Winter wheat cultivars (Triticum aestivum) were evaluated for
their response to water stress and adaptability to a wide range of
moisture limited environments. Grain yield was analyzed for ten
cultivars at six locations across four years. Regression of the cultivar
mean on the year-location mean was used as a measure of the
environmental response of a particular cultivar, Plant water status
(leaf osmotic potential and water potential) during the grain filling
period, along with grain yield and yield components were determined
at three locations for fifteen cultivars under space-planted conditions
in 1972 and for ten cultivars under solid-seeding in 1973. Five cultivars
were evaluated for plant water status and rates of growth during
reproductive development and for final grain yield and yield components
at a severely stressed location in 1973. Drought resistance, described as yielding ability under severe
soil moisture stress, was found to be the result of avoidance, tolerance
and escape characteristics. Plant water status influenced the
components of yield when their development occurred during a period
of high external stress. However, the relative importance of plant
water status to final yield varied depending on the magnitude of the
direct effect of spike number on yield. Drought avoidance was the
ability to maintain a high plant water status when under high external
stress conditions. In the cultivar Yamhill, the stable growth rate
during inflorescence development and the extended period of grain
filling may have resulted from the ability of this cultivar to avoid
drought. These characteristics, also, may explain the large spikelet
number and the high kernel weight of Yamhill. Even though Hyslop
and Wanser had a moderate and high internal water stress, respectively,
they were relatively tolerant of stress in terms of high growth rates
during the grain filling period. The high yielding ability of Wanser and
McDermid under high stress conditions was primarily related to a
large number of spikes. This trait was considered an escape characteristic
because tillering was established before the onset of severe
stress. The club wheats neither avoided or tolerated internal water
stress under solid-seeded conditions nor did they have sufficient
tillering capacity to compensate for the effect of stress on the laterdeveloped
components.of yield. Nugaines, Hyslop and Rew maintained a moderate internal stress during later development but differed in
their yield component responses. The yield of Nugaines depended
primarily on a high tillering capacity, whereas the yield of Hyslop and
Rew depended on a balance of contributions from all the components of
yield.
Widely adapted cultivars, such as Hyslop and McDermid, not
only yielded well under severe stress conditions but had the genetic
potential to respond adequately to more favorable moisture conditions.
Cultivars specifically, adapted to lower yielding environments, such
as Yamhill and Wanser, were able to avoid, tolerate or escape drought
but were limited in one or more of their yield component responses to
better conditions. Conversely, cultivars specifically adapted to high
yielding environments, such as Nugaines and Paha, lacked the ability
to consistently yield well under high stress environments but had a
high response to more favorable moisture conditions.
Cultivars best adapted to dryland cultivation should have the
highest yield under stress conditions as expressed through the components
of yield by one or a combination of avoidance, tolerance or
escape characteristics. These cultivars should also have the maximum
expression in, and balance among, the components of yield for
maximum response to more favorable moisture conditions. This goal
may be obtained by selection in favorable as well as stressed environments
and by testing throughout the range of the environments where the cultivar is expected to be grown. Because high yield under stress
was found in this study to be manifested through different characteristics
of the plant the breeder should be aware of traits which may
contribute to drought resistance, but he should not be restricted to a
particular ideotype when breeding for yield under conditions of high
moisture stress