Proposed physiological mechanism of mango fruitlet abscission

Fruitlet abscission of mango (Mangifera indica) causes severe production problems worldwide. To understand the chain of physiological events that take place during mango fruitlet abscission, measurable fruit parameters were examined to characterise the onset and time-dependent course of this process. Both, external features (fruit coloration) and internal indicators (carbohydrate concentration, auxin transport capacity, gene expression) of about-to-abscise (ATA) fruitlets were investigated, in addition to seed health and the location of detachment. The results indicate that fruitlets with degenerated seeds in comparison to fruitlets with healthy appearing seeds have a reduced fruit detachment force (FDF). However, no differences in the expression of ethylene receptors, the concentration of sucrose, or the polar auxin transport capacity could be measured and the reduced FDF may depend on smaller, less developed fruitlet. It is interesting to note that fruitlets with healthy and degenerated seeds detached to similar proportions either at the abscission zone (AZ) or somewhere along the pedicel, although under natural field conditions fruitlets never separate outside the AZ. Fruitlets that detached at the AZ compared to those that detached at the pedicel had a highly significantly reduced FDF, a significantly lower concentration of sucrose and an significant increase of the ethylene receptor (MiERS1 and MiETR1) transcripts in the pericarp. It is proposed that these highly significant findings and not the seed status characterise ATA fruitlets detaching at the AZ. Moreover, the results further suggest that a carbohydrate supply limitation might induce natural fruitlet abscission in mango.