Potential Increase in Photosynthetic Efficiency from the Redistribution of Solar Radiation in a Crop

Abstract

The efficiency of the conversion of photosynthetically active radiation by C₃ plants falls off with increasing intensity. Hypothesis: an increase in the productivity of direct solar energy will be achieved if, by redistribution, it is intercepted at a more uniform and lower intensity by a greater proportion of the leaf area of a crop. A model is developed which uses estimates of the proportions of clear and overcast conditions from site records of solar radiation to calculate the resultant photosynthetic productivity. The amounts of diffuse light and direct light are estimated for clear conditions. The model predicts that redistributing direct solar radiation over twice the leaf area at half the intensity would give an increase of 22% in annual productivity. The model gives reasonable values for the productivity reductions reported for two shading regimes. Tomato plants were grown for 21 d in three cabinets under regimes that differed from each other only in the distribution of PAR energy over the daily photoperiod: (a) 103 W m⁻² for half the photoperiod followed by 13 W m⁻², (b) 13 W m⁻² followed by 103 W m⁻² and (c) 58 W m⁻² for the whole photoperiod. The dry matter increase of plants under the uniform regime was 33% greater than the average of those in the two asymmetric regimes. It is suggested that, in protected cultivation, screens of partially reflective material could be used to redistribute solar radiation from leaves exposed to high intensities on to shaded leaves and so raise the photosynthetic efficiency. Assuming an absorption of direct light by the screens of 0.10, the increase in productivity is estimated to be 17%.