Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C(4) 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 CO(2) assimilation [Formula: see text] a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C(4) crops. [Formula: see text] was derived from the initial slope of the response of leaf CO(2) uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO(2) assimilation on an absorbed light basis [Formula: see text] Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. [Formula: see text] and [Formula: see text] declined significantly by 15–27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO(2) assimilation over diurnal courses suggested that the observed decline in [Formula: see text] with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C(4) crops.