Agriculture Reference
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Bierhuizen, 1977), provided that temperatures remain < 25°C. Many plant
developmental processes require a certain thermal time or number of heat units
above an appropriate base temperature to complete, and the concept is widely
used in crop technology.
The elongation of radical and shoot after germination in moist conditions
depends similarly on temperature (see below), and a relationship like Eqn 4.7
predicts the emergence of onion seedlings sown in moist soil (Bierhuizen and
Wagenvoort, 1974). They found that, over the range 3-17°C, 219°C days
above a base temperature (T b ) of 1.4°C was required for 50% emergence of
onions. This base temperature was similar to that of many temperate zone
vegetables, but the heat sum ranked fourth highest out of 31 common
vegetable species - only celery, parsley and leek (requiring 222°C days above a
T b of 1.7°C) had higher heat sums, and the former two species are notoriously
slow to emerge because of inhibitors in the seedcoat.
These results show that onion and leek emergence is comparatively slow:
onions will take 2.25 times as long as turnips to reach an equivalent stage of
emergence after sowing in moist soil. From summing up the heat units
accumulated during a time at each temperature, the same number of heat units
were shown to be predictive of a 50% emergence of onions from soil in fluctuating
temperatures in the range 3-21°C (Wagenvoort and Bierhuizen, 1977).
The above analysis ignores the fact that different seeds germinate and
emerge at different rates and that there is a considerable spread in time for
these events. Various proposals have been suggested for quantifying these rates
for different percentiles of the seed population. If, as is often the case (see Fig.
4.8a), percentage germination follows an S-shaped curve, then this often
conforms to a cumulative normal or log-normal distribution (Covell et al. ,
1986). Moreover, it is frequently the case that, although different percentiles
(G) of the population germinate at different rates, they do not have significantly
different base temperatures, T b . They therefore differ only in the thermal time,
(G) values conform to a normal
or log-normal distribution. Such distributions can be characterized by two
constants, K and
(G) above T b for germination to occur, and
is the standard deviation of the frequency distri-
bution of thermal time to germination in the seed population and K is an
intercept constant at thermal time zero; these constants can be estimated using
probit analysis (Covell et al. , 1986). Figure 4.8a shows the response of
cumulative germination of onions cv. 'White Lisbon' to thermal time at
suboptimal temperatures (10-25°C).
As seed ages and the percentage of all viable seeds declines, the mean time
to germination of surviving seeds at near-optimal temperatures increases (see
Fig. 4.9a). The thermal time to 50% germination at suboptimal temperatures
also increases (see Fig. 4.9b).
Germination rate frequently rises to a fairly sharp optimum and then
decreases with temperature (see Fig. 4.7) but, in other cases, there is a broader
optimum range. In one study of onion germination the optimum temperature for
germination was higher for the quick-germinating seeds, i.e. lower percentiles,
, where
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