The influence of air vapor pressure deficit (VPD) and plant fruit load on the expansion and water relations of young tomato fruits grown in a glass- house were evaluated under summer Mediterra- nean conditions. The contributions of phloem, xylem and transpiration fluxes to the fruit volume increase were estimated at an hourly scale from the growth curves of intact, heat-girdled and de- tached fruits, measured using displacement trans- ducers. High VPD conditions reduced the xylem influx and increased the fruit transpiration, but hardly affected the phloem influx. Net water accumulation and growth rate were reduced, and a xylem efflux even occurred during the warmest and driest hours of the day. Changes in xylem flux could be explained by variations in the gradient of water potential between stem and fruit, due to changes in stem water potential. Misting reduced
Received: 8 April 2005; accepted: 5 July 2005; online publication: 31 October 2005
*Corresponding author; e-mail: bertin@avignon.inra.fr
air VPD and alleviated the reduction in fruit vol- ume increase through an increase in xylem influx and a decrease in fruit transpiration. Under low fruit load, the competition for assimilates being likely reduced, the phloem flux to fruits increased, similarly to the xylem and transpiration fluxes, without any changes in the fruit water potential. However, different diurnal dynamics among treat- ments assume variable contributions of turgor and osmotic pressure in F3 and F6 fruits, and hypo- thetical short-term variations in the water potential gradient between stem and fruit, preventing xylem efflux in F3 fruits.