THE INFLUENCE OF SHADING ON YIELD OF GLASSHOUSE TOMATOES

The proportion of solar radiation incident on plants of four tomato cultivars grown as long-season, layered crops in heated glasshouse compartments was reduced by either 6.4% (light shade) or 23.4% (heavy shade); all other environmental variables were kept the same. The two shade treatments reduced the total fresh weight yield of fruit by 7.5 and 19.9% and the estimated total above-ground biomass by 6.2 and 16.5%, respectively. Over the first 14 weeks of harvest (February to May), yield was accumulated in direct proportion to solar radiation receipt and, regardless of treatment, 2.01 kg fresh weight of fruit were harvested for every 100 MJ of solar radiation incident on the crops from the onset of harvest. Later, the net incident solar radiation was apparently used more efficiently, especially under heavy shade. Furthermore, from February to May, the yield produced in each successive two-week period was proportional to the solar radiation incident on the crop in the preceding 10 weeks; the average duration of fruit development. Fruit number per truss was positively correlated with solar radiation receipt, most noticeably when less than 1.5 MJ m-2 d-1 was received around the time of first anthesis of a truss. The duration of fruit development was unaffected by shade but was influenced by the position of the truss on the plant and of the fruit on the truss. In general, shade reduced average fruit size and so reduced the proportion of fruit in the larger size grades. Heavy shade also reduced the incidence of uneven ripening in summer and so increased the proportion, but not the total weight, of Class I fruit. There were differences between cultivars but no significant interactions between cultivars and treatments. The results stress the need to maximize light transmission in glasshouses, especially when considering the use of energy-saving measures.