LEAF GAS EXCHANGES AFFECT WATER FLOWS TO KIWIFRUIT BERRIES DURING THE DAY

Daily fluctuations in environmental conditions largely affect leaf gas exchanges and leaf water status. These changes impact the force with which leaves are able to attract water from the vascular system, and may affect water flows at whole tree level. This work studies daily leaf and fruit water relations to test whether leaf gas exchanges affect water flows to the fruit during the day. The daily patterns of leaf photosynthesis, stomatal conductance and transpiration were monitored on well-exposed shoot leaves of the cultivar ‘Summerkiwi’ 6 weeks after full bloom. In the same period, phloem, xylem and transpiration flows to/from the fruit were quantified by recording the diurnal patterns of intact kiwifruit berries, which were then girdled and subsequently detached, using highly sensitive, custombuilt fruit diameter gauges. The daily patterns of leaf and stem water potentials and of fruit pressure potential in the xylem were also determined using a pressure chamber. All leaves and fruit were sampled on the east- side of the row. Stomatal conductance and leaf gas exchanges were highly related during the day, being high in the morning and decreasing in the afternoon, in correspondence with the increase in vapour pressure deficit. These changes affected leaf water potential which decreased sharply in the morning, as transpiration rose, reached a minimum at about 10.00 and increased thereafter. On the other hand, the fruit pressure potential, which was closely related to fruit transpiration, reached a daily minimum later than the leaves, so that stem-to-fruit pressure potential gradients were low in the morning and increased during the afternoon. As xylem flows respond to pressure potential gradients in the xylem vessels, the xylem flow to the fruit was low, and even negative in the morning, increased in the afternoon and maintained high values during the night, in accordance to the daily pattern of the stem-to-fruit pressure potential gradient. Phloem flow to the fruit was much lower than the xylem’s during the day and showed a peak during the midday hours. These results show that when leaf transpiration rate is high, as occurs in the morning, most of the water available at whole tree level is directed to the leaves due to their negative water potentials. Then, as leaves close their stomata during the afternoon, an increasing amount of water becomes available for the fruit thanks to an increase in the stem-to-fruit pressure potential gradient. Water flows to kiwifruit berries are highly affected by leaf gas exchanges during the day.