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A difficulty in identifying traits that help crop plants maintain their yield under droughted conditions is distinguishing between those traits that contribute to yield stability under drought and traits that do not affect yield. With the development of molecular markers for many crops it is now possible to identify major quantitative trait loci (QTL) regulating specific drought responses. By comparing the coincidence of such QTL for specific traits it is possible to test much more precisely than before whether a particular constitutive or adaptive response to drought stress is likely to be of significance in improving drought resistance. We have used this approach to identify QTL for ABA content and other traits likely to be important in determining drought response in maize. Eighty-four RFLP markers were mapped in an F2 population of 81 plants from a cross between parents, Polj17 (drought resistant) and F-2 (drought sensitive), that differ markedly in many constitutive and adaptive responses to drought stress. In a soil glasshouse experiment, from which water was withheld for 3 weeks after anthesis, flowering time, stomatal conductance, tissue ABA contents, leaf water relations parameters and fluorescence characteristics, root pulling force, and nodal root number were measured. The minimum number and location of genes having major effects on the traits were determined and possible causal relationships amongst them tested. Comparing the coincidence of QTL for ABA content and stomatal conductance showed that xylem ABA content was more likely to have had a regulatory effect on the stomatal conductance of those plants than the whole leaf ABA content. However, both xylem and leaf ABA contents were significantly associated with root characteristics, suggesting that the rooting behaviour (either constitutive or adaptive) was important in regulating stress responses, particularly in determining xylem ABA contents. We also found that Fm (a measure of the activity of photosynthetic reaction centres) was positively associated with chlorophyll concentration per unit area. Different methods for comparing QTL are presented and discussed.
Lebreton et al. (Sun,) studied this question.