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(T6) (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’ (Tb) 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 cm2. Using this,
the leaf area of an onion plant that has been growing for a while following emergence can be predicted by the equation:
loge(leaf area) = loge(0.5) + 0.0108 DD (Eqn 4.17a) and seedling dry weight is predicted by:
loge(Dry Weight) = loge(W0) + 0.0111 DD (Eqn 4.17b) DD is the summation of day-degrees between 6 and 20°C accumulated since emergence and W0is the dry weight at seedling emergence. Eqn 4.17b with loge(Wo) equal to –6.086, indicating a dry weight at emergence of 2.27 mg, applied to field growth of onion seedlings in central UK (Brewster et al., 1977).
A model for growth in seedling shoot dry weight (W) in g, which includes the effects on growth rates of a daily income of photosynthetically active radiation, PAR (R), as well as daily mean temperature (t) – again in terms of accumulated day-degrees above a base temperature (tb) – was developed by Table 4.5. The relative growth rate (RGR) of seedlings of various species growing exponentially in near-optimal temperatures with abundant water and mineral nutrients, compared with that of onion, cv. ‘Hyton’ or cv. ‘Hygro’. The data come from a number of experiments which varied in the light environment used. To account for this, the absolute values of RGR recorded were ‘scaled’ relative to a value of unity attributed to onion in each experiment (from Grime and Hunt, 1975;
Brewster, 1979; Brewster and Sutherland, 1993).
Species RGR relative to onion
Stinging nettlea(Urtica dioica) 2.50
Spring cabbage (Brassica oleracea var. 1.96 capitata cv. ‘Hornet’)
Lettuce (Lactuca sativa cv. ‘Penlake’) 1.91
Brussels sprout (Brassica oleracea var. gemmifera) 1.69 Cauliflower (Brassica oleracea var. botrytis cv. ‘White Rock’) 1.56
Perennial ryegrassa(Lolium perenne) 1.49
Carrot (Daucus carota cv. ‘Nanco’) 1.41
Celery (Apium graveolens cv. ‘Lathom Self-blanching’) 1.37
Red beet (Beta vulgaris cv. ‘Monopoly’) 1.34
Onion (Allium cepa cv. ‘Hyton’ or cv. ‘Hygro’) 1.00 Leek (Allium ampeloprasum cv. ‘Winterreuzen’) 0.89 Leek (Allium ampeloprasum cv. ‘Early Market’) 0.83 Leek (Allium ampeloprasum cv. ‘Winter Crop’) 0.80 Japanese bunching onion (Allium fistulosum cv. 0.85
‘Common Bunching Multi-stalk’)
Chive (Allium schoenoprasum) 0.73
Sitka sprucea(Picea sitchensis) 0.25
aValues for these species were estimated indirectly; see Brewster (1979, p.356).
Scaife et al. (1987). Here, ‘Effective Day Degrees’ (EDD) are used instead of day-degrees (DD) in Eqn 4.17b:
loge(Dry Weight) = loge(W0) + p EDD (Eqn 4.18) where p is a parameter that expresses relative growth per EDD and has units per EDD.
An ‘Effective Day Degree’ is a day-degree adjusted or weighted for the daily total PAR impinging on the plants (termed the ‘radiant exposure to PAR’).
EDDs are calculated according the equation:
EDD–1= DD–1+ f R–1 (Eqn 4.19a)
this can be also expressed in the equivalent form:
EDD = DD/(1 + (f DD)/R) (Eqn 4.19b)
R is the daily radiant exposure to photosyntetically active radiation, usually expressed as MJ/m2
f is a parameter that determines sensitivity to PAR levels and has units MJ/m2/DD.
Parameters p and f are characteristic of a species. From experiments under controlled PARs and temperatures, the value for onion of p was 0.0160/EDD, f Fig. 4.22. (a) The effect of temperature on the Relative Growth Rate (RGR) of whole plant dry weight (solid symbols) and on the Relative Leaf Growth Rate (RLGR) (open symbols) of cv. ‘Hygro’ during early exponential growth (from Brewster, 1979). RGR is the rate of increase in dry weight per unit of existing dry weight (RGR = 1/W.dW/dt, where W = dry weight and t = time). Similarly, RLGR is the rate of increase of leaf area per unit of existing leaf area. (b) The effect of temperature on the rate of initiation of leaves by the main shoot apex (i.e. not counting leaves on side shoots) of cvs ‘Hygro’, ‘Hyton’ and ‘Rijnsburger’, all ‘Rijnsburger’ types, growing in controlled environments (unpublished data).
was 0.136 MJ/m2/DD and Tb, the base temperature for growth and calculating DDs, was 5.9°C (Brewster and Sutherland, 1993). An appropriate value for logeW0, the log of the shoot dry weight (g) at emergence, is 6.086. The value of f was larger than for other species, indicating greater sensitivity to light income, probably because the leaves are erect and do not intercept a high proportion of PAR in the early stages of growth. The difficulty of growing onions satisfactorily in conditions of low light is a familiar problem to experimenters, who have to raise them in glasshouses during the winter. The value of p was one-half to two-thirds that for faster-growing vegetable seedlings like cabbage and lettuce, indicating an inherently low relative growth rate for onion.