Response And Adaptation To Drought Stress During Vegetative Stages In Relation To Plant Growth And Grain Yield Formation In Wheat

Wheat(Triticum aestivum L.)being a widely adapted cereal crop is the staple food of more than 35%of the global population and occupies 32%of the total area under cereal cultivation.Due to increasingly aridifying and warming climatic trends drought has become the greatest threat to wheat production in most of the areas of wheat cultivation.Although drought hampers wheat performance at all growth phases,substantial yield penalties(up to 58-92%)are endured when drought occurs during the grain filling period.Therefore,studying plants' capability to adapt to and recover from drought during vegetative growth periods may provide the essential information for water-saving management and selection of cultivars with better ecological performance.Similarly,investigating whether pre-drought experienced "pre-drought primed" plants could sustain grain filling process against the most yield-fatal post-anthesis drought conditions can provide the practical guidelines for the research efforts seeking for an improved drought tolerance development,especially at reproductive phase in wheat.The main objective of this study was to look into(1)the capability of winter wheat cultivars to adapt and recover the drought stress during vegetative growth periods and(2)whether moderately pre-drought treated wheat plants can display a faster and stronger defense mechanism to sustain the grain development process by regulating source and sink capacities against the post-anthesis severe drought conditions.1.Genotypic variations in adaptability to and recovery from drought stress are key indicators to drought tolerance.To evaluate the different morpho-physiological adaptations to drought stress and recovery after re-watering,two winter wheat cultivars Luohan 7(drought-tolerant)and Yangmai 16(drought-sensitive)were pot-cultured under three water levels including severe(35-40%field capacity,FC)and moderate drought stress(55-60%FC)and well-watered(75-80%FC)conditions.Drought stress was applied at tillering(Feekes 2 stage)and jointing(Feekes 6 stage),respectively,followed by re-watering,and we observed changes in leaf characteristics,growth and physiological activities during water stress and rewatering periods.Results showed that drought stress adaptability associated with reduced leaf area,higher leaf thickness,chlorophyll,leaf dry matter and maintenance of leaf water potential were pronounced during vegetative stages in both cultivars with relatively higher potential in Luohan 7 than Yangmai 16.Under moderate stress both cultivars exhibited a small decrease in leaf gas-exchange and chlorophyll fluorescence activities,followed by rapid recovery.Under severe drought stress,Yangmai-16 displayed relatively less adaptability to drought,with a slower recovery after re-watering and a greater decrease in crop growth rate and grain yield.It was concluded that even though crop growth rate completely recovered after re-watering,the final dry matter and grain yield outcomes were affected by pre-drought stress,which were dependent on the drought intensity,adaptability and recovery differences of the cultivars and growth stage.It was also concluded that genotypic variations in adaptability to and recovery from drought stress are the indicators of drought tolerance and grain yield sustainability in wheat.Reversible decline in leaf water potential,relative water content,turgor potential,and membrane stability in both cultivars at both vegetative stages.Drought stress led to an increased production of reactive oxygen species and caused lipid peroxidation.The production of soluble sugars,proline and total free amino acids and the anti-oxidative activities of superoxide dismutase,catalase,peroxidase,and ascorbate peroxidase increased during drought stress.On rewatering,the leaf water potential,relative water content,membrane stability,reactive oxygen species generation rate,anti-oxidative activities and the lipid peroxidation returned to near normal values under moderate stress but remained incompletely recovered under severe stress.The degree of plant functions maintenance during drought period and recovery rate after re-watering were dependent on the drought severity,crop growth stage,and genotypic response differences of the cultivars to drought stress.Relatively higher recovery in all metabolic changes was observed when drought occurred at moderate level,at tillering stage and in drought-tolerant cultivar.The results concluded that the capability of plant functions maintenance during drought and recovery after re-watering at vegetative stages significantly determine the final productivity in wheat.2.Moderate water deficit applied at vegetative stages improves post-anthesis photosynthesis,dry matter mobilization,and water utilization in wheat.To investigate whether water deficits applied during vegetative growth periods of winter wheat could affect the grain yield and water utilization,post-anthesis photosynthesis,flag leaf characteristics,dry matter mobilization,and water utilization traits were evaluated in both cultivars.The results showed that water deficits applied during vegetative growth stages improved the post-anthesis photosynthesis,leaf chlorophyll content,re-translocation of pre-anthesis dry matter content,grain filling rate,and shortened the grain filling duration.Moderate water deficit during both stages in Luohan 7;however,only during tillering in Yangmai 16 increased WUE and water