Effects of Plant Growth Regulator Treatments in Blooming Period on Rheological Properties and Cell Wall Components of Mature Grape Berries of ‘Shine Muscat’

Abstract

In Japan, seedless grapes are obtained by applications of plant growth regulators such as gibberellic acid (GA₃) and 1-(2-chloro-4-pyridyl)-3-phenylurea (CPPU). To analyze the effect of these chemicals on the palatability of the texture of unpeeled ‘Shine Muscat’ grape berries in the mature stage, we treated plants with GA₃ plus CPPU on 1 day after full bloom and GA₃ at 10–12 days after full bloom (GA₃ 10DAFB). Then, we investigated the rheological properties and plant cell wall components. For texture analysis, mechanical properties were examined using a puncture test, and the peeling ability as a skin property was determined a tensile test. To assay cell wall components, berries were divided into three parts (central part of mesocarp, pericarp around skin, and skin). CPPU treatment of more than 5 ppm at full bloom led to inferior mechanical properties of berries. Furthermore, CPPU treatment of more than 5 ppm at full bloom increased cellulose contents of the skin and pericarp around the skin. GA₃ treatment increased berry firmness and strengthened adhesive forces between the skin and pulp. Simultaneously, uronic acid contents in the 0.05 N sodium carbonate solution-soluble fraction and total sugar contents in the 4 M KOH soluble fraction of the pericarp around the skin on GA₃ treatment were higher than those of the control. Therefore, the increase of cell wall components by GA₃ treatment made the berry texture firm and strengthened adhesive forces between the skin and pulp. These results suggest that each plant growth regulator affects the cell wall components of grape berries and varies their texture.