Physiological Failures in Zea Mays during Water-Stress: Opportunities for Improvement.

Maintaining high photosynthetic yield in water-stressed maize plants is a present priority for agriculture, and will likely increase in importance as key food producing regions become drier in the future.  Although several physiological responses to water stress in maize have been studied in isolation, we address here the whole-plant response to water stress and ask: what are the key physiological failures that occur in maize and how do these failures correlate with reductions in net CO₂ assimilation?  Xylem conductance, whole-plant conductance, stomatal conductance, rate of electron transport (ETR), maximal catalytic rate of phosphoenolpyruvate carboxylase (V_pmax), and net CO₂ assimilation (A_net) were measured in maize plants subjected to contrasting levels of water stress in a greenhouse during their vegetative phase of growth.  Plants were dried down gradually to assess the entire range of physiological response to decreasing leaf water potential (Ψ_leaf).  Photosynthesis (ETR, V_pmax) decreased proportionately and significantly with Ψ_leaf, exhibiting a decline of 80% at the end of the dry-down (Ψ_leaf ~ ‑4.0 MPa).  These reductions in photosynthetic functioning were closely aligned with similar reductions in whole-plant conductance and stem xylem conductance.  Close alignment among xylem, photosynthetic, and stomatal functioning suggest that enzyme activity and CO₂ supply are closely coordinated with the rate of water supplied through the xylem.  Whole-plant transpiration rates recovered to ca 90% of maximal values 4 d after re-watering, suggesting quick recovery of xylem functioning following severe water stress (Ψ_leaf ~ -4 MPa).  Possible strategies for improving the species may include reduced stomatal sensitivity to internal and external cues, as well as xylem that is less susceptible to damage at low Ψ_leaf.