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Environmental and Stress Physiology

Effects of Water-Deficit Stress on Photosynthesis, Its Components and Component Limitations, and on Water Use Efficiency in Wheat (Triticum aestivum L.) ¹

Department of Agronomy, Oklahoma State University, Stillwater, Oklahoma 74078-0507

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 (_w). Light- and CO₂-saturated A, c.e., and g_s decreased with decreasing leaf _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 (_w = –1.6 MPa). However, it was predicted that A₃₅₀/g_s would decrease with more severe stress due to inhibition of c.e.

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