The Effect of Constraining the Intensity of Solar Radiation on the Photosynthesis, Growth, Yield and Product Quality of Tomato

Recent studies propose the combination of electrical energy and plant production, in which often only radiation peaks were used for energy production. Then, the effect on plant growth is assumed to be negligible. However, photosynthesis is known to be a monotonically increasing function of radiation. We studied the response of tomato to constraining the intensity of solar radiation. Tomato crops in greenhouse compartments were shaded when the outside photosynthetic photon flux density (PPFD) exceeded 640 and 1280mol m(-2) s(-1), resulting in a 57% and 34% reduction in the PPFD integral over the growing period compared to the non-shaded control. Constraining the intensity of solar radiation significantly reduced photosynthesis, growth and yield of tomato plants. Model-derived estimates of reduction in crop dry matter increment were 50% and 28% for the strongly and moderately PPFD constrained crops. However, measured plant dry matter increment decreased only by 31% and 19%, respectively, that is, light use efficiency increased markedly. This indicates a strong adaptation of the plant's metabolism to cope with the limitation in light availability such as increasing the specific leaf area and reducing respiration. Surprisingly, this was only of little concern to the fruit quality, because no effect of constraining PPFD on the concentration of total dry matter, sugars and lycopene in the fruits could be observed. The concentration of titratable acids was significantly increased, however, when constraining PPFD, while ss-carotene was slightly decreased. When combining plant and energy production, yield reductions in systems that reduce the supply of solar radiation to crops only at high irradiances will be less considerable than in systems that permanently shade the crop. However, yield losses in tomato production remain significant in both system types.