Leaf gas exchanges and water relations affect the daily patterns of fruit growth and vascular flows in Abbé Fétel pear (Pyrus communis L.) trees

Daily patterns of leaf and stem water potentials, leaf gas exchanges (net photosynthesis, stomatal conductance and transpiration), fruit growth, vascular (phloem and xylem) and transpiration in/outflows were determined in two subsequent years at regular time intervals during the season on pear trees of the cv. Abbé Fétel. Leaf gas exchanges showed higher values in the morning, while fruit shrank due to high transpiration water losses which were not balanced by xylem flows. Despite the high rates of transpiration, fruit were fully rehydrated in the afternoon thanks to high xylem flows, while phloem import occurred mostly in the midday hours. During most of the season, fruit growth was sustained above all by the xylem flux which represented upto 85% of fruit total inflows. Later, fruit growth rate decreased while dry matter accumulation increased following a rise in the phloem relative contribution to growth, which reached 50% of fruit total inflows. This is the first report to show how the biophysical mechanism of fruit growth in “Abbé Fétel” pear is based on high water exchanges by xylem and transpiration during most of the season. The simultaneous monitoring of leaf gas exchanges and fruit vascular flows during the day suggests how until about 95 days after full bloom (DAFB), leaves are stronger “sinks” for water in the morning while fruit import water mainly during the afternoon, thanks to the increase in leaf water potential following stomatal closure.