Agriculture Reference
In-Depth Information
there may be potential for selecting genotypes that have a greater capacity for morpho-
logical adjustment. However, the extent of any genotypic variation in this response has
not yet been quantifi ed.
Ramularia collo-cygni
is interesting in another respect, as it is present in the plant
systemically long before it becomes symptomatic, that is as an endophyte (Walters
et al.
2008b). Other endophytes have been found to modify defence reactions and the toler-
ance of some abiotic stress factors. For example,
Piriformospora indica
when it invades
barley does not transition to a pathogenic state, rather it confers greater disease resistance
and salt tolerance and results in higher yield (Waller
et al.
, 2005). It effectively 'primes'
the defence mechanisms with elevated anti-oxidative capacity due to an activation of the
glutathione-ascorbate cycle. It is thought to achieve the mutualism of the endophytic
interaction through interference with host cell death mechanisms (Deshmukh
et al.
, 2006).
Since interactions with non-pathogenic organisms can modify the response to pathogens,
crop tolerance should be defi ned in its functional context with all the relevant community
of interacting organisms in its normal environment. This highlights further some of the
potential diffi culties when applying results derived from glasshouse experiments to the
fi eld situation.
7.5.4
Source-sink relations and storage reserves
The seed or grain yield potential of annual crops is often analysed in terms of source
versus sink limitation (Borrás
et al.
, 2004). It has been hypothesised that crops with a
large potential assimilate supply relative to the storage capacity of grains will be relatively
tolerant of post-fl owering loss of green leaf area to disease (Kramer
et al.
, 1980; Gaunt,
1995). This could arise from the possession of a large canopy size and/or water soluble
carbohydrate (WSC) storage reserve relative to the number of grains. The relative balance
between source and sink during the post-fl owering period appears to differ between crop
species and production regions. On reviewing a large number of experiments on wheat,
soybean and maize grown in high light environments, Borrás
et al.
(2004) concluded
that in most cases yield was limited by the potential grain size and not by the supply of
assimilate for grain fi lling. By contrast, under the relatively dull conditions of the United
Kingdom, post-fl owering source and sink capacities of wheat seem to be in close balance
(Beed
et al.
, 2007). The effects of foliar disease on grain sink capacity are relatively small,
whilst those on post-anthesis assimilation are large and thus for UK crops disease yield-
loss relationships can be explained well in terms of post-anthesis assimilate availability
(Paveley
et al.
, 2001). The tolerance of these crops might therefore be improved by selec-
tion of traits that enhance post-anthesis radiation interception, RUE, or the utilisation of
storage reserves in the presence of disease.
There is evidence that potential assimilate supply generally exceeds the capacity for
storage in barley, even in low light environments, and that the extent of the imbalance can
differ widely between sites and years (Bingham
et al.
, 2007). This suggests that tolerance
of late season disease could be greater in UK barley crops than wheat. Disease earlier in
the season (pre-fl owering) can reduce the development of potential grain sites in barley as
well as healthy leaf area, so that radiation interception per unit grain number during grain
fi lling may not be altered substantially (Bingham
et al.
, unpublished data).
There is evidence of intra-specifi c variation in the concentration of stem WSC reserves
