Impact of root architecture and transpiration rate on drought tolerance in stay-green sorghum

Sorghum [Sorghum bicolor (L.) Moench] yield loss due to terminal drought stress is common in semiarid regions. Stay-green is a drought adaptation trait, and a deeper understanding of stay-green-associated traits is necessary for sorghum breeding. We hypothesize that the stay-green trait in sorghum may be associated with the root architecture and transpiration rate under drought stress. The objectives were to (i) identify the relationship among stay-green-associated traits, (ii) compare the root system architecture and transpiration rate of stay-green (B35 and 296B) and senescent (BTx623 and R16) genotypes under drought stress, and (iii) quantify the impacts of reproductive stage drought stress on gas exchange and grain yield of stay-green and senescent genotypes. A series of drought experiments were conducted with these genotypes. Under drought stress, the stay-green genotypes had an increased total root length in the top 30-60 cm (18%) and 60-90 cm of soil (45%) than the senescent genotypes. In contrast, under progressive soil drying, stay-green genotypes had a decreased transpiration rate (9%) than senescent genotypes by an early (similar to 1 h) partial closure of stomata under high vapor pressure deficit conditions. The increased seed yield (43%) in stay-green genotypes is due to an increased photosynthetic rate (30%) and individual seed size (35%) than senescent genotypes. Overall, it is concluded that stay-green phenotypes had two distinct drought adaptive mechanisms: (i) increased root length for increased soil exploration for water and (ii) an early decrease in the transpiration rate to conserve soil moisture. Identifying genomic markers for these traits would accelerate drought-tolerant sorghum breeding. Sorghum genotypes with contrasting leaf senescence traits were evaluated under drought stress for stay-green.Stay-green and nodal root angle has been positively associated with terminal drought tolerance in sorghum.Stay-green genotypes had a significantly increased root length than senescent genotypes under drought stress.Stay-green genotypes had an earlier decrease in the transpiration rate than senescent genotypes under drying soil.