Has yield stability changed with genetic improvement of wheat yield?

The effect of plant breeding on yield and their physiological determinants has been widely studied in wheat. However, it is poorly understood how, and to what extent, yield stability has been modified. To attempt a direct ana lysis of changes in absolute and relative yield stability, data of yield of cultivars released in different eras in different environments were obtain ed from records from our lab and from the literature. Depending on the avai lability of data, effects of plant breeding on yield stability of cultivars released in Argentina, Australia, Italy and the United Kingdom were evalua ted using a quantitative approach. In this paper it was assumed that the sl ope of yield vs, environmental index estimates the instability of the culti vars. In addition, a more qualitative approach for Mexico, and the former U SSR complemented this analysis. There was a clear decrease in yield stability assessed in absolute terms as a consequence of wheat breeding. In Argentina, Australia, Italy and the UK this decrease was related to the magnitude of yield increases. However, th e decrease in yield stability in Argentina and Australia was less than for Italy and the UK, particularly so during the last 30 years. Modern cultivar s released in Argentina and Australia showed a trend to maintain yield stab ility as a percentage of their yield similar to that of their predecessors, while the two European countries analysed tended to a slight decrease in y ield stability even in relative terms. The complementary, less quantitative evaluation of Mexico and the former USSR appeared to confirm the quantitat ive trends described for the other countries, i.e. a general decrease in yi eld stability (assessed in absolute terms) with genetic gains in yield pote ntial.