A biophysical model of fruit development with distinct apoplasmic and symplasmic pathways

Abstract

A biophysical model of fruit development is described, based on the current understanding of transport pathways into the developing fruit, and the apoplastic/ symplastic structure of the fruit. The model explicitly includes a pedicel, and distinct apoplasmic and symplasmic pathways into the fruit. Water and sugars enter the fruit vasculature by mass flow through the xylem and phloem of the pedicel. Water from the xylem then flows directly into the apoplasm of the fruit, from which it can enter fruit symplasm across the plasma membrane. Water from the phloem can also reach all parts of the fruit by crossing semi-permeable membranes, but for sugar transport two different pathways are used. Phloem sap can enter the symplasm directly by mass flow through plasmodesmata (the symplasmic pathway), or alternatively it can be unloaded across a membrane into the apoplasm and then be taken up into the symplasm by active (or passive) transport across the plasma membrane (the apoplasmic pathway). The relative importance of these two pathways can change during the season. An explicit apoplasmic pathway means that the sugar concentration and osmotic potential of both the symplasm and the apoplasm can be modelled and compared with experimental results. Issues arising from non-zero sugar concentration in the apoplasm, including the effect of changing properties of the plasma membrane, and the change in sugar concentration between the xylem and the fruit apoplasm, are discussed. Measurements of fruit growth, pedicel xylem resistance, turgor and fruit surface conductance in kiwifruit are used to parameterize and test the model. It is shown that the model can be used to place bounds on the concentration of sugars in the phloem of kiwifruit necessary to maintain observed patterns of fruit fresh weight and dry weight accumulation.