productivity(WP)without affecting the grain yield as compared to well-watered treatments.In contrast,severe water deficit during both stages decreased the plant dry biomass,grain yield,number of spikes,grains spike-1,and 1000-grain weight;however,during tillering,it increased WUE and WP in both cultivars.From these results,it was concluded that appropriately applied water deficits during vegetative stages resulted in higher grain yield sustainability and efficient water utilization,especially when applied at moderate level in drought tolerant cultivar during the tillering stage.These results suggest a sustainable winter wheat production under water scarce conditions by transforming the irrigation scheduling and crop management practices.3.Moderate pre-drought exposure "priming" triggers a faster and stronger defense mechanism against the subsequent severe drought stress.To explore whether moderately pre-drought experienced "pre-drought primed" plants during tillering and jointing stages of wheat could enhance the crop potential to tolerate the subsequent drought stress by triggering a faster and stronger defense mechanism,a subsequent severe drought stress was applied from 7-14 days after anthesis.The results showed that primed plants of both cultivars showed higher potential to tolerate the post-anthesis drought stress through improved leaf water potential,more chlorophyll,and ribulose-1,5-bisphosphate carboxylase/oxygenase contents,enhanced photosynthesis,better photoprotection and efficient enzymatic antioxidant system leading to less yield reductions.The primed plants of Luohan-7 showed higher capability to adapt the drought stress events than Yangmai-16.The positive effects of drought priming to sustain higher grain yield were pronounced in plants primed at tillering than those primed at jointing.In consequence,upregulated functioning of photosynthetic apparatus and efficient enzymatic antioxidant activities in primed plants indicated their superior potential to alleviate a subsequently occurring drought stress,which contributed to lower yield reductions than non-primed plants.However,genotypic and priming stages differences in response to drought stress also contributed to affect the capability of primed plants to tolerate the post-anthesis drought stress conditions in wheat.4.Pre-drought priming sustains grain development process by regulating the levels of plant growth hormones against post-anthesis drought stress.To investigate whether and how pre-drought primed wheat plants could sustain the grain filling process by regulating the favorable levels of growth hormones in grains under post-anthesis drought conditions,the plant growth hormones dynamics and grain development process were studied.The results showed that the drought-stressed plants of both cultivars exhibited a decline in flag leaf water potential,chlorophyll contents,photosynthetic rate,initiation of size and filling of grain as compared to well-watered plants;however,decline in these traits was less in pre-drought primed plants than those nonprimed plants.Under drought stress,the primed plants regulated higher concentrations of zeatin and zeatin riboside,indole acetic acid,gibberellins,and lower abscisic acid content in grains,resulting in higher endosperm cell division,grain size establishment,grain filling rate,and finally higher grain dry weights as compared to nonprimed plants.The primed plants of both cultivars showed higher potential to tolerate the post-anthesis drought stress and this potential was more pronounced in the plants of drought-tolerant than the-sensitive wheat cultivar.From these results,it was concluded that pre-drought priming could facilitate the wheat plants to sustain their higher sink size development and its subsequent filling against the most yield-fatal post-anthesis drought conditions by regulating the levels of growth hormones.5.The activities of sucrose synthesizing and sucrose-to-starch conversion enzyme in pre-drought primed against post-anthesis drought stress.It was also looked into whether pre-drought priming could sustain grain filling in wheat through regulating the key enzyme activities involved in grain development process.It was observed that pre-drought primed plants exhibited higher content of water-soluble carbohydrates and sucrose in the stem,and upregulated activity of sucrose-phosphate synthase and invertase in the flag leaf.Pre-drought primed plants upregulated the sink-capacity of developing grains by maintaining higher sucrose-to-starch conversion activity of ADP-glucose pyrophosphorylase,sucrose synthase,soluble-starch synthase and granule-bound starch synthase enzymes,resulting in higher starch accumulation,grain weight,and yield gains as compared to non-primed plants.From these results it was concluded that pre-drought priming regulated the activities of enzymes involved in sucrose synthesis and sucrose-to-starch conversion,which sustained higher grain filling and grain yield against the post-anthesis drought stress in winter wheat.On the whole,results of the study concluded that winter wheat plants have a potential to adapt and recover the drought,especially the moderate stress efficiently during the vegetative stages,particularly during tillering stage.Moreover,moderately applied drought stress during vegetative stages can provide not only the possibilities to increase the water utilization without affecting the final grain yields but can also ensure a sustainable high grain development and yields under the subsequently occurring post-anthesis drought conditions.