Physio-biochemical Responses,Yield And Quality Of Fragrant Rice Under Different Abiotic Stresses And Role Of ?-Amino Butyric Acid(GABA) In Stress Tolerance

Abiotic stresses is a multi-facet field,encompasses different abiotic factors/stressors occur in the environment that generally impose stress on a wide variety of species.Plants often face multiple abiotic stresses during its life cycle that seriously affect its growth and development.Crop productivities would perhaps the most vulnerable to these abiotic stresses in agricultural production systems,thus studies about the responses of crop plants to different abiotic stress are of ecologically important under changing climate scenario.Aromatic/fragrant rice,a small but best quality rice group,is globally famous due to its special aroma and unique flavor and mainly cultivated in Asian and Middle East countries and exported to international markets worldwide.Aromatic rice cultivars are comparatively low in yield than other rice types whilst imposition of any abiotic stress would thus result in severe yield losses and quality issues.Generally,aromatic rice is sensitive to heavy metals,salinity and drought stress and its specific response to each of the multiple abiotic stresses?to which they are exposed?is difficult to assess.Inter and intra cellular network of plant signaling mechanisms is generally responsible for a suite of responses of plant systems to stressful environments.The singling molecules and/or stress metabolites translate external stimuli into physiological output that ultimately are responsible for growth and yield dynamism in plants.?-Aminobutyric acid?GABA?is a four-carbon non-protein amino acid has been recognized as endogenous plant signaling molecule that regulates plant response to a range of stresses.Despite of surfeit literature on the useful effects of exogenous application of different growth regulators/phyto-hormones on plants under abiotic stresses,the roles of exogenous GABA application in regulating the physio-biochemical responses,yield and quality characters and to confer the harmful effects of different abiotic stresses in aromatic rice has rarely studied.Studies were conducted in rain-protected wire-house to assess the physio-biochemical,yield and quality responses,photosynthesis and gas exchange capacities under the effect of different abiotic stresses viz.,lead(Pb 400-1200 mg kg^-1 of soil),lead(Pb,800 mg kg^-1)&cadmium(Cd,75mg kg^-1)individual and combined,alternate wetting and drying?soil water potential 0 to-25/-30 kPa?,salinity(EC,10 dS m^-1)and drought?soil water potential-40 to-45 kPa,at different growth stages?to investigate the possible roles of exogenous GABA application in abiotic stress tolerance in aromatic rice.It was hypothesized that exogenous GABA application might have potential to induce abiotic stress tolerance in rice by regulating physio-biochemical mechanisms,reducing oxidative damage,protecting photosynthesis and photosynthetic pigments,modulating anti-oxidative defense mechanism?s?,and reducing yield losses.The salient findings of these studies aresummarized below.?1?Pb toxicity significantly reduced the early growth and enhanced oxidative stress in all fragrant rice cultivars viz.,Meixiangzhan 2?MXZ-2?,Xiangyaxiangzhan?XYXZ?,Guixiangzhan?GXZ?,Basmati-385?B-385?,and Nongxiang-18?NX-18?.Pb somewhat enhanced proline and soluble sugars but severely inhibited protein contents.Activities of superoxide dismutase?SOD?,peroxidase?POD?,catalase?CAT?,and ascorbate peroxidase?APX?,reduced glutathione?GSH?and oxidized glutathione?GSSG?contents showed differential behavior,nonetheless,GXZ showed better anti-oxidative defense under Pb toxicity than other cultivars.Roots stored maximum Pb contents followed by stems and leaves in all rice cultivars.Pb contents showed significant but negative correlations with plant height,tillers per plant and plant dry biomass for all rice cultivars whereas the values of translocation factor?TF?from stems to leaves were higher than root to stems.In sum,GXZ proved better able to tolerate Pb stress than all other rice cultivars under study.?2?Pb toxicity increased H₂O₂,lipid peroxidation and electrolyte leakage while inhibited photosynthetic pigments production,but such increment was higher in NX-18than GXZ.Furthermore,Pb toxicity variably affected protein,proline and soluble sugars and the activities of SOD,POD,CAT and APX as well as GSH and GSSG contents in both cultivars;however reductions in osmolyte accumulation and antioxidant activities were more severe in NX-18 than GXZ.Yield,quality and dry biomass of rice were reduced under Pb toxicity;however the reductions were more apparent NX-18 than GXZ.Furthermore,NX-18 accumulated less Pb proportions in roots,and transferred more towards stems,leaves,ears?at panicle heading?PH??and grains?at maturity?MAT??than GXZ.Hence,the ability of GXZ to retain higher Pb proportions in roots and less towards upper plant parts,higher osmolyte accumulation and antioxidant activities and less yield reduction as compare to NX-18 might be an adaptive response of GXZ under Pb toxicity.?3?Pb toxicity reduced photosynthetic pigments and caused oxidative stress with increased production of H₂O₂,malondialdehyde?MDA?and leaves leachates;while such effects were more apparent in XYXZ than MXZ-2 and B-385.Pb stress variably affected the production protein,proline and soluble sugars whilst altered enzymatic and non-enzymatic defense mechanisms in all rice cultivars at both heading and maturity stages.Pb obviously reduced the yield and yield components of all rice cultivars;nonetheless such reductions were observed as XYXZ>MXZ-2>B-385 and resulted in grain quality deterioration.Significant and positive correlations among rice yields with productive tillers/pot and grains per panicle while negative with sterility percentage were also observed.Higher