Synchrony between flower opening and petal-color change from red to blue in morning glory, Ipomoea tricolor cv. Heavenly Blue

Petal color change in morning glory Ipomoea tricolor cv. Heavenly Blue, from red to blue, during the flower-opening period is due to an unusual increase in vacuolar pH (pHv) from 6.6 to 7.7 in colored epidermal cells. We clarified that this pHv increase is involved in tonoplast-localized Na(+)/H(+) exchanger (NHX). However, the mechanism of pHv increase and the physiological role of NHX1 in petal cells have remained obscure. In this study, synchrony of petal-color change from red to blue, pHv increase, K(+) accumulation, and cell expansion growth during flower-opening period were examined with special reference to ItNHX1. We concluded that ItNHX1 exchanges K(+), but not Na(+), with H(+) to accumulate an ionic osmoticum in the vacuole, which is then followed by cell expansion growth. This function may lead to full opening of petals with a characteristic blue color.