In modern orchards, young trees are protected from weeds and pests, fertilized heavily, and irrigated to induce rapid filling of their allotted spaces. Then, in the third or fourth leaf, growers expect trees to decrease vegetative growth and begin flowering and fruiting, so the orchard will be profitable and tree size will be limited. Growers rely on size-controlling rootstocks to switch trees from a high-vigor to a precocious state. This strategy is not always successful. For example, even in the presence of size-controlling stocks, apple cultivars such as 'Jonagold', 'Mutsu', and 'Fuji' may be excessively vigorous for intensive plantings. In other species, such as peach and sweet cherry, size-controlling stocks suitable for North America are still under test and not yet widely used by the industry.
To solve these problems, numerous PGRs have been tested and registered for use in commercial orchards. The underlying hypothesis in vigor control is that there is a "hormonal balance" in the young tree. When that balance favors vigor, flowering, and hence fruiting, is suppressed. When vigor is reduced, flowering and fruiting will follow.
Regulation of vigor in young trees is modulated using PGRs that shift the tree balance from vegetative to reproductive. Direct inhibitors of gibberellin synthesis are compounds such as paclobutrazol and uniconazole. These directly reduce extension growth, induce terminal bud formation, and lead to an increase in flower bud initiation. Once flowering and fruiting begin, cropping controls vigor further, so that a tree can be managed within its allotted space (Zimmerman, 1972). Applications are typically made as a soil drench or spray early in one growing season, with the goal of enhanced yields in the subsequent season.
PGRs that act indirectly to enhance flowering are compounds such as ethephon. Ethephon is applied early in the growing season to nonbearing trees and is metabolized to release ethylene. The ethylene released then suppresses extension growth, induces radial growth and branch stiffening, and enhances flower bud initiation.
Selecting one of the previous approaches for managing young trees depends on crop, PGR cost, and grower preference. Pome fruit growers are likely to use ethephon, while stone fruit growers are more likely to choose gibberellin biosynthesis inhibitors. Controlling tree vigor is more difficult in stone fruit than in pome fruit due to the lack of adequate size-controlling rootstocks; thus, direct inhibition of growth through suppressing gibberellin biosynthesis is required. Additionally, application of ethephon can cause phytotoxicity in stone fruit trees. Characteristic symptoms induced by ethephon are leaf abscission and gummosis.
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