Auxins promote vascular function and reduce bitter pit of ‘Honeycrisp’ apples

Bitter pit is a physiological disorder of apple characterized by dark lesions on the surface of fruit.
Despite the association of bitter pit incidence with localized calcium deficiency, corrective measures to improve calcium content in fruit do not entirely prevent the disorder.
Calcium transport, in planta, occurs solely in the xylem which is driven by transpiration.
Calcium deficiency is primarily the result of gradual dysfunction of xylem during fruit development.
The plant hormone, auxin, is known to promote xylogenesis; therefore, we treated ‘Honeycrisp’ apple trees with multiple, early-season sprays of native or synthetic auxins at 5, 10, and 20 ppm and examined effects on xylem function and bitter pit incidence of fruit.
All auxin treatments increased xylem function.
IAA produced more functional vascular bundles at harvest as concentration increased, especially in the calyx end of the fruit where bitter pit tends to develop.
In general, NAA had greater efficacy than IAA; however, NAA at 20 ppm resulted in lower bundle functionality, decreased fruit mass, fewer mature seeds, and higher bitter pit incidence than 5 or 10 ppm NAA. Depending on the compound and concentration, bitter pit ranged from ~36 to 51% of control levels both at harvest and after storage.
The most efficacious treatments were 20 ppm IAA and 5 and 10 ppm NAA. A comparison between the 20 ppm IAA and control treatments for nutrient content of fruit, indicated that the xylem transported nutrients, Fe and Ca, were increased in peel tissues by IAA. The improved vascular function and marked reduction of bitter pit by exogenous auxins suggest that these compounds have potential for the management of bitter pit.

XIV International Symposium on Plant Bioregulators in Fruit Production

C. Griffith, R. Beaudry, T. Einhorn

1-naphthaleneacetic acid, indole-3-acetic acid, xylogenesis

https://doi.org/10.17660/ActaHortic.2022.1344.10