Changes in cytokinin levels of Phalaenopsis leaves at high temperature

The effect of high temperatures on cytokinin levels in Phalaenopsis hybrida leaves was investigated. Endogenous cytokinins were identified and quantified in Phalaenopsis leaves grown under high temperature conditions (30/25 °C day/night) using high performance liquid chromatography, bioassay and gas chromatography-selected ion monitoring-mass spectrometry. After 5 and 20 d of low temperature (25/20 °C day/night), zeatin, zeatin riboside and dihydrozeatin levels in the leaves were higher than that in leaves subjected to high temperature treatments. When Phalaenopsis leaves were exposed to low temperatures, about 76 % of the free cytokinins detected were of the zeatin-type. Glucoside cytokinins in the leaves increased significantly 5 d following high temperatures, and the rate of increase in glucoside cytokinins corresponded to the duration of high temperatures. At the same time, zeatin riboside and dihydrozeatin declined significantly following high temperature application. A significant accumulation of glucoside cytokinins, zeatin-9-glucoside, zeatin-O-glucoside, zeatin riboside-O-glucoside, and dihydrozeatin-O-glucoside was observed 20 d following high temperatures. These results suggest that high temperatures lead to an accumulation of glucoside cytokinins and a reduction of free base and riboside cytokinins.