12.5.3 Flower bud differentiation and fruit set

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Due to the perennial nature of fruit trees, environmental conditions in one season can have carry-over effects on growth and development during the subsequent season. In seasonally deciduous trees and vines, flowers that emerge each spring were actually initiated during the previous growing season, and conditions prevailing at that time will have influenced the extent and intensity of flower bud differentiation. In temperate horticulture, flower initiation is not a continuous process but may be restricted to several weeks during mid to late summer, followed by several months of continuing development before passing into dormancy over winter. Floral development is then completed during the following spring ahead of flower emergence. In addition to changes in fruit quality of current crops, canopy shade during summer will also reduce flower bud initiation and differentiation.

Flower bud reduction due to shading seems to be a general phenomenon in perennial fruit crops and has been reported for apple, pear, apricot, peach, coffee, cacao and grape (Buttrose 1974). With grape, the number and size of cluster primordia generally increases with increasing light. In this case plants are responding to light quantity rather than quality (expressed as changes in the red/far-red ratio; Williams et al. 1994).

Cultivars differ in their response to shade. Sultana (syn. Thompson Seedless) requires approximately 30% full sun for cluster initiation compared to 10% for Rhine Riesling. Timing of shading is also important for grapes. Shading over a four-week period during bud development of grape in late spring was found to have the greatest negative effect on flower initiation (May and Antcliff 1963).

In citrus, shading may not of itself reduce flowering but can induce heavy fruit abscission. Similarly, shading whole apple trees 14–28 d after full bloom can result in very heavy fruit loss. Two days shading with shade cloth transmitting 8% light was sufficient to reduce fruit set by 80% (Byers et al. 1991). So effective is this treatment that deep shade could work as a non-chemical fruit thinner! In sour cherry, fruit set is adversely affected by light levels within the canopy below 20%, and flower bud initiation requires an irradiance of 15–20% full sun. Shading of whole vines or shoots of kiwifruit during summer can reduce flowering the following spring. Again, as with grapes, changes to the red/far-red ratio did not alter the flowering response of kiwifruit vines to total irradiance. Yields of kiwifruit vines next to shelterbelts are significantly lower than those from vines in the centre of fruit blocks, due presumably to a reduction in flowering as a result of shade from the shelterbelt. This yield depression can be as high as 67% (Manson et al. 1994).

While shading generally reduces fruit quality, excess solar radiation can lead to serious downgrading of fruit quality. Apples and kiwifruit suffer from sunburn, particularly where the fruit has been growing in the shade and then is thrust into full sunlight by movement of branches due to weight of fruit or to summer pruning or defoliation removing foliage that previously shaded the fruit. This tends to be a much larger problem in regions of high solar radiation receipt than cloudy areas.