic conductivity quantifies the efficiency of a plant to transport water from root to shoot and is a major constriction on leaf gas exchange physiology. Mulberry (Morus spp.) is the most economically important crop for sericulture industry. In this study, we demonstrate a finely coordinated control of hydraulic dynamics on leaf gas exchange characteristics in 1-year-old field-grown mulberry genotypes (Selection-13 (S13); Kollegal Local (KL) and Kanva-2 (K2)) subjected to water stress by withholding water for 20 days and subsequent recovery for 7 days. Significant variations among three mulberry genotypes have been recorded in net photosynthetic rates (Pn), stomatal conductance and sap flow rate, as well as hydraulic conductivity in stem (KS) and leaf (KL). Among three genotypes, S13 showed significantly high rates of Pn, KS and KL both in control as well as during drought stress (DS) and recovery, providing evidence for superior drought-adaptive strategies. The plant water hydraulics-photosynthesis interplay was finely coordinated with the expression of certain key aquaporins (AQPs) in roots and leaves. Our data clearly demonstrate that expression of certain AQPs play a crucial role in hydraulic dynamics and photosynthetic carbon assimilation during DS and recovery, which could be effectively targeted towards mulberry improvement programs for drought adaptation.

Amelioration of drought-induced negative responses by elevated CO2 in field grown short rotation coppice mulberry (Morus spp.), a potential bio-energy tree crop.

Present study describes the responses of short rotation coppice (SRC) mulberry, a potential bio-energy tree, grown under interactive environment of elevated CO2 (E) and water stress (WS). Growth in E stimulated photosynthetic performance in well-watered (WW) as well as during WS with significant increases in light-saturated photosynthetic rates (A Sat), water use efficiency (WUEi), intercellular [CO2], and photosystem-II efficiency (F V/F M and ∆F/F M') with concomitant reduction in stomatal conductance (g s) and transpiration (E) compared to ambient CO2 (A) grown plants. Reduced levels of proline, H2O2, and malondialdehyde (MDA) and higher contents of antioxidants including ascorbic acid and total phenolics in WW and WS in E plants clearly demonstrated lesser oxidative damage. Further, A plants showed higher transcript abundance and antioxidant enzyme activities under WW as well as during initial stages of WS (15 days). However, with increasing drought imposition (30 days), A plants showed down regulation of antioxidant systems compared to their respective E plants. These results clearly demonstrated that future increased atmospheric CO2 enhances the photosynthetic potential and also mitigate the drought-induced oxidative stress in SRC mulberry. In conclusion, mulberry is a potential bio-energy tree crop which is best suitable for short rotation coppice forestry-based mitigation of increased [CO2] levels even under intermittent drought conditions, projected to prevail in the fast-changing global climate.

A novel aldo-keto reductase from Jatropha curcas L. (JcAKR) plays a crucial role in the detoxification of methylglyoxal, a potent electrophile.

Abiotic stress leads to the generation of reactive oxygen species (ROS) which further results in the production of reactive carbonyls (RCs) including methylglyoxal (MG). MG, an α, ß-dicarbonyl aldehyde, is highly toxic to plants and the mechanism behind its detoxification is not well understood. Aldo-keto reductases (AKRs) play a role in detoxification of reactive aldehydes and ketones. In the present study, we cloned and characterised a putative AKR from Jatropha curcas (JcAKR). Phylogenetically, it forms a small clade with AKRs of Glycine max and Rauwolfia serpentina. JcAKR was heterologously expressed in Escherichia coli BL-21(DE3) cells and the identity of the purified protein was confirmed through MALDI-TOF analysis. The recombinant protein had high enzyme activity and catalytic efficiency in assays containing MG as the substrate. Protein modelling and docking studies revealed MG was efficiently bound to JcAKR. Under progressive drought and salinity stress, the enzyme and transcript levels of JcAKR were higher in leaves compared to roots. Further, the bacterial and yeast cells expressing JcAKR showed more tolerance towards PEG (5%), NaCl (200mM) and MG (5mM) treatments compared to controls. In conclusion, our results project JcAKR as a possible and potential target in crop improvement for abiotic stress tolerance.

Stage-Specific Fatty Acid Fluxes Play a Regulatory Role in Glycerolipid Metabolism during Seed Development in Jatropha curcas L.

