PHYSIOLOGY OF CROP GROWTH, DEVELOPMENT AND YIELD - Onions and other Vegetable Alliums - page 116

Newly germinated seedlings subjected to water potentials sufficient to
prevent root or shoot elongation were able to fully maintain their elongation
potential for 35 days at 20°C (Whalley et al. , 2001). This contrasts with
seedlings prevented from elongating by mechanical impedance, which lose
most of their capacity to elongate over 35 days (see Fig. 4.20).

VEGETATIVE GROWTH

Seedling growth rates

As with germination and emergence, the growth rate of seedling alliums after
emergence is slow compared with most crop species. However, many non-crop
species are slower growing, particularly woody species and those adapted to
non-productive environments (Brewster, 1979). Table 4.5 compares the
Relative Growth Rate (RGR) of species growing in near-optimal temperatures
during the exponential, seedling phase of growth with that of onion. From Table
4.5 it is clear that onion is the fastest growing of the edible alliums studied, but
that the RGR of onion is only about half that of spring cabbage or lettuce. This
means that, starting at the same weight and growing under the same con-
ditions, onion will take nearly twice as long as spring cabbage or lettuce to reach
a given weight.
Growth rates are strongly dependent on temperature. Figure 4.22 shows
the relationships between RGR, Relative Leaf Growth Rate (RLGR) and leaf
initiation rates and temperature. RGR and RLGR increase linearly over the
range 6-20°C, whereas leaf initiation rates have a lower base temperature and
increased linearly over the range 3.5-23°C.
Hence, growth rates are given by the simple equations:

RGR = 0.0111(T 6) (Eqn 4.16a)

RLGR = 0.0108(T-6) (Eqn 4.16b)

Here, RGR and RLGR are in per day, T is temperature in °C and 6°C repre-
sents the 'base temperature' (T b ) below which growth ceases. If mean tempera-
tures do not exceed the optimal range, i.e. not above about 27°C (see Fig. 4.22),
this relationship implies that growth can be predicted using 'thermal time' as
described for predictions of seed germination and emergence (Eqns 4.6 and 4.7).
Thus, seedling dry weight and leaf area will be linearly related to accumulated
day-degrees between 6 and 20°C. This accords with field studies in which log
seedling dry weight was a linear function of accumulated day degrees between
6 and 20°C, whereas leaves initiated increased linearly with day-degrees above
2°C (Brewster et al. , 1977). A straight onion cotyledon, at the end of the
emergence phase, would typically have a leaf area of about 0.5 cm 2 . Using this,