Diffuse light and wheat radiation-use efficiency in a controlled environment

Radiation-use efficiency (dry matter produced per unit absorbed radiation) of a spring wheat (Triticum aestivum L., cv. Veery-10) was 40% higher in controlled growth chamber experiments than under optimal field conditions. Simulations with CERES-Wheat, a field model modified to account for growth chamber conditions, suggest that the observed increase in radiation-use efficiency was due to the large fraction of diffuse light in the experimental chamber. Under optimal conditions in the field, the highest crop growth rates occur when the daily photosynthetic photon flux (PPF) is at its highest levels (50-60 mol m-2 d-1). However, these high growth rates do not appear to be associated with the highest radiation-use efficiency. High PPF levels in the field occur on clear days when the fraction of direct radiation is high and the diffuse fraction is low. In controlled environments with reflective walls, high PPF levels with a large fraction of diffuse radiation can be obtained. Diffuse radiation penetrates to the lower leaves of a canopy better than direct radiation, with the result that the upper leaves are less light saturated and the lower leaves receive more light, increasing radiation-use efficiency, and thus growth rates. The data and model simulations presented here suggest that when diffuse light is a high fraction of the total PPF crop productivity can exceed the highest values attainable in the field under optimal conditions.