Assessment of Drought Resistance in Pearl Millet [Pennisetum americanum (L.) Leeke]. 11* Estimation of Genotype Response to Stress

The finding that the more than 50070 of the variation in grain yield of pearl millet breeding lines in two different drought stress treatments could be attributed to variation in yield potential and time of flowering was used to develop a drought-response index (DRI) based on the residual variation in grain yield, adjusted for experimental error. DRI was positively correlated to measured yield in the drought treatments, and independent of both yield potential and time to flowering. DRI in both midseason and terminal stress treatments was unrelated to yield component structure in the irrigated control treatment, indicating that selection for plant type under non-stressed conditions will not influence drought response. DRI was correlated to both grain number per panicle, and grain yield per panicle in both stress treatments, suggesting differential ability to maintain normal grain number, and/or that grain yield per panicle was an important factor in response to stress. Maintenance of panicle number did not seem to be important for maintenance of yield under drought stress. The correlation of DRI and individual panicle yield was of sufficient magnitude for the latter to serve as a selection index in terminal stress. The use of a DRI as a component of breeding for better adaptation to stress is discussed. Introduction In the previous paper of this series, it was argued that a retrospective analysis of the reasons for genotype yield differences in a drought-stressed environment (black box analysis, Fischer 1981) is a useful initial step in developing a breeding program to produce improved genotypes for drought environments (Bidinger et al. 1987). Such an analysis demonstrated very marked, although contrasting, effects of phenology in midseason and terminal stress conditions in pearl millet [Pennisetum americanum (L) Leeke]. These included both direct effects on grain yield drought escape and associations between phenology and yield component structure that largely determined the relationship of yield components and grain yield under stress conditions. The combined effects of phenology and yield potential accounted for more than 50070 of the observed variation in grain yield among the test entries in both stress treatments. If drought resistance is considered to be a consequence of advantages conferred by one or more physiological or morphological characteristics (Turner 1982) and is to be manipulated in a breeding program as an independent genetic character (Blum 1979; Richards 1982; Quisenberry 1982), then the assessment of drought resistance should be free from the confounding effects of yield potential and phenology. Yield potential improvement is a universal breeding objective, and phenology can be easily manipulated where it offers opportunIties to increase or stabilize yields in stress conditions. A procedure for assessing drought resistance should identify genotypes whose performance under stress is better than that predicted from the combined effects of their yield potential and phenology. This is particularly true if such assessments are to be used to identify plant characteristics that confer advantages in stress conditions (Fischer and Wood 1979), otherwise correlation analyses using drought-resistance estimates may identify yield potentialor phenology-related characteristics rather than ones related to drought resistance. Fischer and Maurer (1978) defined an index of drought susceptibility (5) based on the relationship of the change in relative yield (yield in drought/yield in the absence of drought) of an individual cultivar to the change in mean relative yield, across a range of stress intensities, of all cultivars in the comparison. Yields measured in the drought were adjusted for differences in phenology before S was calculated. This approach was successful in combining data from multiple-drought treatments to obtain an estimate of genotypic response to a range of stress intensities, and in removing the effects of phenology, but the S calculated in this fashion was not independent of yield potential (Fischer and Maurer 1978). As a consequence, S was found to be positively correlated to traits associated with high yield potential in the material studied in the trials (Fischer and Wood 1979). This paper presents a different approach to the calculation of a drought response index (DRI), and examines correlations of this index and yield components in advanced breeding lines of pearl millet. The index presented here is independent of both yield potential and phenology effects. As used here, it is based on a single comparison between stress and nonstress treatments and as such is of limited value for assessment of individual genotypes. It is of use, however, for assessing the effects of specific physiological or morphological characters on response to drought, if the number and diversity of the genotypes tested are sufficiently large. As the objective of the experiments was to examine the variation in drought response, and the factors associated with it, in the current ICRISA T pearl millet breeding program, this index was quite suitable for this purpose.