Agriculture Reference
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and Ellis, 1991). After hook formation, cotyledon extension growth is
exponential and the relative rate of extension growth is linearly related to
temperature above the same base temperature (see Fig. 4.15b). The primary
root also elongates exponentially, and the relative root extension rate is
linearly related to temperature above a base of 3.1°C (see Fig. 4.15c; Wheeler
and Ellis, 1994).
These relationships applied over the temperature range 5-20°C, and
therefore to emergence in spring-sown crops in temperate regions. They were
derived from seedlings with cotyledons up to 25 mm long and roots up to 30 mm
Fig. 4.15.
The effects of temperature on seedling elongation rates after germination.
(a) The relationship between temperature and cotyledon elongation before hook
formation for newly germinated onion seedlings cv. 'White Lisbon' (from Wheeler
and Ellis, 1991). (b) The relationship between temperature and the relative rate of
elongation after hook formation for the same seedlings as in (a). (c) Relationships
between the relative rate of root elongation and temperature for the 20th, 40th,
60th and 80th germination percentiles from a seedlot of onion cv. 'White Lisbon'.
The solid lines are linear regression of best fit for all percentiles from three different
seedlots; the broken lines are the individual linear regressions for each data set
(from Wheeler and Ellis, 1994. Courtesy of
Seed Science and Technology
).
