Abstract

Approximately 3 weeks after pollination, the water import rate per unit tomato fruit area generally decreases linearly with fruit radius. In order to explain why, in the first 3 weeks, the water import rate of the fruit is lower than predicted by this regression, a model of water flow through the pedicel and a model of water import based on the potential of water entering the fruit and calyx transpiration were formulated. Using data sets available in the literature, these models predict a water potential drop along the pedicel that decreases sharply during the first 3 weeks, while the calculated hydraulic conductivity of the pedicel phloem, which is presumed to be the main pathway of the water imported in the tomato fruit, increases sharply in the lower range of values known for plant phloem conductivity. These models also predict an increase in water import into young fruit when calyx transpiration is decreased, which is consistent with data from the literature. In order to explain the increasing pedicel phloem conductivity, a model of water flow in the pedicel sieve tubes was formulated based on the literature data for the fruit stalk of Lupinus albus. It was suggested that the conductivity might increase because of the development of pores in the sieve plates. Using this hypothesis, the increase in pore radius values within an acceptable range was calculated. This study shows that, under a wide range of conditions, water import in young tomato fruit is limited by the low potential of the water entering the fruit due to pedicel resistance and calyx transpiration. It provides a model to predict young tomato fruit expansion and a testable hypothesis, which can be checked by measuring the size of the phloem component in the fruit pedicel.