Monitoring fruit daily growth indicates the onset of mild drought stress in apple

This work tests the possibility to monitor fruit growth as a potential physiological indicator for tree water status and productive performance in apple, for potential implementation in decision support systems for irrigation scheduling. Starting from 10 weeks after full bloom (WAFB), a wide range of plant water statuses was induced in a Gala apple orchard by applying different shading (with red-50%, white-50%, and black-20% nets) and irrigation levels (severe-SS, moderate-MS and no-NS stress). For each net*irrigation treatment combination, midday stem water potential (mSWP) was assessed weekly, while fruit diameter variations were continuously monitored using automatic fruit gauges, from 10 WAFB until harvest. Leaf gas exchanges were also monitored at 14 and 16 WAFB. As expected, the different net*irrigation treatment combinations widely affected mSVVP, leaf gas exchanges and fruit growth. On all dates of measurement, leaf gas exchanges were tightly correlated with mSVVP, while daily fruit growth showed significant but weaker correlations with mSVVP. In all cases, these relationships indicated the onset of drought stress below the threshold of about 1.2 g fruit(-1) day(-1), which corresponded to mSVVP below(-1) MPa. Almost no correlation was found between mSWP and the other parameters derived from the fruit daily growth pattern (midday, maximum and minimum absolute growth rates (AGR) and fruit shrinkage). Based on these results, we can conclude that, although fruit daily growth rate is not related to mSWP as tightly as leaf gas exchanges, it represents a promising physiological indicator to be implemented in a decision support system for irrigation scheduling. Specific fruit growth thresholds indicating the onset of drought should be defined, depending on the orchard conditions and productive target.