Coordinated decline in photosynthesis and hydraulic conductance during drought stress in Zea mays

Present efforts to improve maize performance in water-limited environments are aggressively pursuing leaf-level traits, such as quantum efficiency, mesophyll osmoregulation, and stress-protein responses. However, it is possible that improvement of these traits will lead directly to hydraulic failure if hydraulics and photosynthesis are closely aligned. Here, we address the whole plant response to drought stress and ask if photosynthetic and hydraulic traits appear bundled together as a "coordinated" response, or if these traits operate largely independently of one another. Xylem conductance of leaves and stems, whole plant conductance, stomatal conductance, rate of electron transport (ETR), maximal catalytic rate of phosphoenolpyruvate carboxylase (V-pmax) (EC 4.1.1.31), and net CO2 assimilation (Amax) were measured in maize plants subjected to contrasting levels of drought stress in greenhouse and field experiments. Photosynthetic traits (A(max), ETR, V-pmax), hydraulic traits (whole plant and stem conductance) and stomatal conductance were all reduced by >80% as leaf water potentials declined below 3.0 MPa. Furthermore, 83% of the variation associated with the photosynthetic and hydraulic traits measured in this study was explained by a single principal component, revealing a remarkable degree of alignment among them. Whole plant transpiration rates recovered to ca 90% of maximal values 4 d after lifting severe drought stress ('Y leaf- leaf 3.5 MPa). Closely aligned hydraulic, photosynthetic, and stomatal responses to drought stress suggest that improvements to individual traits, in isolation to each other, may lead to the loss of plant functioning (e.g. water transport) rather than leading to marked improvements in growth. Published by Elsevier GmbH.