Agriculture Reference
In-Depth Information
of post-anthesis loss of leaf area and photosynthetic activity (Foulkes
et al.
, 2002; Ehdaie
et al.
, 2006). However, there is no direct evidence to indicate that selection for large reserve
deposition will improve tolerance of disease. Tolerance of septoria leaf blotch was neg-
atively correlated with the amount of soluble carbohydrate reserves in wheat (Foulkes
et al.
, 2006). As yet the reason for the negative correlation has not been established. Barley
genotypes also differ in their concentration of stem WSC (Gay
et al.
, 1999). In spring
barley, disease reduced the amount of storage reserves deposited, but increased the propor-
tion subsequently used for grain fi lling (Gaunt & Wright, 1992). However, contrary to the
hypothesis that crops with large storage reserves and low yield potential (small grain num-
ber) would be more tolerant of disease (Gaunt & Wright, 1992), no relationship was found
between yield potential and yield loss caused by leaf rust (Whelan
et al.
, 1997).
7.6
Is there a physiological or ecological cost
to tolerance?
Several reports have highlighted a negative correlation between the tolerance of a genotype
and its yield potential in the absence of disease (Kramer
et al.
, 1980; Lim & Gaunt, 1986;
Parker
et al.
, 2004), which suggests that there may be a physiological cost associated with
tolerance. If tolerance is to be exploited as part of a disease management programme it
will be important to identify possible tolerance traits that are not negatively associated
with yield potential.
Although crop canopies with planophile leaf habits and high extinction coeffi cients may
be more tolerant of disease in the lower canopy, intrinsically, they have a lower RUE under
conditions of high solar radiation in the absence of disease (Angus
et al.
, 1972). This is
because photosynthesis of the upper leaves is light saturated, whilst the lower leaves are
shaded. In canopies with more erect upper leaves, there is a greater transmission of radiation
to the lower canopy, but at the same time high rates of photosynthesis can be maintained
by the upper leaves with a smaller degree of light saturation. The same arguments may also
apply to a constitutive increase in size of the fl ag leaf of cereals accounting for the negative
correlation observed between fl ag leaf size and yield of healthy wheat genotypes (Foulkes
et al.
, 2007). An ability to make compensatory adjustments in leaf growth in response to
disease would not be expected to be deleterious to the yield of healthy crops. However, as
discussed above, if this is associated with a reduction in partitioning of biomass to the root
system it could make the diseased crop more susceptible to drought.
Upregulation of photosynthesis in response to defoliation or pathogen infection is often
interpreted in terms of the relief of sink limitation of photosynthesis (Williams & Ayres,
1981; Zuckerman
et al.
, 1997). If this mechanism is correct, we would expect tolerance to
be greatest in genotypes with the greatest sink limitation. Such sink limitation, however,
represents a reduction in yield potential in the absence of disease. Indeed, increasing
grain numbers and grain size of cereals has been suggested as a possible breeding target
to reduce sink limitation of photosynthesis, thereby increasing RUE and yield (Reynolds
et al
., 2007). This could lead to further reductions in tolerance of pathogens. One trait
that may confer tolerance without impairing yield in the absence of disease is delayed
disease-induced leaf senescence (Sabri
et al.
, 1997). Until the mechanism of the delayed
senescence is understood we cannot determine what its metabolic cost is. However, there
is no reason to suspect that it involves a cost in the absence of disease.
