Decrease in vacuolar pH during petunia flower opening is reflected in the activity of tonoplast H+-ATPase

Flower color is determined by two major factors, pigments present in the vacuole, and intra-vacuolar environment (vacuolar pH and metal ion content). Several reports demonstrate the importance of vacuolar pH in determining flower color, and show that increase in vacuolar pH changes-the color of the petal. We study vacuolar pH in petunia flower as a model system, and the H+ pumps (H+-ATPase and H+-PPase) using flower pH mutants. Two lines of petunia are being used in this study, a red line (R) with a low vacuolar pH (flower sap pH 5.6) and a purple line (P) with a high vacuolar pH (flower sap pH 6.1). ATP hydrolyzing activity of the tonoplast H+-ATPase of petals from the R line was three times higher than that from the P line. Western blot analysis showed similar amounts of the 70 kD and 58kD subunits of vacuolar H+-ATPase in the two lines; however, subunit E amounts differed between the lines. The difference in the pH of the flower sap manifested with higher amounts of subunit E in the R line. The two lines did not differ in their organic acids synthesis. Genetic analysis of R and P lines showed that vacuolar pH is partially controlled by a single dominant gene (Ph gene).