Fruit-load-induced starch accumulation causes leaf chlorosis in “off” ‘Nadorcott’ mandarin trees

Abstract

Leaf chlorosis often develops in low-fruiting (“off”) trees of healthy, severely alternate bearing ‘Nadorcott’ mandarin (Citrus reticulata Blanco) trees in commercial orchards in South Africa, whereas leaves of heavily-fruiting (“on”) trees maintain a healthy green colour. This study investigated the role of fruit load and leaf carbohydrate content on the development of leaf chlorosis in “off” trees under a broad spectrum of source/sink relationships. Leaf chlorosis in “off” trees was mediated by low sink activity, and leaf total chlorophyll content strongly correlated with leaf starch content [r2=(−)0.74 (P<0.001)]. A decrease in leaf total chlorophyll content in “off” trees coincided with a peak in leaf starch content during winter, after activity of all the major sinks reached a minimum. In “on” trees, leaf starch content was significantly lower compared to “off” trees, the major carbohydrate component was sugars, and leaves maintained a healthy green colour. Summer de-fruiting of “on” trees instantly resulted in accumulation of leaf starch, but loss in leaf total chlorophyll content was delayed until autumn. Lower leaf nitrogen (N) and lower leaf sugar content do not appear to be the cause of leaf chlorosis in “off” trees, but rather a response to the disruptive effect of fruit-load-induced starch accumulation on the leaf chloroplasts. Results can be used to prevent unnecessary winter N applications in “off” ‘Nadorcott’ mandarin trees. This phenomenon was previously artificially induced at the branch-level, but this is the first report of fruit-load-induced citrus leaf chlorosis on a whole-tree level.