proportions of Pb contents in above ground plant parts in XYXZ possibly lead to severe yield and quality losses.?4?Four different water regimes i.e.,a)CP:continuous ponding b)AWD-BF:alternate wetting and drying?AWD?before flowering);c)AWD-AF:AWD after flowering;and AWD-TM:AWD till maturity?soil water potential kept-25 to-30 kPa for all AWD treatments?were employed in GZX and NX-18 with 800 mg kg^-1 of soil Pb level.It was found that water dynamics induced oxidative stress while also regulated the both enzymatic and non-enzymatic activities in both rice cultivars,however the oxidative damage was more in NX-18 whilst GXZ proved efficient for antioxidant responses under Pb stress.AWD conditions markedly reduced the Pb contents and translocation factor?TF?for root,stems,leaves,ears?at grain filling?and grain?at maturity?compared with CP.Means of all AWD treatments depicted 28.15 and 17.33%reduced grain Pb contents than CP in GXZ and NX-18,respectively.AWD-TM for GXZ and all AWD treatments for NX-18 resulted in significant yield reductions whereas relative changes?RC?of grain yields significantly correlated with grains panicle^-1.?5?AWD treatments at vegetative stage?VS?,reproductive stage?RS?and VS+RS were found statistically similar?P>0.05?for net photosynthesis and gas exchange,nevertheless substantially affected soluble sugars,proline and protein accumulation as well as the enzymatic and non-enzymatic antioxidant activities.Furthermore,AWD treatments variably affected yield and yield components,plant dry biomass,quality traits nonetheless,on average,all yield and related attributes were found higher in GXZ than Nongxiang-18.AWD-further improved grain 2-AP contents with highest at AWD-RS.Hence,AWD-treatments either at VS or RS individually could promote rice aroma without any severe consequences on rice yield and quality.?6?Pb stress promoted oxidative damage in terms of enhanced MDA,EL and H₂O₂contents in both GXZ and NX-18 rice cultivars while exogenous GABA application provide protection against oxidative stress,leaf chlorophyll contents,and modulated the anti-oxidant activities i.e.,SOD,POD,CAT,APX and GSH significantly under Pb toxicity.Glutamine synthetase?GS?and nitrate reductase?NR?activities were variably affected due to GABA application under Pb stress whilst significant improvements were noticed in proline,protein and GABA contents in both GXZ and NX-18 under GABA application.GABA also improved photosynthesis and gas exchange in both fragrant rice cultivars under Pb stress.Exogenous GABA application improved yield and yield components considerably as the percentage reduction under Pb exposed plants?without GABA?were higher than only GABA applied plants.Exogenous GABA application in rice reduced Pb contents in all plant parts compared with Pb exposed plants without GABA.?7?Individual and combined Pb and Cd toxicity was found to enhance ROS-triggered oxidative stress;nevertheless,exogenous GABA application was remained effective in this regard.Temporal data showed GABA reduced chlorophyll degradation and induce osmo-regulation under both Pb and Cd toxicity,individual and combined,in fragrant rice.The Pb+Cd toxicity reduced protein contents substantially and the degree of damage were severe under non-GABA applied plants.Exogenous GABA application enhanced leaf GABA contents and modulated anti-oxidant activities.GS activity was enhanced under Pb/Cd stress either under the effect of GABA or not whilst highest NR activity was found higher under Pb+Cd+GABA than all other treatments.Pb/Cd toxicity reduced photosynthesis and gas exchange in rice;however GABA application showed an alleviatory effect on photosynthesis and gas exchange.Exposure of rice plant to Pb and Cd readily increased root,stems,leaves and grain metal contents at maturity;however GABA application reduced the both Pb and Cd concentration in all plant parts.Exogenous GABA application further improved grain yield under Pb/Cd toxic conditions.?8?Exogenous GABA application provided protection against oxidative stress and protected chlorophyll contents and carotenoids under salt stress in GXZ and NX-18.Photosynthetic capacities,proline,protein,endogenous GABA and relative water contents?RWC?contents were considerably improved under stress and non-stress conditions.Enzymatic and non-enzymatic anti-oxidants activities were significantly affected by GABA application under salt stress.Salinity altered the C-N metabolism by altering GS and NR activity in both rice cultivars;however GABA helped to stabilize GS and NR activities.Imposition of salt stress led to considerable increase in leaf Na⁺contents whilst have counter affect the K+contents in both rice cultivars where as high Na⁺/K⁺ratio in NX-18than GXZ.Foliar applied GABA improved yield and related attributes appreciably under salt stressed and non-stressed conditions as well as grain 2-AP contents,specifically under S+GABA treatment.?9?Drought stress elevated the extent of lipid peroxidation,membrane damage and H₂O₂ contents significantly;however GABA exerted a retarding effect on oxidative stress.Exogenous GABA application at tillering,booting and flowering stages enhanced leaf chlorophyll contents in rice under well-watered and drought conditions,nevertheless such increment was highest in GABA-treated plants under well watered conditions.Application GABA promoted anti-oxidant activities noticeably under drought conditions.Drought stress disturbed GS and NR activities in rice;however GABA application improved GS and NR activities.GABA also resulted in improved proline and protein contents as well as net photosynthesis,stomatal conductance,inter-cellular CO₂,and transpiration rate under drought stress and well-water conditions.Drought stress at all stages under study significantly reduced yield and yield components,however GABA application improved yield and related attributes substantially.