The effect of shading on the growth of young 'Fuji' apple trees

Abstract

Young apple trees were grown for two growing seasons under 100, 52.9, 34.2, 15.8 or 5.0% natural photosynthetic photon flux density (PPED) to assess the difference of growth on dry weight basis. Total dry weight gains of trees decreased with increasing shade, except for trees grown at 5.0% PPFD which showed a net loss at the end of the second growing season; because dry weight of the root stem, and trunk decreased. Although the growth increment in the root system was negative at 5.0% PPFD, new roots were produced in small amounts in both seasons. Shading neither decreased the photosynthetic light compensation points of leaves nor increased the initial slopes of the light response curves. Specific leaf weight declined significantly with the decrease in light level to 5.0% PPFD. In trees receiving a PPFD level above 15.8%, 42.1 to 49.5% of the total increment of dry weight was partitioned to the old trunk and branches, respectively. A high correlation between total increment and trunk thickness (r = 0.983 **) was found, but not in trees under 5.0% PPFD. Therefore, the minimum light condition required to sustain a growth in dry weight increment in succeeding years appeared to be between 5.0 to 15.8% of natural sunlight in the northern part of Japan.