Differential tolerance in wheat (Triticum aestivum L.) genotypes to metribuzin

Abstract

Field and glasshouse experiments were undertaken at 2 locations in South Australia to evaluate wheat (Triticum aestivum L.) tolerance to metribuzin. Sloop SA barley (Hordeum vulgare L.) was used as a standard in the evaluation. Linear and logistic regression models were used to describe the response of wheat genotypes to metribuzin. Parameter estimates of B (slope) and ED₅₀ (dose required for 50% inhibition) estimated by the models were used to compare the responses of the genotypes with that of the known sensitive cv. Spear. In the field, wheat cvv. Blade, Kite, EGA Eagle Rock, and Sloop SA barley showed tolerance to metribuzin, exhibiting little change in seedling density and anthesis dry matter (DM) when treated with metribuzin (187.5 and 375 g/ha). In contrast, wheat cv. Spear, which has 50% common parentage with Blade, showed a strong negative response to metribuzin for both seedling density (B = –0.308) and anthesis DM (B = –0.482), indicating sensitivity to the herbicide. Sonora 64 and Tezanos Pinto Precoz, parental lines of Blade, were also sensitive to metribuzin (>187.5 g/ha), showing similar negative responses to that of Spear for anthesis dry matter (B = –0.307 and –0.387). However, Kite, which is another parent of Blade, showed excellent tolerance to this herbicide, exhibiting only a small response (B = –0.076), which was statistically (P < 0.05) different from that of Spear. In the second field study, cvv. Blade, EGA Eagle Rock, and Sloop SA barley were again far more tolerant to metribuzin than the sensitive cv. Spear, requiring almost double the dose of metribuzin to induce 50% inhibition (ED₅₀). Glasshouse studies, where metribuzin was highly active in a light sandy potting soil at what would be considered low rates in the field (50 and 100 g/ha), confirmed the sensitivity differences among wheat genotypes observed in the field. Wheat genotypes Blade, Kite, EGA Eagle Rock, and daughter line RAC 0824 were consistently tolerant to metribuzin. Sensitivity to metribuzin (50 g/ha) was observed in Spear, and most parental lines of Blade with the exception of Kite. Interestingly, Kite showed equivalent tolerance to its progeny, Blade and EGA Eagle Rock, exhibiting similar ED₅₀ and B (slope) values. It is noteworthy that EGA Eagle Rock has recently been specifically bred for tolerance to metribuzin using Blade as a parent.

Considering Kite was the only parent to show tolerance to metribuzin in these field and glasshouse studies, it would appear to be the major contributor to metribuzin tolerance in cv. Blade. At present, Kite is not favoured by wheat breeders due to the presence of a rust-resistant gene (SR26) linked to yield penalty. Further research is required to determine alleles responsible for metribuzin tolerance in wheat and to identify alternative sources of metribuzin tolerance.