Abstract

Firmness is an essential quality parameter of table grapes (Vitis vinifera) for consumers, with grapebunches that contains soft berries less preferred, resulting in a reduction in the market price. The soften-ing of grape berries has been commonly associated with cell walls, especially the disassembly of pecticpolysaccharides. However, the process of berry softening is not completely understood. To investigate thesoftening process of grape berries, we compared the Thompson Seedless variety, which suffers significanteconomic losses due to fruit softening, and NN107, a new variety with a significantly higher level of berryfirmness. The composition of the cell wall during the berry development of these two grape varietieswas compared. NN107 berries had a greater amount of calcium and uronic acids in the cell wall materialthan Thompson Seedless grapes, suggesting a special role for calcium bridge formation in NN107. Addi-tionally, polyacrylamide carbohydrate electrophoresis (PACE) analysis suggested differences betweenthese varieties in pectin structure. Thompson Seedless grapes showed increased pectolyase hydrolysablesite dynamics in the cell wall material and higher polygalacturonase activity than NN107. Immunohis-tochemistry focusing on the pectin structure confirmed the roles of both calcium bridge formation andcell wall integrity as they relate to a firmer grape berry phenotype.