Citrus developmental research: A historic, conceptual perspective

As long as microscopes were the major research tool, developmental anatomy and morphology led the way. But then, towards 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; flowering, fruit development and ripening, abscission, rooting and vegetative growth were all attributed to one, specific plant hormone or to the perplexing 'hormonal balance' concept. The pioneer of plant growth regulators use in citriculture was the American researcher C.W. Coggins Jr. (born 1930). Synthetic plant growth regulators were exploited in every possible way to solve all practical problems. A major representative of this 'Hormonal' era was the renown Israeli citrus researcher S.P. Monselise (1920-1986). The emergence of the source-sink concept (ca. 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' phenomenon. Although the effects of girdling and fruit thinning seem to support the sink-source concept, direct control of flowering and fruit set by carbohydrate levels has not been unequivocally demonstrated. The Spanish researcher J.L. Guardiola (1943-2010) played a major role in this debate. But then, rather soon, came the molecular-genetic era, forcing us all to acquire a new language and follow its logic. Citrus research is still a few steps behind Arabidopsis but, here we are, still doing our best in order to solve the remaining mysteries of citrus.