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.