EFFECTS OF PARTIAL DEFOLIATION AT VARIOUS STAGES OF THE GROWING-SEASON ON FRUIT YIELDS, ROOT-GROWTH AND RETURN BLOOM OF KIWIFRUIT VINES

The effects of partial defoliation at various stages of the growing season on fruit yields, root growth and return bloom for kiwifruit (Actinidia deliciosa var. deliciosa cultivar 'Hayward') were examined using vines in a commercial orchard. An arbitrary distinction was made between shoots arising from the "replacement cane zone", the 1.5 m wide horizontal area between the trellis support wires, and shoots arising from the "fruiting zone", the remaining area of the vines. On control vines, the total fruit yields in the first season were 3.8 kg m-2, with an overall leaf to fruit ratio of 4.8:1. The replacement cane zone included 60% of the total leaf area but only 30% of the fruit yield. The average fruit weight was about 8% larger within the replacement cane zone than within the fruiting zone. Defoliation of all shoots within the fruiting zone reduced fruit numbers and average fruit weights in that zone, decreasing fruit yield there by up to 44%, but had no effect on fruit yields elsewhere in the vine. This treatment had very little effect on root growth but reduced flowering in the following season by up to 29%. Defoliation within the replacement cane zone had no effect on fruit numbers but reduced average fruit weight by up to 45% and also reduced the average fruit weight within the fruiting zone by 15%. The effects of this treatment on root growth were greater than those on fruit yields, while flowering in the following season was totally eliminated except where this defoliation was imposed very late in the growing season. The effects of defoliation on fruit yields within the vine clearly demonstrated the importance of sink proximity to source leaves. The effects of the treatments on the growth of various component organs of the kiwifruit vine were consistent with a ranking of competitive strengths of the assimilate sinks as (in descending order), shoots, fruit, roots, return bloom. The large reductions in return bloom following defoliation indicated that flowering depended on the accumulation of a critical level of assimilate rather than the reception of some stimulus at a discrete point in time. The overall sensitivity of the vines to partial defoliation for even short periods of the growing season suggested that kiwifruit production was limited by a tight carbohydrate budget. Severe defoliation resulted in the appearance of leaves characteristic of juvenile plants. This phase change appeared to follow a depletion of carbohydrate reserves and may have been related to a response towards root growth following the earlier reductions in root growth. © 1990.