Bulb dormancy in onion. II. The role of cytokinins in high-temperature imposed sprout inhibition

Abstract

SummarySprouting of onion bulbs is inhibited at high storage temperatures. To investigate the role of cytokinins in this phenomenon small bulbs of the ‘Rijnsburger’ cv. Augusta were stored from four weeks after harvest at 5, 15 and 30°C for a period of 18 weeks. Bulb samples were investigated for time to rooting and subsequent sprouting in moist vermiculite at 15°C and assayed for endogenous cytokinins at six-week intervals. Root dormancy gradually decreased during storage, the fastest at exposure to 30°C; rooting was followed by sprouting. After 18 weeks of temperature exposure, no visible sprouting had occurred in dry storage; internal sprouts had relative lengths of 0.29, 0.72 and 0.31 at 5, 15 and 30°C, respectively. Cytokinin activity was low during the first six weeks of temperature exposure. At 5 and 15°C a sudden rise in cytokinin level was found after 12 weeks and a further increase after 18 weeks. At 30°C, however, cytokinin levels increased very little during the storage period. Injection of plant growth regulators into large bulbs of the ‘Rijnsburger’ cv. Hyton that had been stored for 28 weeks at 25°C showed that benzyladenine strongly promoted sprouting. The data indicate that inhibition of sprouting at high temperature is caused by low levels of endogenous cytokinins.