Agriculture Reference
In-Depth Information
resolution than can be achieved with gas exchange techniques, has confi rmed that in some
pathosystems photosynthesis can be impaired in green tissue beyond the region of the
lesion and before a loss of chlorophyll occurs (Scholes & Rolfe, 1995). Bastiaans (1991)
developed the concept of the virtual lesion to account for the relationship between visible
lesion size and the photosynthetic rate of infected leaves. The virtual lesion is the area of
the leaf within which the photosynthetic rate is negligible. A ratio of virtual lesion area to
visible lesion area (
in Bastiaans' model) greater than 1.0 implies that photosynthesis is
inhibited outwith the visible lesion. Values of
β
ranging from 1.3 to 11 have been reported
for rust and mildew infected cereals (Rossing
et al.
, 1992; Robert
et al.
, 2005, 2006), but
the value can vary depending on the stage of lesion development and the nature of the
symptom assessed. Values were greatest when only the sporulating area of rust-infected
wheat was included in the visible lesion area and least when sporulating, necrotic and
chlorotic tissue was included (Robert
et al.
, 2005, 2006). The virtual lesion concept has
been widely used to model the effects of disease on canopy photosynthesis (Bastiaans &
Kropff, 1993; Robert
et al.
, 2004).
Genotypic variation in the extent to which photosynthesis and respiration are modifi ed
by infection in symptom-less areas of leaves will lead to apparent variation in disease
tolerance if tolerance is expressed as AUDPC, HAD, or healthy area PAR interception
and yield. Some inter-specifi c variation has been found in cereals. Powdery mildew-in-
fected leaves of a wild oat genotype showed a smaller reduction in net photosynthetic rate
than two cultivated oat genotypes, even though the severity of infection was greater in the
wild oat (Sabri
et al.
, 1997). The wild oat leaves also showed a slower rate of disease-
associated senescence. Similar, but less striking effects have been found in a comparison
of wild and cultivated barley genotypes (Akhkha
et al.
, 2003). However, to date, the effect
of this variation on tolerance at the scale of the crop canopy has not been determined. In a
comparison of winter wheat genotypes infected with rust (
Puccinia recondita
Rob. f. sp.
tritici
), no difference was found in the rate of photosynthesis expressed per unit of remain-
ing green area, but genotypes did differ in the rate of rust-associated leaf senescence
(Spitters
et al.
, 1990). There is also some limited evidence of intra-specifi c variation in
the response of spring wheat leaves to infection by septoria leaf blotch. Zuckerman
et al.
(1997) observed a 3.5-fold increase in the rate of photosynthesis in the remaining green
tissue of the upper three leaves of the variety Miriam compared to Barkai and this was
associated with a smaller reduction in average grain weight in Miriam.
Increased photosynthetic activity of non-infected leaves in response to infection else-
where on the plant has been observed in several pathosystems (Ayres, 1981a; Williams &
Ayres, 1981; Roberts & Walters, 1986; Rooney & Hoad, 1989; Murray & Walters, 1992).
The scale of response reported differs between studies, and is often greater for dicoty-
ledonous plant pathosystems than monocots. Similar responses have been reported for
plants subjected to herbivory or mechanical defoliation (Prins & Verkaar, 1992; Tiffi n,
2000; Macedo
et al.
, 2006). However, increased photosynthetic activity is not a universal
response to partial defoliation. In some studies the rate of photosynthesis was unaffected
or even temporarily decreased by defoliation (Prins & Verkaar, 1992; Zangerl
et al.
,
1997). It is not clear what mechanisms are underlying these responses and why such
a range of responses has been observed. Some of the variation may be associated with
the growth stage at which treatments are imposed and the relative capacities of source
and sink tissues. Inoculations of wheat with
Septoria nodorum
at the young vegetative
β
