Short-period changes in weather conditions affect xylem, but not phloem flows to young kiwifruit (Actinidia deliciosa) berries

Weather conditions are known to affect many physiological parameters like canopy gas exchanges, tree water relations and fruit growth. However, their effects may be different depending on plant species and developmental stage. This study investigates the effects of sudden weather worsening on the daily dynamics of kiwifruit berry growth, at different times during the season. Phloem, xylem and transpiration flows to/from the berry were continuously monitored for several days at about 5, 9, 11 and 15 weeks after full bloom (WAFB), on two consecutive years, while air temperature, relative humidity, solar radiation and rain were simultaneously recorded. From these data vapour pressure deficit (VPD) was also calculated. For each period considered, the daily amounts and the daily patterns of kiwifruit berry relative growth rate (RGR) and vascular flows were compared between two subsequent days having high (HVPD) and low (LVPD) mean VPDs, respectively. During all periods, daily water exchanges were linearly related to VPD although these relationships showed decreasing slopes during the season. At 5 and 9 WAFB daily and hourly rates of xylem and transpiration flows were highly reduced by LVPD conditions, while phloem flow was not affected by such changes during the whole season. Also, LVPD conditions affected the typical daily fluctuations in the berry RGR by reducing its morning shrinkage and slowing down its afternoon growth rates. This study shows how in the early stages of kiwifruit berry growth, weather worsening highly reduces the berry water exchanges by xylem and transpiration, but does not affect its phloem imports in the short-period.