The petal of blue morning glory, Ipomoea tricolor cv. heavenly blue, changes its color from purplish red to light blue during blooming. The pigment, heavenly blue anthocyanin, was distributed in the vacuoles of upper and lower epidermis and not in parenchyma. We measured pHv of living petal cell directly using proton selective-microelectrode. The pHv of purplish red buds was 6.6 and that of blue open flower was 7.7. This study provides the clear evidence that the color change is caused by the increase of pHv during blooming. The protoplasts were prepared from the pigment cells and then the vacuolar membrane were provided. By immunoblot technique H^+-ATPase and H^+-PPase retained both in the buds and the open flower petals. PPi- and ATP-dependent H^+-transport activities in vacuolar membrane vesicles from the buds and the open flower petals were almost in the same level, although no activities in those from the faded flower petals. HBA produced three isomers by irradiation of UV-B light in the acidic methanol solution, of which the outer two caffeic acids were isomerized to Z-configuration. The inner one was not because it might be strongly stacking to the anthocyanidin nucleus. Anthocyanin is generally stable in strong acidic condition. However, under UV-irradiation, HBA in the acidic medium was not stable, but in neutral condition relatively stable. It is concerning that the intramolecular stacking of HBA between caffeic acid and the chromophore is stronger in the neutral condition than in acidic medium. HBA showed very strong fluorescence in neutral aq. solution, while in acidic very weak. The strong fluorescence may play a role of UV-light quenching and protecting system.
