The effect of plant growth regulators on xylem differentiation, water and nutrient transport, and bitter pit susceptibility of apple

Abstract

Bitter pit, a physiological disorder of pome fruit, causes substantial annual losses of apple fruit, especially for the commercially important cultivar ‘Honeycrisp.’ Despite being classified as a calcium deficiency disorder, methods for increasing fruit calcium do not fully prevent bitter pit. Bitter pit is a complex disorder which has been connected with fruit xylem dysfunction, mineral ratios, water stress, and cell membrane stability. Each of these factors are influenced by hormones and plant growth regulators (PGRs), notably auxins, gibberellins (GAs), and abscisic acid (ABA), though other compounds are likely involved. Before auxins were determined to promote xylem function and, therefore, calcium transport, two experimenters in the 1950’s applied several auxins to apple trees to remedy boron deficiency and bitter pit. GAs have been demonstrated to exacerbate bitter pit by reducing membrane stability and calcium translocation. GAs also increase vegetative growth resulting in greater competition for calcium between vegetative and reproductive sinks. ABA is a GA antagonist that mitigates bitter pit by reducing transpiration, improving xylem function and, ultimately, facilitating calcium transport to fruit. The connection between other classes of hormones and PGRs such as cytokinins, ethylene, jasmonates, and salicylic acid to bitter pit has not yet been firmly established, but their effects on plant growth and cellular function suggest potential for bitter pit mitigation. The objective of this review is to synthesize the physiological, anatomical and biochemical effects of endogenous and exogenous PGRs on xylem function, mineral transport and calcium-related disorders of apple and other species. We suggest that hormone concentrations and interactions ultimately control xylem dysfunction that results in the often characterized nutrient imbalances linked to bitter pit.