Light Level Influences Spur Quality and Canopy Development and Light Interception Influence Fruit Production in Apple

Abstract

To model canopy growth and fruiting of apple trees, a thorough understanding of the interaction of light levels and leaf and fruit growth is required. Light distribution within a tree canopy is determined by spatial aspects of leaf development, which is usually influenced by horticultural practices such as pruning and tree training. However, individual leaf growth is itself directly influenced by light penetration to the site of leaf development. In this paper we discuss the influence of light levels on the development of individual vegetative and fruiting spurs of apple and the relationships of light interception, leaf development, and fruit production for whole trees. Apple tree canopy development within a season and over the life of an orchard is crucial to the production of high quality fruit each season and to the early and sustained productivity of the orchard. Canopy development within a season and, in many cases, its development the following season can be influenced by cropping level, mineral nutrition, moisture level, plant growth regulators, pruning and training, scoring, light distribution, rootstock, and cultivar. The effect of these factors on canopy development was recently reviewed by Forshey and Elfving (1989). However, few studies have examined the influence of these factors on the development of individual spurs, the site of fruiting on apple trees. Spur leaves develop before shoot leaves (Cain, 1973; Forshey et al., 1987; Lakso, 1984). When spur leaf area is reduced, fruit set, fruit size, and fruit Ca level are also reduced (Abbott, 1960; Ferree and Palmer, 1982; Rom and Ferree, 1984a). A greater than 2-fold difference in leaf area per spur at bloom (Rom and Ferree, 1984c) and during midsummer (Barritt and Schonberg, 1990; Oh et al., 1987) has been found among cultivars. Canopy position and light levels strongly influence leaf area per spur at full canopy (Barritt et al., 1987; Ferree, 1989; Ferree and Forshey, 1988). Spur leaf area has been increased and decreased with growth regulators (Curry and Williams, 1983; Ferree and Schmid, 1988). Unpruned trees have greater total spur leaf area and less shoot leaf area than pruned trees (Lakso, 1984). Fruiting spurs have smaller leaves and less leaf area per spur than vegetative spurs (Barritt et al., 1987; Lakso, 1984). Horticulturists generally believe that the key to high early production in high density orchard systems is the rapid development of leaf canopy in the first few years after planting. High leaf area early in the life of the orchard is achieved by planting at high tree densities, by planting large, well-branched (feathered) trees, and by minimal pruning. Few studies have examined the relationships of leaf area/ha (orchard leaf area index), light interception, and yield with diverse apple orchard systems. With apple, light interception has been associated with leaf area/ha (Barritt, 1989; Jackson and Palmer, 1974; Palmer and Jackson, 1977) and with yield/ha (Barritt, 1989; Jackson, 1978; Jackson and Palmer, 1974; Robinson and Lakso, 1989; Wagenmakers and Callesen, 1989; Wertheim et al., 1986). The objectives of the studies reported here are to 1) describe the seasonal development of 'Delicious' fruiting and vegetative spurs, 2) determine the influence of canopy position and light level on spur development of 'Delicious' and 3) determine in two seasons the relationship of leaf area, light interception, and yield with 'Granny Smith.' Horticulture/Landscape Architecture Paper no. 89-15. INFLUENCE OF LIGHT LEVEL ON SPUR DEVELOPMENT Seasonal development of leaves of fruiting and vegetative spurs was examined in a commercial orchard in Manson, Wash., with 17-year-old 'Oregon Spur Delicious'/MM.104 trees spaced 3 × 5.5 m in north-south rows. Trees were central leader-trained to a pyramid shape (Heinicke, 1975) and were ≈ 3.5 m wide at the base and 4 m tall. There were three data collection positions near the central leader on each of 24 trees: 1 m (bottom), 2 m (middle), and 3 m (top) above ground level. Spurs were collected within a 50-cm spherical radius of the central leader at each canopy position on 10 dates throughout the 1986 growing season: 23 Apr., 5 May [full bloom at harvest 22 Sept. (FB + 141). On each date one fruiting spur and one vegetative (nonflowering) spur were collected from each canopy position on each of 24 rep-licate trees. By collecting only one spur per position per tree per date the impact of spur removal on subsequent development of the remaining spurs was minimized. Leaf number (LNO/SP), area (LA/ SP; cm 2) and dry weight (LDW/SP; mg) were determined for each fruiting and vegetative spur. From these values, area per leaf (LA, cm 2), dry weight per leaf (LDW; mg) and leaf dry weight per unit leaf area (LDW/LA, mg·cm-2) were calculated. With fruiting spurs, the primary leaves, those which emerged first in a close spiral directly below the terminal flower cluster, were measured separately from bourse leaves, those which emerged later in the axil of a primary leaf either on short bourse shoots or as a rosette of leaves.