IDENTIFICATION OF QTL FOR DROUGHT RESPONSES IN MAIZE AND THEIR USE INTESTING CAUSAL RELATIONSHIPS BETWEEN TRAITS

A difficulty in identifying traits that help crop plants maintain thei r yield under droughted conditions is distinguishing between those tra its that contribute to yield stability under drought and traits that d o not affect yield, With the development of molecular markers for many crops it is now possible to identify major quantitative trait loci (Q TL) regulating specific drought responses, By comparing the coincidenc e of such QTL for specific traits it is possible to test much more pre cisely than before whether a particular constitutive or adaptive respo nse to drought stress is likely to be of significance in improving dro ught resistance. We have used this approach to identify QTL for ABA co ntent and other traits likely to be important in determining drought r esponse in maize. Eighty-four RFLP markers were mapped in an F-2 popul ation of 81 plants from a cross between parents, Polj17 (drought resis tant) and F-2 (drought sensitive), that differ markedly in many consti tutive 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 fo rce, and nodal root number were measured, The minimum number and locat ion of genes having major effects on the traits were determined and po ssible causal relationships amongst them tested. Comparing the coincid ence of QTL for ABA content and stomatal conductance showed that xylem ABA content was more likely to have had a regulatory effect on the st omatal conductance of those plants than the whole leaf ABA content. Ho wever, both xylem and leaf ABA contents were significantly associated with root characteristics, suggesting that the rooting behaviour (eith er constitutive or adaptive) was important in regulating stress respon ses, particularly in determining xylem ABA contents. We also found tha t F-m (a measure of the activity of photosynthetic reaction centres) w as positively associated with chlorophyll concentration per unit area, Different methods for comparing QTL are presented and discussed.