Agriculture Reference
In-Depth Information
was 0.136 MJ/m
2
/DD and T
b
, the base temperature for growth and calculating
DDs, was 5.9°C (Brewster and Sutherland, 1993). An appropriate value for
log
e
W
0
, 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.
Carbon dioxide concentration and growth and
photosynthetic rates
Elevated CO
2
concentrations increase growth in dry weight and leaf area in
onions (Daymond
et al.
, 1997; Wurr
et al.
, 1998; Wheeler
et al.
, 2004; Fig. 4.23a,
b). At the stage of bulb initiation, the light-saturated rate of photosynthesis in the
youngest fully expanded leaf was greater in onions grown at 560 rather than at
350
mol/mol CO
2
, particularly in plants grown in warmer temperatures (see
Fig. 4.24). A higher photosynthetic rate was probably the basis of the faster
growth at elevated CO
2
. During darkness, CO
2
concentrations of 0.6% have been
measured in the air within the hollow cavity of onion leaves, about eight times
the level in ambient air, presumably as a result of the retention of respiratory CO
2
(Darbyshire
et al.
, 1979). This might conserve respired CO
2
and 'recycle' it in
photosynthesis with the onset of light.
Water relations of growth and photosynthesis
Leaf relative growth rates and stomatal conductance decline as leaf water
potential and the associated turgor pressure decline (see Fig. 4.25). The
maximum turgor pressure developed within onion leaves was found to be 0.4
MPa, rather low compared with other crop species, where a maximum turgor
of 1 MPa is common. Stomatal conductance decreases rapidly when turgor
declines from 0.15 to 0.05 MPa, the rate of decline being three times faster
than in
Phaseolus
beans over the same range of turgor (Millar
et al.
, 1971).
Therefore, both transpiration and photosynthesis will drop rapidly with such
decreases in turgor. When the roots of onion plants are exposed to saline
solutions, the plants compensate for the increased osmotic potential in the root
medium by increasing the leaf osmotic potential by only about half as much.
Hence, leaf turgor is reduced in such a saline solution. In contrast, bean and
