Agriculture Reference
In-Depth Information
In some legume crops it is possible to identify the mechanisms by which tolerance
operates. Allen (1983) provided several examples of legume crops that have the
ability to compensate for damage, to both vegetative and reproductive structures.
Beans can compensate for infection, at or below ground level, by
Fusarium solani
f.sp.
phaseoli
through the production of adventitious roots. In other crops
physiologically surplus foliage is an effective compensation mechanism. Pigeon pea
varieties that have an indeterminate growth pattern can tolerate some infection
damage caused by rust (
Uredo cajani
); varieties with a determinate growth pattern
can suffer much more extreme yield losses. Similarly, alternately branched varieties
of groundnut can compensate for defoliation caused by cercospora leaf spot but
sequentially branched varieties cannot (Allen, 1983). Compensation for damage
caused by disease is likely to have a physiological mechanism which is independent
from 'true' resistance and the extent to which this compensation can be made will
certainly be affected by the stage at which the damage occurs and the extent of that
damage. Furthermore, it is likely to be a function of the partitioning and distribution
of assimilates within the plant (Allen, 1983).
The physiological mechanisms underlying tolerance and the ability of certain
crop varieties to produce high yields under disease severity levels that might be
expected to constrain these yields have been examined in cereal cultivars.
Zuckerman
et al
. (1997) found that under equivalent disease progress curve and
disease severity levels the tolerant cultivar 'Miriam' showed significantly smaller
losses in thousand kernel weight than the non-tolerant cultivar Barkai. In this
host-pathogen system, it appears that the rate of carbon fixation per unit area of
chlorophyll and per residual green leaf area of the tolerant cultivar was higher
than in healthy plants. This enhancement of photosynthesis in residual green
tissue compensated for the loss of photosynthesizing tissue due to septoria tritici
blotch.
As is the case for horizontal resistance, arguments against the use of tolerance in
crops stem from difficulties in breeding. In a practical sense, whether a cultivar
yields well in the presence of disease due to tolerance or resistance makes little
difference. However, where tolerance is operative and crop losses are dispropor-
tionately less than those commensurate with observed levels of infection, it is only
possible to measure tolerance effectively by comparing symptom expression at the
same stage of host development with the final yield of plants (Parlevliet, 1979).
Since such assessments would prove both time-consuming and difficult, selection
would not be possible among early segregating generations (Buddenhagen and de
Ponti, 1983). As a result, breeding for disease resistance has, in the past, generally
been limited to the selection of host resistance and little attention has been paid to
the selection of host genotypes showing evidence of tolerance to infection. More
recently however, with the introduction of molecular marker technologies, it has
been possible to identify, for example, quantitative loci for tolerance to virus
diseases (Jin
et al
., 1998). This suggests that the process of selection, identification
and transfer of genes controlling complex traits such as tolerance to disease into new
lines, could be accelerated.
