Genetic Variation for Nitrogen Efficiency in Oilseed rape (Brassica napus L.)

Oilseed rape is produced today under high N fertiliser application. However, the crop is known to be having a low N efficiency mainly due to low translocation into the seed during the reproductive growth phase, despite the high N uptake early in the season until flowering. This results in a low N harvest index leaving behind a high N content in the strew which following mineralisation finds its way to ground water and thus contribute to environmental hazards.N efficiency can be splitted into two components, namely N uptake (the efficiency with which the soil N can be taken up by the plant) and N utilisation (the seed dry weight produced pro unit of absorbed N fertiliser). Genotypic variation in N efficiency could generally be attributed to high N uptake and/ or high utilisation. A better understanding of the physiological basis for genetic variation in N efficiency would be very helpful to develop selection strategies to improve N efficiency. Therefore a large number of traits were measured which are thought to be related to N efficiency and could eventually serve as traits of indirect selection in improving N efficiency in oilseed rape.In the first experiment 54 DH lines from the cross Apex x Mohican (cross 1) were tested in three years at seven environments. The genotypes x nitrogen interaction (G x N) was highly significant and the size of their variance components was about 40% of the genetic variance. which is nearly the same size as the genotype x environment (G x E) interaction. The lines with highest yield at high N supply showed sometimes rather low yields at the reduced N supply. At low N supply 62% of the variation in N efficiency was contributed by differences in N uptake efficiency, whereas at high N supply only 36% was contributed by differences in N uptake and N utilisation was more important.In the second experiment 99 DH lines from the cross Mansholt x Express (cross 2) were tested in two years at four environments. There was significant G x N interaction for all the traits except days to begin of flowering, end of flowering, N in dropped leaves and seed N uptake. The correlation between the two N levels for yield was R2 = 0.53 but after excluding three very low yielding lines the correlation was only R2 = 0.35. Yield/flowering interval and yield/end of flowering were significantly correlated at the reduced N supply but not at the high N supply.In the third experiment 44 divergent bulks of cross 1, cross 2 and cross 3 (Bristol x Lirajet) were tested in one year in 5 locations. The divergent bulks were composed from about 10 lines each which differed extremely for performance in morphological or agronomic traits. The bulks of cross 1 differed significantly for all the traits except N uptake at end of flowering and seed N uptake. the bulks of cross 2 differed significantly only for oil content, the bulks of cross 3 showed significant genetic variation for plant height. oil and protein content. There was a clear relationship between protein content and high yield throughout the three crosses, the low protein containing lines proved to be high yielding.