The present study describes the changes in lipid profile as well as fatty acid fluxes during seed development in Jatropha curcas L. Endosperm from 34, 37, and 40 days after anthesis (DAA), incubated with [(14)C]acetate, showed significant synthesis of phosphatidylcholine (PC) at seed maturation. The fatty acid methyl ester profile showed PC from 34 DAA was rich in palmitic acid (16:0), whereas PC from 37 and 40 DAA was rich in oleic acid (18:1n-9). Molecular species analysis of diacylglycerol (DAG) indicated DAG (16:0/18:2n-6) was in abundance at 34 DAA, whereas DAG (18:1n-9/18:2n-6) was significantly high at 40 DAA. Triacylglycerol (TAG) analysis revealed TAG (16:0/18:2n-6/16:0) was abundant at 34 DAA, whereas TAG (18:1n-9/18:2n-6/18:1n-9) formed the majority at 40 DAA. Expression of two types of diacylglycerol acyltransferases varied with seed maturation. These data demonstrate stage-specific distinct pools of PC and DAG synthesis during storage TAG accumulation in Jatropha seed.

Differential responses in photosynthesis, growth and biomass yields in two mulberry genotypes grown under elevated CO2 atmosphere.

This study was aimed to examine the responses of two mulberry genotypes (Morus alba L.), which include a drought tolerant (DT) Selection-13 (S13) and a drought susceptible (DS) Kanva-2 (K2) grown under elevated atmospheric CO2 concentration ([CO2]) of 550 µmol mol(-1). Although both genotypes exhibited positive responses to elevated CO2, S13 showed higher light saturated photosynthetic rates (A') and apparent quantum efficiency (AQE), suggesting better Rubisco carboxylation. Increased water use efficiency (WUEi) in elevated CO2 grown S13 (ES13) was due to reduced stomatal conductance (gs) and transpiration (E). Elevated CO2 significantly increased chlorophyll a fluorescence characteristics including maximum quantum yield of primary photochemistry (FV/FM) and performance index (PI(ABS)) suggesting an improved photosystem-II efficiency in both genotypes compared to their respective controls. Even though ES13 showed superior photosynthetic performance, accumulation of soluble and insoluble sugars (starch) were significantly low compared to elevated CO2 grown K2 (EK2), demonstrating higher sink capacity in ES13, which in turn resulted in better biomass yields. We conclude that S13 could be a potential genotype for mulberry-based short rotation forestry (SRF) to mitigate increasing atmospheric [CO2] as well as for the production of carbon neutral renewable bio-energy.

Delayed flowering is associated with lack of photosynthetic acclimation in Pigeon pea (Cajanus cajan L.) grown under elevated CO2.

In the present study, we investigated the likely consequences of future atmospheric CO2 concentrations [CO2] on growth, physiology and reproductive phenology of Pigeonpea. A short duration Pigeonpea cultivar (ICPL 15011) was grown without N fertilizer from emergence to final harvest in CO2 enriched atmosphere (open top chambers; 550µmolmol(-1)) for two seasons. CO2 enrichment improved both net photosynthetic rates (Asat) and foliar carbohydrate content by 36 and 43%, respectively, which further reflected in dry biomass after harvest, showing an increment of 29% over the control plants. Greater carboxylation rates of Rubisco (Vcmax) and photosynthetic electron transport rates (Jmax) in elevated CO2 grown plants measured during different growth periods, clearly demonstrated lack of photosynthetic acclimation. Further, chlorophyll a fluorescence measurements as indicated by Fv/Fm and ΔF/Fm' ratios justified enhanced photosystem II efficiency. Mass and number of root nodules were significantly high in elevated CO2 grown plants showing 58% increase in nodule mass ratio (NMR) which directly correlated with Pn. Growth under high CO2 showed significant ontogenic changes including delayed flowering. In conclusion, our data demonstrate that the lack of photosynthetic acclimation and increased carbohydrate-nitrogen reserves modulate the vegetative and reproductive growth patterns in Pigeonpea grown under elevated CO2.

Persistent stimulation of photosynthesis in short rotation coppice mulberry under elevated CO2 atmosphere.

