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Genotypic differences in yield between breeding lines or cultivars may be estimated with the aid of functions of the regression of their individual yields, in different environments, on the mean yields of all the lines tested in the respective environments. CONSIDERATIONSThe progress made in any breeding programme is dependent on the recognition of superior genotypes. Selection for disease-and insect-resistance, maturity, height, shape or colour of fruits etc. can be successfully done in a few nursery tests. However, the expression of some quantitative characters, and in particular that of yield, are strongly influenced by genotype × environment interaction effects. Therefore, selection for these characters has to be based on evaluations at many different environments. This can be obtained by conducting the tests at various sites and under different managements, e.g., crop rotation, fertilization, irrigation, seeding dates and rates, etc.If these environments would comprise a representative sample of the population of environments for which the breeding programme is intended then the mean phenotypic performance of any line, averaged over these environments, should provide a reliable estimate of its genotypic value (CoMsTOC~: and MOLL, 1963). In practice such a representative sample of environments can hardly be achieved. It seems, however, feasible to conduct a series of tests under environmental conditions which include the range of conditions for which selection is being done. PROPOSED METHODIt is suggested to fit for each line a function of the regression of its individual yields, at the different environments, on the mean yields of all the tested lines at the respective environments. Data presented by FINLAY (1968) and our own investigations indicate that a good fit to a linear regression may be obtained either from the actual yield data or from their transformed values.The regression functions for a breeding line i and for an established cultivar cv, included in the trials as check, would be Yt = at + btx and Ycv = acv + bc~x, respectively. The value of Yt at that level of x for which x~ c~ equals the average yield of this cultivar, under the conditions which the breeding programme is aiming at, may be regalded as an estimate of the genotypic value of line i (Fig. 1). 121

Near-isogenic wheat (Triticum aestivum L.) lines differing in height-reducing (Rht) alleles were used to investigate the effects of temperature on endogenous gibberellin (GA) levels and seedling growth response to applied GA3. Sheath and lamina lengths of the first leaf were measured in GA treated and control seedlings, grown at 11, 18, and 250C, of six Rht genotypes in each of two varietal backgrounds, cv Maris Huntsman and cv April Bearded. Endogenous GA, levels in the leaf extension zone of untreated seedlings were determined by gas chromatographymass spectrometry with a deuterated intemal standard in the six Maris Huntsman Rht lines grown at 10 and 250C. Higher temperature increased leaf length considerably in the tall genotype, less so in the Rhtl and Rht2 genotypes, and had no consistent effect on the Rhtl+2, Rht3 and Rht2+3 genotypes. In all genotypes, endogenous GA1 was higher at 250C than at 100C. At 10°C the endogenous GA1 was at a similar level in all the genotypes (except Rht2+3). At 250C it increased 1.6-fold in the tall genotype, 3-fold in Rhtl and Rht2, 6-fold in Rht3, and 9-fold in Rhtl+2.Likewise, the genotypic differences in leaf length were very conspicuous at 250C, but were only slight and often unsignificant at 11°C. The response of leaf length to applied GA3 in the Rhtl, Rht2, and Rhtl+2 genotypes increased significantly with lowering of temperature. These results suggest the possibility that the temperature effect on leaf elongation is mediated through its effect on the level of endogenous GA, and that leaf elongation response to endogenous or applied GAs is restricted by the upper limits set by the different Rht alleles.Most modem high-yielding wheat cultivars have short straw and are referred to as 'dwarf or 'semi-dwarf.' The majority of these cultivars owe their short stature to the presence of one or both of the Norin 10-derived, height-reducing genes, RhtJ and Rht2 (5 Culm and leaf elongation of tall (rht) cultivars that do not carry any of these Rht alleles are promoted by application of GA3, whereas genotypes carrying these Rht alleles are relatively insensitive to applied GA3 (4,12). In near-isogenic lines in the genetic background of cv Maris Huntsman the presence ofthe RhtJ allele has been shown to reduce the responsiveness of the second leaf sheath to applied GA3, whereas the line containing the Rht3 allele was totally unresponsive compared with the normal tall rht line (9). This information regarding responses to GA3 application was obtained at temperatures of about 20°C.Work with GA-deficient mutants of maize and pea has shown that GA, is essential for shoot elongation (10). The concentration of biologically active GAs in wheat cultivars containing RhtJ, Rht2, Rhtl+2, and Rht3 alleles, however, is much greater than that of rht, tall cultivars (12, 18). In addition, in Maris Huntsman near-isogenic lines, the level of GA, has been shown to be 4.5-fold and 25-fold greater (compared with the tall line) in expanding stem internodes containing RhtJ and Rht3 alleles, respecti...

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