ENERGY COSTS OF FRUITING AS A YIELD-LIMITING FACTOR WITH SPECIAL REFERENCE TO AVOCADO

An indication of yield potential can be gained from comparing the energetic costs of fruting to the photosynthesizing tree. Energy costs are calculated from the total carbohydrate, protein, and lipid contents of whole fruits, including rind and seed. Such comparisons reveal major differences between sugar-storing and oil-storing fruits. If the potential average yield of an intensively-managed high density apple planting on dwarfing rootstock is 100t ha^-1, the equivalent energetic cost for avocados is 32,5t ha^-1. This appears to be a realistic target yield for currently available germplasm, but implies harvesting at a low oil content, and significantly intensified orchard management. Energetic implications of various management aspects are discussed.

In field crops, yield improvement has been attributed mainly to increased harvest index, rather than to improved photosynthetic efficiency, reduced photorespiration and/or dark respiration, or improved canopy architecture. Tree crops have the C₃ photosynthetic pathway in common, and do not appear to differ widely in photosynthetic efficiency on an annual basis.

Little is known of assimilate partitioning in the relatively unselected tropical and subtropical fruit crops. The potential for increasing canopy leaf area is limited by lack of dwarfing rootstocks, and varying plasticity to crowding from a yield viewpoint. Significant yield improvement will nevertheless most likely arise from selection for harvest index, and smaller trees with more efficient radiation interception and assimilate partitioning.

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