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 (gs), maximum photosynthetic rate (Amax, 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 gs (r = 0.94, P < 0.01). Compared with the overall yield increase of 27%, gs increased 63%, Amax increased 23%, and canopies were 0.6°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 Amax. However, causal links to the yield progress, and the accompanying increase in kernels per square meter, are not clear. It is concluded that gs and CTD should be further investigated as potential indirect selection criteria for yield.

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A new hypothesis is presented to explain the major molecular process that regulates the efficiency of light harwesting by chloroplast membranes. It is proposed that in excess light the decrease in the thylakoid lumen pH causes an increase in aggregation of the light harvesting complexes of photosystem II resulting in formation of an efficient pathway for non-radiative dissipation of excitation energy. The aggregation is potentiated by the conversion of violaxanthin to zeaxanthin. This hypothesis is based upon (i) similarity between ~he spectroscopic changes associated with energy dissipation and those observed upon aggregation of isolated ligh~ harvesting complex; and (ii) the link between changes in light scattering and increased energy dissipation.

The extent of energy-dependent quenching of chlorophyll fluorescence in broken spinach chloroplasts has been quantitatively related to the size of the thylakoid proton gradient as measured by the quenching of 9-aminoacridine fluorescence by titration at constant irradiance with the uncoupler nigericin or by change in irradiance. It was found that chloroplasts prepared from leaves that had been pre-illuminated with strong light for 30 min showed energy-dependent quenching at a lower proton gradient than chloroplasts prepared from dark-adapted leaves. Measurement of the carotenoid composition of the thylakoids showed that light treatment raised the ratio of zeaxanthin:violaxanthin. The possible dependence of energy-dependent quenching on xanthophyll composition and the physiological implications of this light-activation process to the regulation of photosynthetic electron transport are discussed.

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