Agronomic And Physiological Mechanisms Of Water Deficit Tolerance In Cowpea In Condition Of Symbiotic Relation With Rhizobia

The increase in global climatic change coupled with drought stress incidence and events expose plants to multiple abiotic stresses at different stages of their growth which can result in higher risk of yield damage.In legumes such as cowpea,drought stress affects not only their growth but also the efficacy of their symbiosis with rhizobia.The impact of soil water regimes on physiological responses and water use efficiency?WUE?for cowpea inoculated with rhizobia still remains implicit.The legacy of drought priming stress against the later drought stress has been well reported in many crops species.However,this was not confirmed yet in cowpea,and the impact of drought priming on subsequent different soil water status remains unclear.Therefore,understanding the mechanisms of drought tolerance in cowpea is a great of importance to maintain or sustain grain yield and productivity.The objectives of this study were:?i?to investigate the effect of water stress and rhizobia inoculation on leaf gas exchange,plant water relation,water use efficiency as well as carbon and oxygen isotope compositions(?¹³C,?¹⁸O),?ii?to determine the effect of drought priming during the early growth stage of cowpea on physiological responses,drought signaling and WUE in response to subsequent drought stress during the later growth stage.Pot experiments were carried out in 2017 and 2018 in a temperature-controlled glasshouse at Chinese Academy of Agricultural Sciences?CAAS?,Beijing,China.In order to investigate the effect of water stress and rhizobia inoculation on leaf gas exchange,plant water relation,water use efficiency as well as carbon and oxygen isotope compositions(?¹³C,?¹⁸O),cowpea was inoculated with rhizobia under three different soil water levels.The treatments included soil water regimes at three levels?90%,70%,and 50%of soil water holding capacity?SWHC??and two inoculation forms?inoculated and non-inoculated with rhizobia?.The results showed that across the inoculation treatments,reduced soil water regimes depressed both stomatal conductance?g_s?and photosynthesis?A_n?of the leaves,nonetheless,the decrease of g_s was more pronounced compared with the reduction in A_n.Consequently,the intrinsic water use efficiency?WUE_i?was improved in the treatments under decreased soil water conditions.Plant WUE was also improved when soil water contents decreased as exemplified by the increased leaf?¹³C and?¹⁸O,indicating the enhanced plant WUE was mainly attributed to the decrease of g_s.Significant interactions between soil water regimes and rhizobia treatments for root water potential?RWP?,leaf water potential?LWP?,and g_s were found due to the different responses of rhizobia to varied soil water regimes.Inoculation could improve plant water status and g_s under 70%and 90%SWHC compared to 50%SWHC with negative effect from rhizobia.A moderate soil water regime is suggested for cowpea production in terms of high WUE with a minor biomass reduction.Further,before the application of rhizobium strains in the field,investigations should be made to examine the inoculation efficiency of rhizobium strains and their relations with soil environments for specific crops.To examine the effect of drought priming during the early growth stage of cowpea on physiological responses and WUE in response to subsequent drought stress during the later growth stage,two cowpeas varieties with contrasting responses to drought stress?sensitive and tolerant?were used.Plants were submitted to drought priming by withholding water for 12 days followed by water recovery and then subjected to subsequent drought stress?80%,60%and 40%SWHC?.The results showed that,under subsequent drought stress,primed plants showed higher photosynthesis rate,stomatal conductance,chlorophyll content,WUE,N content and better plant water status compared to non-primed plants in both cultivars.The results indicated that cowpea pre-exposed to drought acquired a stress imprint that alleviates the effect of subsequent drought stress which occurs during the later growth stage as exemplified by the improvement of biomass production of primed plants.Drought stress during the later growth stage resulted in significant increases in stem?¹³C and leaves?¹³C of both plants,however,primed plants maintained lower stem?¹³C?more negatives values?under 60%SWHC compared to non-primed ones.Primed plants maintained also significantly lower leaf?¹³C under 40%and 60%of SWHC in V1 whereas in V2,the primed plant showed significantly high leaf?¹³C?less negatives values?as compared to non-primed plants.The study also showed that under the subsequent drought stress during later growth stage,primed plants maintained lower[ABA]_leaf and higher[IAA]_leaf,[Ga3]_leaf and[ZR]_leaf as compared to non-primed plants which are the results of better plant water status.The results revealed that drought priming at the early growth stage in cowpea could modulate against[ABA]_leaf increase under drought stress,as the increase of[ABA]_leaf is the main reason for stomatal limitation thereby decreasing photosynthesis and leading to great yield loss.Primed plants saved 32 and24%of water use in V1 and V2,respectively,which significantly increased WUE while decreased WUE_i of primed plants as compared to non-primed plants.This indicates that drought priming at the early growth stage can be used as a promising strategy to save water use for irrigation while improving WUE of crops in the region where water is scarce.In general,such important information could be used in breeding program for drought tolerance in cowpea.However,most of the studies on plant drought priming and stress memory including ours have been done in a controlled environment which may be different from the natural environmental conditions.Further studies in field conditions should be performed to compare the results obtained from controlled environment and natural conditions.