TEMPERATURE AND GENOTYPIC EFFECTS ON THE EXPRESSION OF POL CYTOPLASMIC MALE-STERILITY IN SUMMER RAPE

The effect of high temperature treatment on the expression of cytoplasmic male sterility (CMS) was studied using inbred lines of four summer rape (Brassica napus L.) cultivars. The nuclear genotypes of inbred lines from each of the cultivars Karat, Lergo, Marnoo and Regent were evaluated for their ability to maintain pollen sterility in the pol cytoplasm. Seven days' exposure to day/night temperatures of 30/24-degrees-C in a controlled environment led to increased pollen production in all of the F1 populations (F1's) in the study. Maximum reversion to male fertility occurred 6-13 d after removal from the high temperature treatment. There was a significant linear relationship between daily mean temperatures in the field and mean male sterility index (MSI). In Lergo, Marnoo and Regent F1's, the effect of high temperatures on male sterility expression in terms of days to maximum pollen production in the field was accurately predicted by the values obtained in the controlled environment study. For the Karat F1's, the maximum response in the field occurred later than in the controlled environment. Variation in the stability of male sterility was evident in the F1's, indicating the presence of different maintainer genes among the inbred lines of each cultivar. The absence of environment-by-line interactions in the F1's of three cultivars indicated a high degree of stability for this trait. Selection for maintenance of high levels of male sterility may therefore be possible.