Cultivar and temperature effects on starch in the primary thickening meristem of onion bulbs

SummaryThe effect of cultivar and temperature on the temporal pattern of starch occurrence in the primary thickening meristem (PTM) of onion (Allium cepa L. var. cepa) bulbs was studied. Bulbs of four cultivars differing in their sprouting behaviour in dry storage were analysed during bulb development for the concentration of starch in the PTM and inner sprout growth by histochemical analysis and determination of leaf ratio, respectively. Generally, in all cultivars the concentration of starch in the PTM was high during active growth, decreased to a low value during sprout dormancy, and increased before or concomitant with inner sprouting. The duration of the low starch period varied between cultivars. If onions were stored at low (18C), intermediate (158C), or high (308C) temperature, inner sprouting was delayed at low and high temperature compared with the intermediate temperature, whereas starch increased before sprouting at low and intermediate temperature, but became undetectable at high temperature. Moreover, if benzyladenine was injected into onions stored at high temperature, sprouting was readily induced, but starch in the PTM remained undetectable and no root initiation occurred. Since the PTM is not only responsible for the stem thickening but also for root initiation we conclude (i) that low starch in the PTM primarily indicates root dormancy and only indirectly sprout dormancy, and (ii) that high temperature-imposed sprout dormancy is actually based on root dormancy.