Increasing fruit size in Citrus. Thinning and stimulation of fruit growth

TLDR: Understanding the mode of action of the applied bioregulators shall ensure more reliable results from their application and has resulted already in the formulation of some general recommendations on their use.

Abstract: The importance of fruit size as a parameter of quality of citrus fruits has increased markedly in recent times. The consumer’s marked preference for large fruit causes huge differences in price between large and small fruit to the point that the income from the smaller fruit is often less than picking and hauling costs. Fruit size has become as important as yield in the determination of the profitability of citrus plantings, and an economic premium is usually obtained through the increase in fruit size even at the expense of a reduction in crop yield. This applies not only to the small fruited mandarins and hybrids but also to large fruit species such as lemons, oranges and grapefruit. To increase fruit size beyond the limits which may be obtained through the optimization of the standard cultural practices (fertilization, irrigation, pruning), several techniques have been tried such as hand thinning [80], chemical and hormone thinning [49, 77] and the hormonal stimulation of fruit growth rate by synthetic auxins [69, 70, 71]. The earlier investigations on this subject were reviewed extensively by Coggins and Hield [12], Monselise [57] and Wilson [78], which pointed out some drawbacks encountered with the use of these techniques in practice. Particularly, the application of synthetic auxins to increase fruit size often resulted in too erratic results to justify the use of these compounds by the growers [57]. An exception was the successful use of NAA and, in recent times, IZAA (5-chloroindazol-8-acetic acid; ethylcholozate) to thin satsuma mandarins in Japan [49, 50]. This success resulted from a nation wide research in which the physiological aspects of the response to these compounds were carefully characterised. In the present report, we summarise the most recent developments and our present knowledge on the way fruit size is determined and the different approaches to increase it through the use of plant growth regulators. Understanding the mode of action of the applied bioregulators shall ensure more reliable results from their application and has resulted already in the formulation of some general recommendations on their use. Some aspects on the regulation of fruit size and the response to synthetic auxins have been reviewed recently [38, 46]. Recent research carriedout in Japan has been summarised by Iwahori [54] and Yamashita [79].