Agriculture Reference
In-Depth Information
Assessments of fungal biomass or DNA also do not measure pathogen challenge. For that,
gene expression would have to be measured, particularly that of defence genes, and would
require better knowledge than is currently available as to the costs and trade-offs associ-
ated with such gene induction (Walters & Heil, 2007). However, we do know that under
fi eld conditions there may be a general high-level expression of defence genes compared
with glasshouse-grown plants (Pasquer
et al.
, 2005) which will have an energetic and
therefore possible yield cost to the plant (Smedegaard-Petersen & Tolstrup, 1985). The
ability of the plant to recognise potential pathogens and prime rather than fully express
defence pathways would reduce the metabolic cost and may be important for maximising
tolerance.
7.5
Based on theoretical considerations of radiation interception, RUE and partitioning,
several plant and crop traits can be identifi ed that might infl uence the tolerance of pathogen
infections. However, for many of these traits, experimental evidence supporting their role
in tolerance is insubstantial or absent. There are some parallels between the physiologi-
cal responses of plants to leaf loss caused by herbivory and that resulting from pathogen
infection, although plant-pathogen interactions are generally more complex (Ayres,
1992; Gaunt, 1995). As such, the more extensive literature on tolerance of herbivory
can, if interpreted with care, provide useful insights to guide investigations on pathogen
tolerance. Historically, however, studies of both herbivory and pathogen tolerance have
suffered from the same short-coming; relatively few have measured tolerance and expres-
sion of candidate mechanisms in the same experiment (Tiffi n, 2000).
Potential crop traits conferring tolerance
7.5.1
Canopy size and structure
The fraction of incident PAR that is intercepted by the crop depends on the size and structure
of its canopy. It can be estimated from the canopy size (GAI, green area index) and the extinc-
tion coeffi cient for PAR transmission (
k
) according to an analogy of Beer's law (Bingham
et al.
, 2007). Large canopies will intercept a larger proportion of the incident radiation
than small ones, but because the relationship between size and interception is non-linear,
each increment of GAI results in a progressively smaller increase in radiation interception
(Figure 7.3). The interaction between canopy size and impact of disease-induced 'loss'
of GAI on radiation interception will be highly dependent on the vertical distribution of
disease (Bastiaans & Kropff, 1993; Bancal
et al.
, 2007). If disease is located in the lower
leaf layers the loss of green area will have a negligible effect on total radiation intercep-
tion by healthy tissue in a large canopy, but a signifi cant effect in a small canopy. This
is because in the former, relatively little of the radiation penetrates to the lower leaves.
However, if disease is distributed more evenly throughout the canopy, including the upper
leaf layers, a reduction in green area through disease will reduce radiation interception by
healthy tissue in both large and small canopies. Disease tends to be located in the lower
leaf layers in tall crops infected with splash-dispersed pathogens. It also occurs during
canopy expansion in pathosystems (e.g. powdery mildew of cereals) where successive
leaves emerge more rapidly than pathogens can infect and lead to symptom develop-
ment on the emerged leaves. In these cases there may be some scope for increasing the
