Light studies in a spacing trial with apple on a dwarfing and a semi-dwarfing rootstock

Abstract

Light interception by the orchard canopies in a spacing trial with Golden Delicious and Jonathan on M IX and on M II was measured in 1968, 1969 and 1970. In 1969 and 1970 light distribution over the Golden Delicious hedges in the high-density plots was also measured and related to fruit and foliage distribution and to fruit quality. Simultaneously, integrated light readings at 100–200 locations were taken over 24-hour periods, using a photochemical method. Although all plots attained their ceiling yield level in 1967, the 6th year after planting, light interception continued to increase from 1968 to 1970. In the low-density plots — 1100 trees per ha on M IX and 660 trees per ha on M II — the canopies intercepted roughly half of the incident light and yields levelled off at 40 tons per ha. The high-density M IX plot (3300 trees per ha) intercepted two-thirds, the high-density M II plot (2260 trees per ha) more than three-quarters of incident light. Both plots yielded more than 70 tons per ha in 1967 and 1968, but thereafter the yield level dropped, especially for trees on M II, presumably because of mounting inter-tree competition for light. Light levels in the hedges in the high-density plots at equal distances from the hedge perimeter were very much the same for both varieties, for both rootstocks and in the two years. Light levels fell sharply towards the interior and bottom parts of the hedges. The main advantage of the M IX hedge was its smaller cross section, leaving little room for a poorly illuminated centre. At lower light levels weight per fruit and especially fruit colour declined, but the relation between light level and fruit quality was not consistent for both rootstocks and in both years. These inconsistencies could be explained on the basis of differences in pruning system, affecting the type of wood on which the fruit is borne. Fruit was concentrated in the interior part of the hedges, so that a high proportion of the crop suffered from the poor light conditions in this part. Fruit distribution was generally correlated with foliage distribution. In the M IX hedge by far the highest leaf density was found in the centre of the hedge. This is an advantage in comparison with the large leaf area — relatively unproductive and casting much shade — in the top of the M II hedge. Light measurements before and after fruit removal suggested that the shade cast by the fruit is negligible as compared to shading by the foliage. It is concluded that canopy architecture in the high-density M IX plot, with a leaf area index of 2.15, is close to the optimum for single row planting systems in Northwest Europe. Gains in percentage light interception, as in the high-density M II plot, are associated with a greatly inferior light distribution over the canopy, leading to lower yields per unit growth and deterioration of fruit quality. In addition to light, pruning strongly influences fruit quality.