APPLE AND PEACH: A DIFFERENT ROLE FOR FRUIT TRANSPIRATION?

The environments where apple and peach originated are very different in terms of water availability and evaporative demand. This likely determined the evolution of different strategies against drought stress, with ensuing effects on the growth mechanisms of their fruit. This work compares the seasonal biophysical mechanisms of apple and peach fruit growth via an analysis of the vascular and transpiration in/outflows on ‘Gala’ apple and ‘Red Gold’ nectarine fruit. Due to their lower surface conductance, apples attain lower transpiration rates during the season, which are reflected in their lower xylem flows, as these are directly related to transpiration. By contrast, phloem flow is always comparable between the two species both in terms of daily amounts and daily patterns. It is striking that peaches exchange much more water than apples, but receive the same amount of phloem sap. This might be related to the different strategies adopted: apples unload assimilates actively and phloem-to-fruit hydrostatic pressure gradients are of relatively low importance. On the contrary, although the mechanism of phloem unloading in peach fruit is still unclear, some results suggest that it may be partly passive, at least during the cell expansion stage. As fruit turgor pressure is negatively related to transpira¬tion rate, the high water loss of peach fruit may be the way to lower water potential and to create the hydrostatic pressure gradient needed to promote passive unloading of assimilates. Therefore, the high surface conductance and transpiration rates of peach fruit appear to have a key role for fruit growth. This feature may be the consequence of the higher water availability typical of the south-east of China, from where most of the peach genetic material derives. Similarly, the growth strategy of apple fruit, based on low surface conductance, seems related to the dry environment (Kazakhstan mountains) where the apple species originated.