The Role of Xylem in Bitter Pit Incidence of Apple: A Review

Bitter pit is a physiological disorder in pome fruit which causes substantial losses for commercial apple producers. Bitter pit is purportedly associated with calcium (Ca) deficiency, but other nutrients and their ratios with Ca, i.e. magnesium (Mg) and potassium (K), exacerbate the disorder. Symptoms of bitter pit are similar to corking disorders resulting from boron (B) deficiency, resulting in frequent and considerable confusion, especially in the early literature. Ca and B transport in apple, as in other fruits, becomes increasingly more limited as fruit develop by gradual and progressive xylem dysfunction. While the mechanisms that regulate and impair xylem function are still poorly understood, xylem dysfunction of bitter pit-sensitive cultivars occurs earlier than in resistant cultivars. Although phloem plays a compensatory role for water transport to growing apples, phloem tissue is void of free Ca. The focus of this review therefore is on new insights in xylem differentiation, function, and transport and the consequent effects on nutrient deliveries to fruit. We propose a novel approach to mitigate bitter pit and increase xylem differentiation and function via exogenous applications of auxin. The process by which xylem tissue is maintained, and the mechanisms which govern its dysfunction require further elucidation.