ECOPHYSIOLOGY OF THE AVOCADO (Persea americana Mill.) TREE AS A BASIS FOR PRE-HARVEST MANAGEMENT

In spite of selection for thousands of years, the avocado is still a poorly domesticated tree in the early stages of adaptation to an orchard environment. Cultivars based on Guatemalan and Mexican germplasm, for the subtropics and Mediterranean climates, have only been available since the 1920's, and even in the best growing conditions the yield barrier of 30 t·ha is hard to breach. Tropical “West Indian” (lowland) avocado cultivars and production technology is less well developed. Orchard performance is therefore still largely influenced by evolutionary hangovers and constraints. For “subtropical” types, we have to deal with the residual survival strategies of a late-successional, K-selected, small-gap colonizing mountain cloud forest tree. These include potentially vigorous vegetative growth in flushes, counter-productive to flowering and fruiting; delayed and typically irregular (mast) fruiting; unnecessarily profuse flowering in late winter synchronized by winter cold and drought; short-lived shade-adapted leaves borne increasingly further from the trunk on the tree periphery; energy-expensive fruits for large dispersal agents, hence the exhaustive nature of cropping on reserves, especially in large trees; and the tree’s strategy to efficiently hoard, store and recycle carbohydrate and mineral reserves (hence “mineral-cheap” fruiting). Physiological attributes include potentially rapid C assimilation, but reduced photosynthesis under shade, water of Phytophthora stress; reasonable drought tolerance which however is at the expense of C assimilation and leaf efficiency; and the poorly researched dependence on perseitol rather than sucrose as the main translocation sugar. Manipulation implications center around canopy/tree size/light management and the alleviation of stress at critical times, to optimize C gain and allocation to fruiting on a sustainable basis. The prime need remains breeding for more manageable scion and rootstock combinations.