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

The effect of cultivar and temperature on the temporal pattern of starch oc currence in the primary thickening meristem (PTM) of onion (Allium cepa L. var. cepa) bulbs was studied. Bulbs of four cultivars differing in their sp routing behaviour in dry storage were analysed during bulb development for the concentration of starch in the PTM and inner sprout growth by histochem ical 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 bef ore or concomitant with inner sprouting. The duration of the low starch per iod varied between cultivars. If onions were stored at low (1 degrees C), i ntermediate (15 degrees C), or high (30 degrees C) temperature, inner sprou ting was delayed at low and high temperature compared with the intermediate temperature, whereas starch increased before sprouting at low and intermed iate temperature, but became undetectable at high temperature. Moreover, if benzyladenine was injected into onions stored at high temperature, sprouti ng was readily induced, but starch in the PTM remained undetectable and no root initiation occurred. Since the PTM is not only responsible for the ste m thickening but also for root initiation we conclude (i) that law starch i n the PTM primarily indicates root dormancy and only indirectly sprout dorm ancy, and (ii) that high temperature-imposed sprout dormancy is actually ba sed on root dormancy.