EFFECTS OF WATER-DEFICIT STRESS ON PHOTOSYNTHESIS, ITS COMPONENTS AND COMPONENT LIMITATIONS, AND ON WATER-USE EFFICIENCY IN WHEAT (TRITICUM-AESTIVUM L)

It is of theoretical as well as practical interest to identify the components of the photosynthetic machinery that govern variability in photosynthesis rate (A) and water-use efficiency (WUE), and to define the extent by which the component processes limit A and WUE during developing water-deficit stress. For that purpose, leaf exchange of CO2 and H2O Was determined in two growth-chamber-grown wheat cultivars (Triticum aestivum L. cv TAM W-101 and cv Sturdy), and the capacity of A was determined and broken down into carboxylation efficiency (c.e.), light- and CO2-Saturated A, and stomatal conductance (g(s)) components. The limitations on A measured at ambient CO2 concentration (A350) were estimated. No cultivar difference was observed when A350 was plotted versus leaf water potential (PSI(w)). Light- and CO2-saturated A, c.e., and g(s) decreased with decreasing leaf PSI(w), but of the corresponding photosynthesis limitations only those caused by insufficient c.e. and g(s) increased. Thus, reduced stomatal aperture and Calvin cycle activity, but not electron transport/photophosphorylation, appeared to be major reasons for drought stress-induced inhibition of A350. WUE measured as A350/g(s) first increased with stomatal closure down to a g(s) of about 0.25 mol H2O m-2 s-1 (PSI(w) = -1.6 MPa). However, it was predicted that A350/g, would decrease with more severe stress due to inhibition of c.e.