Interactions between environment, fruit water relations and fruit growth

In many plant organs the rate of growth decreases when reduced water availability leads to a reduction of the hydrostatic pressure of cells in the tissue (the turgor pressure, P). We have therefore examined the effects of changes of temperature, light and water availability on fruit growth rate and P in tomato fruit. Fruit and leaf mid-rib P values have been determined using a non-destructive method called a pressure probe. The fruit growth rate was simultaneously measured using linearly variable differential transformers (LVDTs). Temperature was regulated by a specially constructed growth cabinet. Although increased temperatures generally lead to increased fruit growth rates (e.g. Pearce et al., 1993), fruit P decreased after the temperature was increased. Therefore such increases in fruit growth rate are not due to elevated P. Furthermore increases in fruit growth rate with temperature are much smaller at temperatures above 25°C and it seems likely that, under these circumstances, decreased fruit P limits fruit growth rate. Diurnal changes in light intensity had no effect on fruit P or fruit growth rate provided the temperature was unchanged. Therefore, effects of light on fruit growth are indirect. Fruit water relations were found to be remarkably isolated from the water relations of the rest of the plant. During soil drying episodes, fruit P and water potential (psi) were largely unaffected until leaf psi fell below fruit psi, when fruit P decreased rapidly. Some plants were also grown in special enclosed pots to allow pressurisation of the plants' roots. Soil drying episodes and root pressurisation experiments demonstrated that fruit water relations of such plants were even less dependent on whole plant water relations than in plants grown in normal "open" pots. These results suggest that the root environment may affect fruit and pedicel vascular development