Drought resistant
varieties as an experimental tool to prove a physiological hypothesis – a
frequent pitfall
It has been and still is a
common approach in physiological investigation of drought resistance to use two
or more crop varieties divergent in their "drought resistance". When
in the experiment the two varieties also differ for a certain physiological
drought-related trait then this trait is concluded to be associated with or
even explain drought resistance. For example, if the two varieties differ in
their root length then long roots are taken as the explanation of drought
resistance in that case. Two problems are recognized here:
1.
The a priori assertion
that these varieties differ in their drought resistance is sometimes not well
substantiated.
2.
Other possible
physiological explanations of the difference in drought resistance between the
test varieties might be overlooked.
A recent example is the
paper in Functional Plant Biology 33:823–837, 2006* where two wheat
varieties were stated to be drought resistant (Seri82) and drought susceptible
(or ‘standard’) (Hartog), respectively. Data
collected in root boxes on these two varieties was used to explain the
difference in drought resistance between them on the basis of their different
root architecture. However, in this case as in others, no information or proof
is given why these two varieties were designated as different in their drought
resistance and no other developmental or physiological factor besides root
architecture was considered as a possible explanation of drought resistance if
indeed it varied between these varieties. These two varieties could differ in
their drought resistance by factors not investigated in this study, such
osmotic adjustment or use of stem reserves for grain filling or any other
contributing trait. These two varieties could differ in their drought
adaptation due to plant developmental traits such as phenology,
plant size, early vigor or other traits. Roots may or may not have been the
explanation of drought resistance in this case – if indeed the two varieties
differed in their drought resistance. The simulation performed in this study
showing that greater soil moisture extraction increases yield under drought
stress does not help convince the reader that root architecture is the reason
for the stated “drought adaptation” of Seri82 and consequently of “wheat” as
stated in the title.
Defining drought
resistance is crucial for such studies. Varieties might differ in their
performance under drought stress for different of reasons and if the definition
of drought resistance ignores these reasons then the use of such varieties to
dissect drought resistance becomes impossible. Poor definition of drought
resistance is not uncommon in such studies. For example, in some cases
differing varieties for drought resistance were simply stated as such by word
of mouth. In another case they were designated by the token of their yield in a
certain dry year. In another case the opinion of the local breeder was cited as
proof. Even when “yield performance under water-limited conditions” is cited as
the basis for defining drought resistance, there is a problem. We already
realize now that plant production under mild stress can sometimes be fully
accounted for by the yield potential of the variety without any bearing on its
drought adaptation.
Hence, when two or more
varieties are stated to be different in their "drought resistance"
and are being used to dissect the physiology of drought resistance,
satisfactory information should be produced to substantiate the basis of their
defined difference in field resistance. For cereals such as wheat, the
definition is ideally based on solid tests of yield performance and yield
reduction under well quantified severity and timing of stress while accounting
(or normalizing) for a possible role of phenology and
gross developmental traits. Therefore the choice of such varieties is crucial
and it should be based not only on their performance under drought but also on
their being similar (not necessarily isogenic) in
terms of whole plant development. A physiological dissection of drought
adaptation in such carefully chosen and well-tested varieties is then most
likely to reveal relevant traits associated with crop productivity under
stress.
(*)
The Role of Root Architectural Traits in Adaptation of Wheat
to Water-Limited Environments.
By
A.M. Manschadi, J. Christopher, P. deVoil and G.L. Hammer