Abstract
Genotypic variability and plasticity in hydraulic anatomy are not well-understood in herbaceous monocots. In this study, we used Sorghum bicolor, a monocotyledonous, tropical grass model, to understand whether differential plant water use is associated with xylem anatomy and if whole-plant xylem anatomy responds to water stress, justifying differential genotypic sensitivity to drought. In a greenhouse environment, we studied four sorghum genotypes that are known to genetically differ in growth and exhibit differential sensitivity to drought. Under well-watered scenario, transpiration variability and plant growth traits correlated with xylem anatomical traits at both the leaf and stem level, including xylem area and predicted xylem-specific hydraulic conductivity. High water use genotypes had inherently higher hydraulic capacity, but under drought, their transpiration declined at higher fractions of transpirable soil water (FTSW) and they showed greater plasticity in hydraulic anatomy. However, lower FTSW thresholds and modest anatomical changes were identified in the low water use genotypes with inherently lower hydraulic conductivity. Drought, induced modular phenotypic plasticity in hydraulic anatomy, whereby plasticity in leaf xylem traits was remarkably higher than stem xylem, while root xylem showed a reverse nature of vascular modification. Xylem traits were in agreement with phloem anatomy, irrespective of water regime. Our study indicates that hydraulic anatomy can be critical for herbaceous monocots in determining limits to plant water use and genotypic response to drought with implications on whole-plant functions and habitat ecology.