Capacity for Proline Accumulation During Water Stress in Barley and Its Implications for Breeding for Drought Resistance1

During controlled water stress applied at the three-leaf stage, the barley (Hordeum vulgare L.) cultivars 'Proctor' and 'Excelsior' showed different rates of free proline accumulation and leaf-firing. Both processes were faster in Proctor (a drought-susceptible cultivar) than in Excelsior (a drought-resistant cultivar). The different rates of proline accumulation during stress in Proctor, in Excelsior, and in a wild barley (H. spontaneum C. Koch) could be attributed to differences in the rate of decline of leaf water potential ( psi leaf); at the same psi leaf value, all three genotypes contained similar amounts of free proline. After non-destructive screening of the F2 generation from a Proctor X Excelsior cross for proline-accumulating capacity under controlled water stress conditions, F3 seed from plants selected for high and low proline accumulation was obtained. When F3 palnts were tested for proline accumulation during controlled stress, the selection for proline accumulation was found to have been effective. Reselection among F3 plants and subsequent testing of the F4 generation confirmed this result. Among individual plants of the F3 generation (which was segregating for the proline accumulation trait) there was a positive association between the amount of free proline accumulated during stress and the severity of leaf-firing. Massive proline accumulation is a symptom of severe internal water stress in barley and apparently has no survival value during drought. Although there is heritable variation in the amount of proline accumulated during stress, it is not of practical value in breeding for drought resistance.ADDITIONAL ABSTRACT:During controlled water stress, rates of proline accumulation and leaf firing were greater in Proctor (drought-susceptible) than in Excelsior (drought-resistant) seedlings. The different rates of proline accumulation among the above varieties and Hordeum spontaneum (intermediate resistance) were attributable to differences in the rate of decline of leaf water potential. At a given value of the latter, similar concentrations of free proline were present in the leaves of the three genotypes. F2 plants derived from a Proctor X Excelsior cross were selected for high or low proline accumulation. F3 and subsequent F4 progenies (obtained by F3 reselection) retained the proline accumulation characteristics of the F2 selections. Both high proline concentration and intensity of leaf firing are symptomatic rather than causative of drought susceptibility. Leaf firing would be a more readily measurable selection criterion.ADDITIONAL ABSTRACT:Studies were carried out determine if high proline accumulation was associated with susceptibility or with resistance to water stress. During controlled water stress applied at the 3-leaf stage the drought-susceptible barley cv. Proctor showed higher rates of free proline accumulation and leaf firing [scorch] than did the drought-resistant cv. Excelsior. The different rates of proline accumulation during stress in Proctor, Excelsior and a wild barley were attributed to differences in the rate of decline of leaf water potential; at the same leaf water potential, all 3 genotypes contained similar amounts of free proline. Progeny from a Proctor X Excelsior cross selected for high and low proline accumulation showed a positive association between the amount of free proline accumulated during stress and the severity of leaf firing. Massive proline accumulation is a symptom of severe internal water stress in barley but has apparently no survival value during drought