Agriculture Reference
In-Depth Information
stage (three-leaf stage) resulted in a signifi cant increase in rate of photosynthesis of
non-inoculated leaves, but no increase was found when plants were inoculated at a later
growth stage (six-leaf stage) (Rooney & Hoad, 1989). It is tempting to speculate that the
lack of response at the later growth stage resulted from a smaller sink limitation of photo-
synthesis due to the presence of the developing ear so that photosynthesis was operating
at a rate close to its full capacity.
An increase in the photosynthetic rate of healthy leaves following defoliation or infec-
tion elsewhere is often interpreted as compensating for the reduction in photosynthetic
activity of the damaged leaves. If this is the case, those genotypes that are better able to
increase their rate would be expected to display a greater tolerance of disease or herbivory
in the fi eld. However, the increase in rate may not be 'compensatory' as such and thus
may not confer tolerance. For example, it could be associated with an enhanced biosyn-
thesis of defence compounds induced by pathogen infection or wounding rather than
production of biomass that is utilised in the formation of yield (Tiffi n, 2000). It would
also be expected to be a resource-dependent response, modifi ed by nutrient availability
or other environmental variables and few studies have attempted to distinguish between
these possibilities.
7.5.3
Compensatory adjustments in growth
Morphological adjustments following herbivory or pathogen infection may help
ameliorate the effects of leaf damage on net assimilation (Prins & Vekaar, 1992; Tiffi n,
2000). A reduction in the proportion of biomass allocated to the root system and an
increase in leaf area ratio (ratio of leaf area to total plant biomass) is a common response
to defoliation and serves to re-establish the photosynthetic surface as rapidly as possible
(Brouwer, 1983; Prins & Verkaar, 1992). An increase in shoot:root biomass ratio has been
observed in several species following infection with foliar pathogens (Walters & Ayres,
1981; Rooney & Hoad, 1989; Farrar, 1992). Adjustments in shoot morphology made in
response to foliar pathogens have been reported less frequently, but increases in leaf area
ratio have been found in some pathosystems (Paul & Ayres, 1986).
Those genotypes that show the greatest morphological plasticity may be able to
maintain high levels of reproductive output following defoliation and hence display
the greatest tolerance. For example, the grazing tolerant grass
Agropyron desertorum
reduced its root elongation following defoliation, whilst the grazing sensitive
A. spicatum
did not (Richards, 1984). Maintaining canopy expansion at the expense of root growth
could potentially predispose the crop to water and nutrient limitations later in the sea-
son (Walters & Ayres, 1981). However, soil conditions can also modify root morphology
(Bingham, 2001) and negate some of the potentially deleterious effects of infection. As
the soil profi le dries from the surface downwards root growth may be enhanced in moist
soil layers. Powdery mildew infection of barley increased the shoot:root ratio of plants in
moist soil, but not those in dry soil (Ayres, 1981b).
The timing of disease epidemics will have a signifi cant infl uence on the potential for
morphological adjustments to contribute to tolerance. In some pathosystems (e.g. barley-
Ramularia collo-cygni
) epidemics tend to occur late in the season at a time when canopy
growth is at or nearing completion. Thus, there is little opportunity for compensatory
adjustments to be made in leaf or tiller growth. By contrast, epidemics of Rhynchosporium
