Carbohydrate Supply as a Critical Factor for Citrus Fruit Development and Productivity

Productivity is the final outcome of a long chain of developmental events. The reproductive processes leading to cropping in a fruit tree may be analyzed at two distinct levels: 1) the reproductive unit, single organ level, and 2) the multi-organ fruit population, tree level. (The orchard, with its tree population, represents a still higher organizational level.) Although a complete “failure” at the organ level precludes crop formation altogether, both types of analysis are biologically meaningful and differ in many respects. Analysis at the organ level will focus on cellular and biochemical processes. Analysis at the tree level will emphasize quantitative aspects, employing statistical and modeling approaches. Taking “fruit set” as an example, for the individual fruit it is an “all-or-none” event, governed by hormonal and biochemical systems. However, when the fruit population of an entire tree is considered, the rate of fruit set assumes a quantitative significance. The carbohydrate economy of plants, more frequently referred to as source-sink relationships, has received considerable attention in recent years, and important biochemical and regulatory aspects have been elucidated. However, such research has been conducted mainly with model plants and with a few annual crop plants. Our knowledge and understanding of these processes in fruit trees, in general, and citrus, in particular, are still very fragmentary. Carbohydrate economy may be more problematic in this respect than other research areas, because of considerable variability among plant genera and the existence of several biochemical alternatives. The source-sink physiology of citrus as related to fruit growth and productivity has been investigated extensively in the last decades, mainly at the quantitative tree level. Certain cellular and biochemical carbohydrate systems have been examined at the organ level, mainly in relation to fruit development and maturation. Most of this research has been reviewed by Goldschmidt and Koch (1996). The present article concentrates on specific aspects of citrus’ source-sink relationships that deserve further discussion, and indicates directions for future research.