Changes in levels of cytokinins in etiolated squash [Cucurbita maxima] seedlings after illumination

When etiolated seedlings of squash (Cucurbita maxima Duch. cv. Houkou-aokawaamaguri) were exposed to light, their cotyledons began to accumulate chlorophyll at a low rate for the first hour but at a high rate therafter. After 0, 1 and 2 h of illumination, cytokinin levels in cotyledons, hypocotyls and roots were analyzed by ELISA (enzyme-linked immunosorbent assay) and GC-SIM (gas chromatography-selected ion monitoring). Cytokinin levels measured by ELISA were about 10 times higher than those measured by GC-SIM, suggesting that ELISA was not suitable for the measurements of cytokinin levels in the extracts from these tissues. Cytokinins found by GC-SIM were t-RZ (t-ribosyl zeatin), c-RZ, t-Z (t-zeatin), c-Z, RiP (ribosyl isopentenyladenine) and iP (isopoentenyladenine). Levels of biologically active cytokinins (t-RZ and t-Z) did not show marked changes after illumination. Levels of c-RZ and c-Z did, however, decrease in cotyledons and increase in hypocotyls after illumination. Promotion of the accumulation of the chlorophyll in detached squash cotyledons by exogenously applied t-Z was partially inhibited by the addition of c-Z, suggesting that the decrease in the endogenous level of c-Z in cotyledons of intact seedlings after illumination permits the light-induced formation of chlorophyll.