Journal of Experimental Botany

The wild progenitors of major C₄ 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 (ΦCO2max,app),(ΦCO2max,app), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C₄ crops. ΦCO2max,appΦCO2max,app was derived from the initial slope of the response of leaf CO₂ uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO₂ assimilation on an absorbed light basis (ΦCO2max,abs).(ΦCO2max,abs). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. ΦCO2max,appΦCO2max,app and ΦCO2max,absΦCO2max,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 ΦCO2max,appΦCO2max,app with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C₄ crops.