Agriculture Reference
In-Depth Information
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GAI
Figure 7.3
Theoretical relationship between the canopy size (GAI) and fraction of radiation intercepted
for canopies with different extinction coeffi cients.
tolerance of the crop through careful management of canopy size. However, any benefi t
in terms of greater tolerance must be balanced against the risk of increased disease sever-
ity. The latter may occur if nitrogen fertiliser is used to manage canopy size (Walters &
Bingham, 2007).
The extinction coeffi cient for transmission of PAR through the canopy is infl uenced
by several factors including leaf inclination, leaf shape, leaf size distribution through the
canopy, elevation of the sun and the relative proportions of direct and diffuse radiation
(Kramer
et al.
, 1980; Campbell, 1986). Crop species and varieties with planophile leaf
habits tend to have a larger extinction coeffi cient than those with erect leaves and a higher
proportion of the incident radiation is intercepted with a small canopy size (Figure 7.2)
(Angus
et al.
, 1972). As canopies with planophile leaves intercept a larger fraction of the
incident radiation in the upper canopy (Angus
et al.
, 1972) we would expect them to be
more tolerant of disease in the lower leaf layers than those with more erect leaves. At pres-
ent there is relatively little experimental evidence to support this hypothesis. However,
results from experiments on spring barley genotypes differing in canopy structure suggest
that tolerance of Rhynchosporium leaf blotch is indeed greater in those with planophile
habits (Bingham
et al.
, unpublished results).
7.5.2
Photosynthetic activity and compensatory adjustments
The impact of fungal pathogens on net photosynthetic activity within an infected leaf
depends on the location within the leaf, as well as the host species and race of pathogen.
Some studies based on measurements of gas exchange have suggested that an increase
in the rate of photosynthesis can occur in symptom-less areas of infected leaves (Last,
1963; Habeshaw, 1984), but more commonly a decrease has been reported (Martin, 1986;
Bastiaans, 1991). Chlorophyll-fl uorescence imaging, which provides a greater spatial
