Principles of Plant Genetics and Breeding

drought. Consequently, phenotypic selection for the trait
may be more efficient under post-flowering drought stress.
Stem reservation utilization
Small grains and cereals store carbohydrates in their
stems. The size of storage depends on favorable growing
conditions before anthesis, and the genotype. The
potential stem storage is determined by stem length and
stem weight density. It has been found that the presence
of Rht 1 and Rht 2 dwarfing genes in wheat limits the
potential reserve storage by about one-half, as a result of
the reduction in stem length. This stem reduction may
be partly responsible for the observed greater drought
susceptibility of the dwarf high-yielding wheat cultivars.
The percentage of grain yield accounted for by stem
reserves varies according to the environment and the
genotype, and ranges between 9% and 100%. Stem
reserve mobilization is a major source of carbon for
grain filling under any stress. High reserve utilization,
however, accelerates shoot senescence – a consequence
of the export of stored carbohydrates into the grain.
This finding suggests that a breeder may not be as successful
in selecting for both traits (delayed senescence
and high reserve mobilization) as a strategy for developing
a cultivar with high grain filling under stress.
Mechanisms of drought resistance
Plant species differ in the stages at which they are most
susceptible to drought stress. Some species are most
prone to stress damage during the early vegetative stage,
while others are most susceptible during the pre- or
postanthesis stage, with others in between. Four general
mechanisms may be identified by which plants resist
drought:
1 Escape. Using early maturing cultivars may allow the
crop to complete its life cycle (or at least the critical
growth stage) before the onset of drought later in the
season. The plants use the optimal conditions at the
beginning of the season to develop vigor.
2 Avoidance. Some plants avoid drought stress by
decreasing water loss, for example by having cuticular
wax or by having the capacity to extract soil moisture
efficiently.
3 Tolerance. In species such as cereals in which grain
filling is found to depend on both actual photosynthesis
during the stage, as well as dry matter distribution
from carbohydrates in pre-anthesis, terminal
drought significantly reduces photosynthesis. This
BREEDING FOR RESISTANCE TO ABIOTIC STRESSES 391
shifts the burden of grain filling to stored carbohydrate
as the source of dry matter for the purpose.
Consequently, such species may be more tolerant of
postanthesis drought, being able to produce appreciable
yield under the stress.
4 Recovery. Because drought varies in duration, some
species are able to rebound (recover) after a brief
drought episode. Traits that enhance recovery from
drought include vegetative vigor, tillering, and long
growth duration.
Approaches for breeding drought resistance
Plant breeders have two basic approaches for breeding
for drought resistance – indirect breeding and direct
breeding.
Indirect breeding
In this strategy, the breeder exposes genotypes to an
environmental stress, even though they are not being
directly evaluated for environmental stress. Indirect
selection pressure is applied to these genotypes by conducting
performance trials at locations where stress
conditions exist. This approach is not advisable if the
cultivars to be released are not intended for cultivation
in the location where the evaluation was conducted.
Under such atypical conditions, it is possible the cultivars
might exhibit susceptibility to other stresses in their
area of production.
Direct breeding
Direct selection for drought is best conducted under
conditions where the stress factor occurs uniformly and
predictably. Temperature and moisture are highly variable
from one location to another and hence are difficult
to predict. There are several methods used for direct
breeding.
Field selection
Field selection is often problematic in drought-resistance
breeding. Water requirement is variable from year to
year, and may be sufficiently severe in one year to cause a
loss of breeding materials. Further, drought in different
seasons can occur at different growth stages. Without
special management for stress, inconsistencies in the
field may result in inconsistent selection pressure from
one cycle to the next. The ideal field selection site would