The growth of young apple fruit and their phloem, xylem and transpiration flows were continuously monitored, from 28 to 41 days after full bloom. The study period was characterized by several days of sunny weather followed by 4-5 days of cloudy and rainy conditions. Fruit transpiration showed positive, linear relationships with vapour pressure deficit (VPD). However, the slope of these daily relationships decreased during the experiment due to the physiological progressive reduction in the fruit epidermis conductance. Xylem flow was related to transpiration by a positive relationship: when transpiration rates were very high, xylem flows to the fruit were probably limited by the hydraulic conductance of the xylem vessels. On the contrary, phloem flow did not show any relationship either with VPD or with fruit transpiration. This can be attributed to the active mechanism of assimilate unloading in apple, which does not need hydrostatic pressure gradients to occur. Once the weather turned cloudy and rainy, transpiration and xylem flows both decreased suddenly whereas phloem flow showed a slower, progressive reduction as well as fruit daily growth, probably due to the decrease in whole canopy assimilation. This study shows how, in apple, fruit water exchanges (via transpiration and xylem flows) are directly affected by changes in weather conditions, occurring both at hourly and daily time scales; whereas effects on phloem flow and on fruit net daily growth are slower and appear only when weather changes are protracted for a few days.
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