| Abstract: | The efficiency of the conversion of photosynthetically activeradiation by C3 plants falls off with increasing intensity.Hypothesis: an increase in the productivity of direct solarenergy will be achieved if, by redistribution, it is interceptedat a more uniform and lower intensity by a greater proportionof the leaf area of a crop. A model is developed which usesestimates of the proportions of clear and overcast conditionsfrom site records of solar radiation to calculate the resultantphotosynthetic productivity. The amounts of diffuse light anddirect light are estimated for clear conditions. The model predictsthat redistributing direct solar radiation over twice the leafarea at half the intensity would give an increase of 22% inannual productivity. The model gives reasonable values for theproductivity reductions reported for two shading regimes. Tomato plants were grown for 21 d in three cabinets under regimesthat differed from each other only in the distribution of PARenergy over the daily photoperiod: (a) 103 W m–2 for halfthe photoperiod followed by 13 W m–2, (b) 13 W m–2followed by 103 W m–2 and (c) 58 W m–2 for the wholephotoperiod. The dry matter increase of plants under the uniformregime was 33% greater than the average of those in the twoasymmetric regimes. It is suggested that, in protected cultivation, screens of partiallyreflective material could be used to redistribute solar radiationfrom leaves exposed to high intensities on to shaded leavesand so raise the photosynthetic efficiency. Assuming an absorptionof direct light by the screens of 0.10, the increase in productivityis estimated to be 17%. Key words: Photosynthesis, C3, canopy, solar energy, solar redistribution, light interception, partial reflection, growth cabinet, glasshouse, tomato |
