Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C4 crops?

The wild progenitors of major C-4 crops grew as individuals subjected to little shading. Today they are grown in dense stands where most leaves are shaded. Do they maintain photosynthetic efficiency in these low light conditions produced by modern cultivation? The apparent maximum quantum yield of CO2 assimilation (Phi(CO2max,app)), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C-4 crops. Phi(CO2max,app) was derived from the initial slope of the response of leaf CO2 uptake (A) to photon flux (Q). Leaf fractional light absorptance (a) was measured to determine the absolute maximum quantum yield of CO2 assimilation on an absorbed light basis (Phi(CO2max,abs)). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus x giganteus and Zea mays following canopy closure. Phi(CO2max,app) and Phi(CO2max,abs) declined significantly by 15-27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO2 assimilation over diurnal courses suggested that the observed decline in Phi(CO2max,app) with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C-4 crops.