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 58 kD 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).