Agriculture Reference
In-Depth Information
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 nettle a ( 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 ryegrass a ( 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 (A llium ampeloprasum cv. 'Winter Crop')
0.80
Japanese bunching onion ( Allium fistulosum cv.
0.85
'Common Bunching Multi-stalk')
Chive ( Allium schoenoprasum )
0.73
Sitka spruce a ( Picea sitchensis )
0.25
a Values for these species were estimated indirectly; see Brewster (1979, p.356).
the leaf area of an onion plant that has been growing for a while following
emergence can be predicted by the equation:
log e (leaf area) = log e (0.5) + 0.0108
DD
(Eqn 4.17a)
and seedling dry weight is predicted by:
log e (Dry Weight) = log e (W 0 ) + 0.0111
DD
(Eqn 4.17b)
DD is the summation of day-degrees between 6 and 20°C accumulated
since emergence and W 0 is the dry weight at seedling emergence. Eqn 4.17b with
log e (W o ) 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 (t b ) - was developed by
 
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