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
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 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
(Functional
Plant Biology 33:823–837, 2006)
Abstract. Better
understanding of root system structure and function is critical to crop
improvement in waterlimited environments. The aims of this study were to
examine root system characteristics of two wheat genotypes contrasting in
tolerance to water limitation and to assess the functional implications on
adaptation to water-limited environments of any differences found. The drought
tolerant barley variety, Mackay, was also included to allow inter-species
comparison. Single plants were grown in large, soil-filled root-observation
chambers. Root growth was monitored by digital imaging and water extraction was
measured. Root architecture differed markedly among the genotypes. The
drought-tolerant wheat (cv. SeriM82) had a compact root system, while roots of
barley cv. Mackay occupied the largest soil volume. Relative to the standard
wheat variety (Hartog), SeriM82 had a more uniform rooting pattern and greater
root length at depth. Despite the more compact root architecture of SeriM82,
total water extracted did not differ between wheat genotypes. To quantify the
value of these adaptive traits, a simulation analysis was conducted with the
cropping system model APSIM, for a wide range of environments in southern
Queensland, Australia. The analysis indicated a mean relative yield benefit of
14.5% in water-deficit seasons. Each additional millimetre of water extracted during
grain filling generated an extra 55 kg ha−1 of grain yield. The
functional implications of root traits on temporal patterns and total amount of
water capture, and their importance in crop adaptation to specific
water-limited environments, are discussed.