Weather effects on canopy gas exchanges, fruit growth and vascular flows in apple during spring and possible consequences on orchard performance

Net carbon exchange (NCER) and transpiration rates (TR) of 'Gala' apple canopies were continuously monitored from 36 to 41 days after full bloom, using a custom built open system. In the same period, fruit growth, vascular and transpiration flows were monitored using custom built fruit gauges, while the main environmental parameters were also recorded. The experiment was characterized by some days of sunny weather followed by 3-4 days of cloudy and rainy conditions. Once the weather turned cloudy, daily NCER and TR showed a 70% sudden decrease, which occurred mainly during the morning hours, and lasted until the end of the period. Similarly, fruit water exchanges by xylem and transpiration flows dropped suddenly to values close to zero. Instead, phloem flow decreased progressively, reaching a 50% reduction only after three days of cloudy/rainy weather. This study shows how, in apple, canopy gas exchanges and fruit water flows (via xylem and epidermis transpiration) promptly respond to weather changes, both at hourly and daily time scales, while phloem flow and fruit growth are not affected by these changes in the short period (one day). However, while cloudy conditions last for 2-3 days, fruit growth may undergo losses of about 0.3 g fruit-1 day-1 of fresh weight. Besides, the permanence of cloudy and/or rainy conditions may also reduce tree carbon balance and fruit calcium accumulation and decrease canopy water needs. Therefore, the weather is an important factor to be considered for orchard management, as it affects both whole tree physiology and its performances.