Sorghum Whole-Plant Responses to Pre-Flowering Drought Stress.

Sorghum (Sorghum bicolor) is a globally important, multipurpose crop that displays drought resistance according to its growth stage. Drought stress during pre-flowering growth is of particular concern as it is the longest state of development and limited water can result in reduced panicle size, grain number and yield. Although sorghum is generally considered to be drought tolerant, there exists tremendous genetic variation for this trait, which may be used to improve performance in water-limited environments. We evaluated morphological, physiological, and biochemical responses of two sorghum varieties that vary in pre-flowering drought resistance [BTx623 (resistant) and SC56 (susceptible)] under drought-stressed and irrigated conditions. Additionally, plant recovery was evaluated after returning drought-stressed plants to fully irrigated conditions. Both genotypes differed in growth habits under irrigated conditions, with resistant BTx623 having a more conservative growth rate, and susceptible SC56 having a faster growth rate and also larger total leaf area, root length and nodal root length. Both varieties had increased root length: leaf area, however BTx623 maintained living leaf tissue while leaves of SC56 developed severe necrosis. Upon rewatering, the resistant variety BTx623 promptly returned to its pre-stress root ratio, potentially through a rapid increase of leaf area in response to the re-addition of water. In contrast to SC56, BTx623 maintained photosynthetic rates for longer periods of time under drought stress. Further, pre-flowering drought stress impacted panicle weight in the susceptible variety SC56 when grown to maturity. Targeted and untargeted metabolomics analysis by liquid chromatography coupled to mass spectrometry revealed significant differences in both primary and secondary metabolism between genotypes under drought stress. Taken together, the integration of morphological, physiological, and biochemical data provides a systems-level view of drought resistance in sorghum that can be used to facilitate breeding for pre-flowering drought tolerance, and these data may be applicable to other cereal crops.