Unlocking Bitter Pit Resistance in Apples

Bitter pit (BP) is a physiological disorder in apples that often emerges during or after cold storage. Classified as a calcium deficiency disorder, BP's development is a complex process influenced by not only the overall calcium (Ca2+) content in the fruit but also proper cellular Ca2+ homeostasis and distribution. Additionally, it has been suggested that rootstocks can significantly affect BP incidence and severity.

In our lab, we are investigating the impact of rootstock on BP development in 'Honeycrisp' apple trees. Specifically, we are examining 14 different rootstocks, including Budagovsky, Geneva, Malling, and Vineland series rootstocks. Our focus is on evaluating BP incidence at harvest and three months after cold storage. Our preliminary observations have indicated that 'Honeycrisp' trees on B.10 rootstock demonstrate a significantly reduced BP incidence compared to other rootstocks. Conversely, trees on V.6 rootstock exhibited the highest percentage of BP both at harvest and after cold storage.

In addition to BP incidence, our research also explores the relationship between rootstock and the fruit's mineral nutrient composition, Ca2+ homeostasis, and cell wall properties. We are particularly interested in the pectin content, pectin de-esterification rate, and pectin methylesterase (PME) activity in fruits from different rootstock-scion combinations.

Our initial findings suggest that fruits from B.10 rootstock have markedly higher water-soluble and insoluble pectin content compared to fruits from G.41 and V.6. Furthermore, we observed increased PME enzyme activity and a higher degree of water-insoluble pectin de-esterification in apples from V.6. Another noteworthy observation was that fruits from B.10 exhibited significantly higher Ca2+ content than those from G.41 and V.6. Furthermore, fruits from B.10 had higher Ca2+ and lower Mg2+ levels in the cell wall and water-insoluble pectin fractions. Interestingly, the ratio of cell wall-bound Ca2+ to total Ca2+ was lower in B.10 compared to G.41 and V.6.

Through our ongoing research, we aim to further elucidate the complex interplay between rootstock, calcium homeostasis, and cell wall properties in the context of BP development in 'Honeycrisp' apples.