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

In spite of seleetion for thousands of years, the avocado is still a poorly domesticated tree in the early stages of adaptation to an orehard environment. Cultivars based on Guatemalan and Mexican germplasm, for the subtropics and Mediterranean elimates, have only been available since the 1920's, and even in the best growing conditions the yield barrier of 30 t.ha-1 is hard to breaeh. Tropical "West Indian" (Iowland) avocado eultivars and produetion teehnology is less well developed. Orehard 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-seleeted, small-gap colonizing mountain eloud forest tree. These inelude potentially vigorous vegetative growth in flushes, counter-produetive 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 bome in creasingly further from the trunk on the tree periphery; energy-expensive fruits for large dispersal agents, hence the exhaustive nature of eropping on reserves, espeeially in large trees; and the tree's strategy to effieiently hoard, store and recyele carbohydrate and mineral reserves (hence "mineral-eheap" fruiting). Physiological attributes inelude 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 effieiency; and the poorly researehed 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 criticar times, to optimize C gain and allocation to fruiting on a sustainable basis. The prime need remains breeding for more manageable seion and rootstock combinations.