How irrigation level affects the daily growth pattern of young pear fruit of the cv. 'Abbé Fetel'

Fruit growth is a key factor to be considered when evaluating overall plant performance, as it represents the economically relevant, integrated result of all physiological processes underway at tree level. Water availability is known to affect important plant functions related to fruit growth such as carbon assimilation, phloem translocation and xylem flows. This work investigates the effect of water restrictions on pear fruit growth in the early stages of fruit development. Four irrigation regimes, corresponding to 100%, 50%, 25% and 0% of the calculated daily evapotranspiration, were applied to several trees of the cv. 'Abbé Fetel', starting from 40 days after full bloom (DAFB) until harvest. At about 50 DAFB, diameter changes of 4 fruit per treatment were continuously monitored over 24 hours, using automatic fruit gauges. On the same trees, leaf and stem water potentials were measured at 10:00,12:00,15:00 and 24:00 hours, while the daily patterns of phloem, xylem and transpiration flows to/from the fruit were estimated only on the 100% and 25% irrigated trees. For each treatment, soil water status was also monitored using tensiometers at different soil depths. In the morning, both stem and leaf water potentials were reduced in the 0% treatment, while from midday on only stem water status was negatively affected by drought conditions and showed differences between the stressed (50% 25% and 0% treatments) and the 100% treatment. Fruit growth patterns over the 24 hours were characterized by the typical daily fluctuation in fruit diameter, alternating periods of shrinkage and swelling and no significant differences were detected among 100%, 50% and 25% treatments. Only fruit on the 0% irrigated trees showed reduced growth rates during the night and longer and deeper fruit shrinkages during the central hours of the day, which were followed by faster rehydration rates during the late afternoon. This change in the growth pattern resulted in a decreased total fruit daily growth, which however, was detected also on the 25% fruit. According to these results, a water supply of 50% of the tree evapo-transpiration may not be limiting for pear fruit growth at this time of the season and in these experimental conditions. Reduced xylem inflows were detected on 25% irrigated fruit at midday and in the early afternoon, the times when the same trees showed a significant reduction in their stem water potential. Stem water status may thus represent a good indicator for upcoming limitations in fruit water inflows due to drought stress.