Apple russeting: the contribution of surface conductance in the early fruit growth development stages

Biophysical fruit growth depends on a balance among the vascular and transpiration flows which enter/exit the fruit via phloem, xylem, and through the epidermis. There is a lack of information about how russet could influence fruit growth in apples by affecting these mechanisms. Two apple cultivars "Renetta Grigia di Torriana" (RGT) and "Golden Delicious" ( GD) were monitored in their first stage of development determining fruit surface conductance (gc), as well as fruit growth parameters as absolute growth rate (AGR) and relative growth rate (RGR). The gc and growth behaviors of the two cultivars were compared to outline if there was a different fruit development pattern, assuming RGT has a more spread russet on its skin while GD is russet-free. The fruit gc was significantly higher in RGT compared to GD, after the manifestation of russeting on the skin surface while daily AGR and RGR trends were relatively similar between the cultivars. Thus, the fruit growth pattern in the initial stage of development suggests that the presence of russet does not affect daily fruit quantitative growth, but it does influence the within-day pattern. Fruit with higher gc have higher transpiration and xylematic fluxes thus inducing in the diurnal growth pattern a greater fruit shrinkage during the day followed by a higher recovery enlargement at night. This suggests a cultivar-dependent contribution of the vascular fluxes within the day that may be a cause-effect of russet development. Besides, this knowledge can provide hints on how to optimize resource inputs to the orchard shifting from a species-base to a cultivar-base water/environment requirement.