Agriculture Reference
In-Depth Information
The equation is:
ln w = ln mx + ln ln[kL
0
+ exp(-rx)]
(Eqn 4.22)
where the symbols and the values derived from fitting the model and the
units are:
w = crop dry weight, g/m
2
x = EDD from the first sampling; the base temperature for EDD was 5.9°C
and the light response term f, was 0.136 MJ/m
2
/°C day (Brewster and
Sutherland, 1993)
m = maximum crop growth rate = 2.92 (g/m
2
/EDD)
k = light extinction coefficient = 0.43
L
0
= initial leaf area index (a measured input)
r = initial relative growth rate = 0.0200 (per EDD)
This equation provides a simple description of onion crop growth rate,
which varies according to changing temperature and light income. The
equation has general validity since it is based on crop physiological theories.
Modification to incorporate some 'optimum temperature' above which EDDs
no longer increased as temperature increased would probably be needed to
apply this approach in warm climates. The model provides no information
about development, just the dry weight growth of a well-adapted cultivar.
A comprehensive mechanistic simulation model of bulb onion crop growth
and development, called 'ALCEPAS', has been developed by de Visser (1994a, b).
This utilizes the general crop growth simulation model SUCROS87 (Spitters
et
al.
, 1989) and combines it with a model for onion development rate from
seedling, through bulbing to ripening and senescence. Figure 4.34 shows the
relational diagram of ALCEPAS; the continuous lines represent flows of dry
matter entering the various material fractions of the crop, e.g. root or bulb +
neck, represented by oblong boxes, and the broken lines represent flows of
information that influence the rate processes in the 'valve' symbol boxes. The
rate processes are determined by equations, the numerical inputs to which
depend on the values of the variables shown as information flows (broken lines).
The Developmental Stage (DVS) between emergence and bulbing was
predicted using equations based on the relationships illustrated in Fig. 4.30
and Equation 4.21, modified by a factor to account for the R:FR effect that
depends on LAI, as discussed in a previous section. DVS from bulbing to
ripening was simply a function of thermal time with 6°C base temperature. An
equation of general applicability to crops, based on the proportion of protein
and mineral ash in a tissue, was used to predict maintenance respiration rate.
A similar equation based on the proportion of carbon and ash in a tissue was
used to predict growth respiration. Material from field experiments was
analysed to determine ash, carbon and protein contents of roots, shoots and
bulbs and how they changed with developmental stage.
Field experiments provided data for: (i) equations for specific leaf area
(SLA), and how it varied with developmental stage and plant density; (ii)
