REGULATION OF TOMATO FRUIT-GROWTH BY EPIDERMAL-CELL WALL ENZYMES

Water relations of tomato fruit and the epidermal and pericarp activit ies of the putative cell wall loosening and tightening enzymes Xyloglu can endotransglycosylase (XET) and peroxidase were investigated, to de termine whether tomato fruit growth is principally regulated in the ep idermis or pericarp, Analysis of the fruit water relations and observa tion of the pattern of expansion of tomato fruit slices in vitro, has shown that the pericarp exerts tissue pressure on the epidermis in tom ato fruit, suggesting that the rate of growth of tomato fruit is deter mined by the physical properties of the epidermal cell walls. The epid ermal activities of XET and peroxidase were assayed throughout fruit d evelopment. Temporal changes in these enzyme activities were found to correspond well with putative cell wall loosening and stiffening durin g fruit development. XET activity was found to he proportional to the relative expansion rate of the fruit until growth ceased, and a peroxi dase activity weakly bound to the epidermal cell wall appeared shortly before cessation of fruit expansion. No equivalent peroxidase activit y was detected in pericarp tissue of any age. It is therefore plausibl e that the expansion of tomato fruit is regulated by the combined acti on of these enzyme activities in the fruit epidermis.