Current study was undertaken to elucidate the responses of short rotation coppice (SRC) mulberry under elevated CO2 atmosphere (550µmolmol(-1)). Throughout the experimental period, elevated CO2 grown mulberry plants showed significant increase in light saturated photosynthetic rates (A') by increasing intercellular CO2 concentrations (Ci) despite reduced stomatal conductance (gs). Reduced gs was linked to decrease in transpiration (E) resulting in improved water use efficiency (WUE). There was a significant increase in carboxylation efficiency (CE) of Rubisco, apparent quantum efficiency (AQE), light and CO2 saturated photosynthetic rates (AMAX), photosynthetic nitrogen use efficiency (PNUE), chlorophyll a fluorescence characteristics (FV/FM and PIABS), starch and other carbohydrates in high CO2 grown plants which clearly demonstrate no photosynthetic acclimation in turn resulted marked increase in above and below ground biomass. Our results strongly suggest that short rotation forestry (<1year) with mulberry plantations should be effective to mitigate raising CO2 levels as well as for the production of renewable bio-energy.

Molecular cloning and characterisation of metallothionein type 2a gene from Jatropha curcas L., a promising biofuel plant.

In the present study, we have cloned a gene encoding JcMT2a protein from Jatropha curcas L., a promising biofuel tree species. Full length sequence of JcMT2a gene was isolated using RACE PCR. Heterologous expression of JcMT2a in Escherichia coli and its purification has shown distinct bands corresponding to the GST and GST-fused JcMT2a protein. Significant tolerance was observed in E. coli cells expressing recombinant GST-JcMT2a for zinc, copper and cadmium metals compared to cells expressing GST alone. JcMT2a also restored Cu and Cd tolerance in the metal sensitive yeast mutants. Quantitative real time PCR showed a significant increase in JcMT2a transcripts with Cu and Cd in the leaf compared to root tissue. Our Scanning electron microscopy and energy dispersive X-ray spectroscopy analysis clearly demonstrates that J. curcas L. could be a potential candidate for phytoremediation to clean heavy metals from the environment, in addition to its non-edible oil seed yields for biodiesel production.

De novo transcriptome analysis of an imminent biofuel crop, Camelina sativa L. using Illumina GAIIX sequencing platform and identification of SSR markers.

Camelina sativa L. is an emerging biofuel crop with potential applications in industry, medicine, cosmetics and human nutrition. The crop is unexploited owing to very limited availability of transcriptome and genomic data. In order to analyse the various metabolic pathways, we performed de novo assembly of the transcriptome on Illumina GAIIX platform with paired end sequencing for obtaining short reads. The sequencing output generated a FastQ file size of 2.97 GB with 10.83 million reads having a maximum read length of 101 nucleotides. The number of contigs generated was 53,854 with maximum and minimum lengths of 10,086 and 200 nucleotides respectively. These trancripts were annotated using BLAST search against the Aracyc, Swiss-Prot, TrEMBL, gene ontology and clusters of orthologous groups (KOG) databases. The genes involved in lipid metabolism were studied and the transcription factors were identified. Sequence similarity studies of Camelina with the other related organisms indicated the close relatedness of Camelina with Arabidopsis. In addition, bioinformatics analysis revealed the presence of a total of 19,379 simple sequence repeats. This is the first report on Camelina sativa L., where the transcriptome of the entire plant, including seedlings, seed, root, leaves and stem was done. Our data established an excellent resource for gene discovery and provide useful information for functional and comparative genomic studies in this promising biofuel crop.

Interdependence of plant water status with photosynthetic performance and root defense responses in Vigna radiata (L.) Wilczek under progressive drought stress and recovery.

The present study investigates the interdependence of plant water status with foliar and root responses in Vigna radiata L.Wilczek under progressive drought. Vegetatively-mature V. radiata plants were subjected to water withdrawal for 3 and 6days (D3 and D6, respectively) and then re-watered subsequently for 6days (6R) for stress-recovery. Changes in plant water status were expressed in terms of leaf and root moisture contents (LMC and RMC, respectively) and leaf relative water content (LRWC). Progressive drought caused apparent decrease in LRWC, LMC and RMC depicting significant level of dehydration of leaf and root tissues. Stomatal limitation alone could not account for the observed decrease in net CO2 assimilation rates (Pn) due to comparatively less decrease in sub-stomatal CO2 (Ci) concentrations with respect to other gas exchange parameters indicating possible involvement of non-stomatal limitations. Analysis of polyphasic chl a fluorescence kinetics during progressive drought showed decreased energy connectivity among PSII units as defined by a positive L-band with highest amplitude during D6. Efficiency of electron flux from OEC towards PSII acceptor side was not significantly affected during drought conditions as evidenced by the absence of a positive K-band. Increasing root-level water-limitation enforced a gradual oxidative stress through H2O2 accumulation and membrane lipid peroxidation in V. radiata roots exhibiting drastic enhancement of proline content and a significant but gradual increase in ascorbic acid content as well as guaiacol peroxidase activity under progressive drought. Expression analysis of Δ(1) pyrroline-5-carboxylate synthetase (P5CS) through real time PCR and enzyme activity studies showed a strong positive correlation between VrP5CS gene expression, enzyme activity and proline accumulation in the roots of V. radiata under progressive drought and recovery. Drought-induced changes in root moisture content (RMC) showed positive linear correlations with leaf water content, stomatal conductance as well as transpirational water loss dynamics and a significant negative correlation with the corresponding drought-induced expression patterns of ascorbate, guaiacol peroxidase and proline in roots of V. radiata. The study provides new insights into the plant water status-dependent interrelationship between photosynthetic performance and major root defense responses of V. radiata under progressive drought conditions.

