Agriculture Reference
In-Depth Information
Fig. 4.7.
The relationship between temperature and the rate and percentage
germination of onion seeds on moist paper. Rates were calculated as reciprocals of
the number of days for 50% of the viable seeds to germinate (from Harrington, 1962).
In Fig. 4.7 the rate of germination in the range 5-25°C is approximated by
a straight line equation:
Germination rate = 1/t = b(T
T
b
)
(Eqn 4.6)
where
t = time to 50% germination, days
T = temperature (5 < T < 25°C)
T
b
= base temperature, where the fitted line projects to zero rate - for these
data 2.5°C
The 'thermal time' or 'heat sum' required for 50% germination,
, is given by:
= (T
T
b
)t
(Eqn 4.7)
The 'thermal time' or 'heat sum' required for 50% germination combines
the effect of time and temperature on germination and quantifies the fact that a
warm day is more effective than a cool day in progressing germination. In the
case of Fig. 4.7, 80°C days above a base temperature of 2.5°C are required for
50% germination. The units °C days are variously know as 'day degrees',
'degree days', 'thermal time units' or 'heat units' above 2.5°C. For example,
80°C days could be achieved by 4 days at 22.5°C or 16 days at 7.5°C. In
fluctuating temperatures time of emergence can be estimated by summing the
heat units accumulated during the time at each temperature (Wagenvoort and
