Due to an oil content of more than 40%, rapeseed (Brassica napus) is of great importance for meeting the world’s edible oil demand. There have been reports of 60% oil content in some rapeseed cultivars in recent plant breeding experiments. Regression and path analysis were carried out to characterize important agronomic traits of high correlation with oil content, pinpoint the traits influencing oil percent, determine the etiological relationships and to introduce the effective oil increasing traits. Some important quantitative and qualitative traits in 29 rapeseed (Brassica napus) fall cultivars were studied in a randomized complete block design of four replications in Research Station, Faculty of Agriculture, Tehran University during the agronomic year 2000-2001. Analysis of variance showed that there were significant differences among varieties for traits under study, indicating the existence of genetic variation among varieties which can be efficiently exploiled in breeding programs. Genotype correlation coefficients indicated that oil percent had a significant negative correlation with seed yield
(rg=-0.483**) as well as with protein percent (rg=-0.179*) at a 1% probability level. Oil content also exhibited a significant negative correlation with days to flowering, flowering period, duration of growth period, number of pods per plants as well as number of seeds per pod at a 1% probability level. Considering oil percent as a dependent variable, while using stepwise regression analysis, 53.2% of oil percent variation was attributed to three traits of duration of growth period, 100-seed weight and protein percent, the model significant at 1% probability level (R2=0.532).Results of path analysis indicated that the duration of growth period exerted the highest direct negative effect on oil percent (p=-0.73.Thus, due to decreasing effect of duration of growth period trait on oil percent that is temperature increases leading to degradation of fatty acids and decrease oil percent with the increase in growing period in during seed formation, early trait should be known as selection criteria for oil percent increase. Negative correlation between oil percent and protein percent results from their competition for occupying seed volume. Thus, selection for decreased protein percent lead to increase in oil percent. 1000-seed weight had high direct positive effect on oil percent (p=0.418) but its genotype correlation with oil percent (rg=0.083) was not significant, which is to carry out some restriction to zero undesirable indirect effects in order to benefit its direct effect. Results of factor analysis showed the qualification of four factors for 14 given traits considering the least character roots. This factor explained %78.3 of total variation in data.
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