Phytoscale transport physics: Insights into xylem flow homeostasis and drought stress

We investigate the flow dynamics of nutrient solution through the xylem vessels of Brassica juncea (Indian mustard) under drought stress. To this end, we perform experiments to obtain morphological traits of xylem vessels under drought-stressed conditions and develop a mathematical framework to model the underlying flow through the xylem, considering several features relevant to plants. Performing experiments using state-of-the-art instruments, we measure the morphology of xylem vessels and the physicochemical and mechanical properties of xylem walls under drought-stressed conditions. Our experimental results unveil that drought reduces both xylem diameter and pit aperture size, implicating hydraulic adaptations of plants to drought stress. We find that the reduced cellulose content in drought-stressed xylem vessels lowers the zeta potential and decreases the elasticity of the vascular region. Additionally, drought stress alters metabolite activity, increases reactive oxygen species, reduces chlorophyll content, and limits the uptake of essential ionic nutrients. Besides, we perform three-dimensional numerical simulations to evaluate the local flow field, mechanical stress, hydraulic conductivity, and radial transport efficiency of xylem vessels under drought-stressed conditions. Simulated results reveal that under drought conditions, resistance to axial flow through xylem vessels increases significantly, which, in turn, promotes radial transport of nutrients, allowing plants to survive even under drought stress. We show that the radial flow efficiency of xylem vessels becomes notably higher under drought stress than in well-watered control plants. Overall, results of this endeavor provide new insights into how geometric adaptations of xylem vessels modify flow behavior under water-deficient conditions, enhancing the plant's ability to survive environmental stress.