Fifty Years of Citrus Developmental Research: A Perspective

An overview of the conceptual and methodological changes in citrus developmental research during the past half century is provided. Beginning with the hormonal era, the extensive use of plant growth regulators (PGRs) is described and the presumed role of gibberellins in various stages of the reproductive processes is emphasized. Introduction of the sourcesink concept opened the way for alternative, carbohydrate-mediated explanations of productivity and alternate bearing. The potential role of ethylene in citrus abscission and fruit ripening is reviewed. The recent adoption of genetic–molecular approaches paves the way for deeper understanding of citrus developmental enigmas. For a long time, morphological observations, microscopes, and simple chemical analyses were fruit tree researchers’ major tools. However, then, toward the middle of the 20th century, horticulturists were enchanted by the plant hormone concept. The hormonal thinking dominated citrus developmental physiology for a long time; rooting, vegetative growth, flowering, fruit development, ripening, and abscission were all attributed to one specific plant hormone or to the perplexing ‘‘hormonal balance’’ concept. Synthetic growth regulators were exploited in every possible way to solve practical problems. The emergence of the source-sink concept ( 1980) partially replaced the hormonal hypothesis. The source-sink hypothesis claims that carbohydrate levels control the major stages of the fruiting process—flowering, fruit set, and fruit enlargement—and are, therefore, responsible also for the ‘‘alternate bearing’’ syndrome. Although the effects of girdling and fruit thinning seem to support the source-sink concept, direct control of flowering and fruit set by carbohydrate levels has not been unequivocally demonstrated. However, then, rather soon, came the molecular–genetic era, forcing us all to acquire a new language and follow its logic. We can now identify the genes responsible for developmental phenomena. Citrus research is still a few steps behind Arabidopsis but here we are, still doing our best to solve the remaining mysteries of citrus. The purpose of the present article is to provide an overview of the conceptual changes that took place in the area of Citrus developmental physiology during the recent half century or so and highlight major achievements. Corresponding changes must have taken place in other fruit tree crops. In the background of this specific area are the far-reaching developments in biological theory and, of no lesser significance, the emergence of new techniques and methodologies. While plant physiological research gradually adopted the new experimental tools, the acquisition of new methodologies by horticulturists was a slow, painstaking process. A schematic illustration of these conceptual changes is provided in Figure 1. Please note that the ‘‘source-sink’’ concept ‘‘invades’’ the scenery but does not fully replace the ‘‘hormonal paradigm.’’ The elliptic ‘‘genetic–molecular era’’ is wider than the circle of the ‘‘hormonal paradigm,’’ indicating its broader biological significance.