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 va cuole, and intra-vacuolar environment (vacuolar pH and metal ion content). Several reports demonstrate the importance of vacuolar pH in determining fl ower 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 petun ia are being used in this study, a red line (R) with a low vacuolar pH (flo wer sap pH 5.6) and a purple line (P) with a high vacuolar pH (flower sap p H 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 a nalysis 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 am ounts of subunit E in the R line. The two lines did not differ in their org anic acids synthesis. Genetic analysis of R and P lines showed that vacuola r pH is partially controlled by a single dominant gene (Ph gene).