Wheat yield progress associated with higher stomatal conductance and photosynthetic rate, and cooler canopies

Grain yields of eight representative semidwarf spring wheat (Triticum aestivum L.) cultivars released in northwest Mexico between 1962 and 1988 have increased linearly across years as measured in this region during 6 yr under favorable management and irrigation. To understand the physiological basis of this progress and possibly assist future selection for grain yield, leaf traits were determined during 3 yr in the same study. Stomatal conductance (g(s)), maximum photosynthetic rate (A(max)), and canopy temperature depression (CTD), averaged over the 3 yr, were closely and positively correlated with progress in the 6-yr mean yield. The correlation was greatest with g(s) (r = 0.94, P < 0.01). Compared with the overall yield increase of 27%, g(s) increased 63%, A(max) increased 23%, and canopies were 0.6 degrees C cooler. Carbon-13 isotope discrimination was also positively associated with yield progress (r = 0.71, P < 0.05), but other leaf traits such as flag leaf area, specific leaf weight, percentage N and greeness were not, nor was crop growth rate around anthesis. The causal basis of the leaf activity interrelationships is reasonably clear, with both increased intercellular CO2 concentration and increased mesophyll activity contributing to the increase in A(max). However, causal links to the yield progress, and the accompanying increase in kernels per square meter, are not clear. It is concluded that g(s) and CTD should be further investigated as potential indirect selection criteria for yield.