Agriculture Reference
In-Depth Information
The base temperature is 3.5°C,
9134 is the K value and 0.1844 the
value (see text above for explanation).
Probits are taken from tabulated values with probit 50% taken as 0 rather
5 as is sometimes done.
At supra-optimal temperatures the equation is:
1/t(G) = ([{4.55
probit(G)}7.58]
T)/19.75
(Eqn 4.9)
T and t(G) have the same meaning as in Eqn 4.8, 4.55 is a fitted constant,
19.75°C day is the thermal time needed for germination at supra-optimal
temperatures and 7.58 is the standard deviation of ceiling temperature for the
seeds. The term '{4.55
probit(G)}7.58' is the ceiling temperature for
germination of the Gth percentile of the seed population.
The response of onion germination rates to temperature over middling
percentiles (within the range for which the data was fitted and where probit
analyses is most accurate) predicted by Eqns 4.8 and 4.9 is shown in Fig. 4.10. The
graphs show that the optimum temperature for germination is higher for the
lower percentiles, i.e. the fastest germinators, and that these seeds also have a
higher ceiling temperature for germination, T
c
, than the lower percentiles. The
graphs also indicate a decreasing percentage of seeds germinating as temperature
increases above about 25°C, as in Fig. 4.7, as the ceiling temperatures, T
c
, for
progressively lower percentiles (i.e. slower-germinating seeds) are exceeded.
Fig. 4.10.
The response of germination rate to temperature in three cohorts
(percentiles) of onion seed cv. 'White Lisbon' as predicted by Eqns 4.8 and 4.9.
