Fruit production starts with the initiation of a flower, pollination, and subsequent fertilization and fruit development. Seedling trees must go through a transition from a juvenile to adult stage before flowering can occur. Grafted trees, however, are adult above the graft from the time they are planted in the orchard, so juvenility is not a consideration in commercial fruit orchards.
In temperate fruit, flowers are formed within the buds during summer and fall, overwinter, and then bloom the following spring. In apple and pear, buds are either vegetative or mixed (containing leaves and flowers) with five to six flowers per bud and seven to eight flowers per bud, respectively. Flowers are typically borne on spurs, which are short shoots on two-year and older wood. Lateral buds on the previous year's growth may also produce flowers, depending on cultivar and growing environment. Apple flowers borne on the preceding year's growth open later and are generally smaller, producing smaller fruit. In peach and nectarine, however, flowers are borne laterally on one-year-old shoots. At each node, one to three buds may be borne, depending on cultivar and the vigor status of the tree. Where one bud is borne, it may be either a fruit or leaf bud. Where two are borne, one is usually fruitful while the other is not, and where three buds are present on a node, the center bud is often a leaf bud with a flower bud on each side.
Many factors affect whether a bud becomes reproductive or remains vegetative. Since flowers are initiated in the buds the year before they bear fruit, events one year can affect cropping the following year. The most obvious example of a year-to-year carryover effect on flowering is biennial or alternate bearing. This is fairly common in apple orchards and occurs when trees are overcropped one year, resulting in poor flower bud development and low crops the following year. For many years, this was thought to be because heavy crops depleted the tree of energy or nutrients, and the tree simply did not have enough energy to produce a sizeable crop the following year. In 1967, however, Chan and Cain published a classic paper demonstrating that the cause of poor flowering following a heavy crop is not heavy cropping per se, but rather the presence of seeds. Now it seems the gibberellins (GA) produced by developing seeds are the source of the inhibition of flower initiation, and some gibberellins (GA7) appear more inhibitory to flowering than others (GA4). Continued research examining the types, amounts, and transportation of various GAs in apple has failed to explain the differences among cultivars in their tendency for biennial bearing. The mechanisms controlling flower formation are complex, and although they have been widely studied, our understanding of the process is still fairly rudimentary. Although biennial bearing remains a problem in many orchards, judicious use of chemical thinning agents can help reduce its severity.
Flower initiation usually occurs during early spring, within approximately one month of the time of bloom. The first visible signs (Figure F1.1) of development occur when the flower begins to differentiate in the bud, usually during mid- to late summer. The transition of the apex of the bud from a flattened to a domed shape indicates that a bud has become floral rather than vegetative, although the invisible signal or switch obviously occurred some time earlier. A concept referred to as the "critical appendage number" embodies the idea that the bud must attain a certain degree of complexity for doming to occur. For apple, 18 to 22 appendages are required in spur buds prior
FIGURE F1.1. Electron micrographs of flower development in buds of 'Delicious' apple: (A) vegetative bud with flattened apex; (B) doming of the apex, the first visible sign of flower initiation; (C) flower bud of apple showing development of the king blossom and three lateral flowers to flower formation, although buds on previous year's growth flower at a low level of complexity.
Following doming of the apex, the flower parts (sepals, petals, anthers, etc.) differentiate in preparation for budbreak the following spring. Some evidence suggests that the degree of differentiation and/or flower size may play a role in the size of fruit produced, and studies are currently progressing to understand this further.
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