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Limiting transpiration rate under high vapor pressure deficit (VPD) and/or progressive soil drying conditions are soil water conservation mechanisms that can play an important drought-adaptive role if water is limiting to support crops at its full potential. In this study, these two important physiological mechanisms were measured on parental pairs of existing Recombinant Inbred Lines (RILs) of sorghum mapping populations; both in experiments run in the glasshouse and growth chambers, and outdoors. In controlled environmental conditions, the RIL1, RIL2, RIL6 and RIL8 showed contrasting transpiration response to increasing VPD. The difference in the soil moisture fractions of transpirable soil water threshold where transpiration initiated a decline were high in RIL1, RIL3 and RIL8 respectively. The exploration of the variation of the evapotranspiration response to VPD was also carried out in a high throughput phenotyping facility in which plants were grown similar to field density conditions. Under high VPD conditions, the RIL parental pairs showed usual transpiration peak during the midday period. At this time period, genotypic differences within parental pairs were observed in RIL1, RIL2, RIL6 and RIL8. The donor parent had lower transpiration than the recurrent parents during the midday/high VPD period. Also, we found variation among parental pairs in leaf area normalized with received radiation and measured plant architecture traits. Across studied genotypes, RIL1, RIL2 and RIL8 showed differences in the plant canopy architecture and the transpiration response to an increasing VPD. Collectively, these results open the opportunity to phenotype the RIL progenies of contrasting parents and genetically map the traits controlling plant water use. In turn, this can act as an important genetic resource for identification and incorporation of terminal drought tolerance components in marker-assisted breeding.

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