Polyphasic chlorophyll a fluorescence kinetics and leaf protein analyses to track dynamics of photosynthetic performance in mulberry during progressive drought.

Modulation of photosynthesis and the underlying mechanisms were studied in mulberry (Morus indica L. genotype V1) under progressive drought stress conditions. Five months old potted mulberry plants were arranged in a semi-controlled glasshouse chamber in completely randomized block design with four replications. On day 1 (D1), the plants were subjected to two watering treatments: well-watered (WW) and water-stressed (WS). In WS plants, watering was completely withheld for next 10days (D1-D10), whereas the WW plants were maintained at 100% pot water holding capacity. Photosynthetic performance was tracked periodically (from D0 to D10) through measurements of leaf gas exchange and chlorophyll a fluorescence (OJIP) transients and additionally leaf protein analyses were performed on D10. Down-regulation in net CO(2) fixation (P(n)) was primarily mediated through stomatal limitation which concurrently reduced transpiration rate (E), stomatal conductance (g(s)) and intercellular CO(2) concentration (C(i)). The OJIP transients and other associated biophysical parameters elucidated the events of photoacclimatory changes in photosystem II (PSII) with progressive increase in drought stress. Down-regulation of PSII activity occurred predominantly due to increase in inactive reaction centers (RCs), decrease in electron transport per RC (ET(O)/RC) as well as per leaf cross-section (ET(O)/CS(m)) and enhanced energy dissipation. The L and K-bands appeared only in the stage of extreme drought severity indicating the ability of genotype V1 to resist drought-induced damage on structural stability of PSII and imbalance between the electrons at the acceptor and donor sides of PSII, respectively. Drought-induced changes in leaf protein analyses revealed significant up-regulation of important proteins associated to photostability of thylakoid membrane including oxygen evolving enhancer, chlorophyll a/b binding proteins, rubisco and rubisco activase. Further, the antioxidative defense proteins including peroxiredoxin and NADH ubiquinone oxidoreductase were also enhanced. In conclusion, our data demonstrate an integrated down-regulation of the photosynthetic process to maintain intrinsic balance between electron transfer reactions and reductive carbon metabolism without severe damage to PSII structural and functional integrity.

Elevated atmospheric CO2 mitigated photoinhibition in a tropical tree species, Gmelina arborea.

Effects of elevated CO2 on photosynthetic CO2 assimilation, PSII photochemistry and photoinhibition were investigated in the leaves of a fast growing tropical tree species, Gmelina arborea (Verbenaceae) during summer days of peak growth season under natural light. Elevated CO2 had a significant effect on CO2 assimilation rates and maximal efficiency of PSII photochemistry. Chlorophyll a fluorescence induction kinetics were measured to determine the influence of elevated CO2 on PSII efficiency. During midday, elevated CO2-grown Gmelina showed significantly higher net photosynthesis (p<0.001) and greater F(V)/F(M) (p<0.001) than those grown under ambient CO2. The impact of elevated CO2 on photosynthetic rates and Chl a fluorescence were more pronounced during midday depression where the impact of high irradiance decreased in plants grown under elevated CO2 compared to ambient CO2-grown plants. Our results clearly demonstrate that decreased susceptibility to photoinhibition in elevated CO2 grown plants was associated with increased accumulation of active PSII reaction centers and efficient photochemical quenching. We conclude that elevated CO2 treatment resulted in easy diminution of midday photosynthetic depression.