Pea breeding to improve effectiveness of symbiotic nitrogen fixation

 Symbiotrophic nitrogen nutrition is the unique property of legumes. Lupine and forage beans, satisfy 80% to 98% of their nitrogen requirements by symbiotic fixation. However, peas bare a low efficiency crop and can fix only 35 to 40% of required nitrogen (Hardarson et. al., 1993). Improvement of symbiotic effectiveness in peas should be possible by breeding. The previous absence of a breeding program to enhance the symbiotic effectiveness of pea in Russia stimulated the development of a special breeding and genetic program. The main aim of the program is to develop new pea genotypes with the high level of symbiotrophic nitrogen nutrition, that will allow us to increase the protein production essentially reducing application of N fertilizers.

We studied 180 primitive pea varieties with respect to nitrogen-fixing activity, plant weight and seed productivity. The best seven lines, Orlus, Orlovchanin 2, Skif, OP-91-829, A3-93-1995, T-111 and l 12/38, were selected and included in a complete diallel crossing scheme. The seven lines were also inoculated by nodule bacteria according to the "Afghanistan" system. The Nord sym2sym2 line was used as the donor of sym2 recessive gene, preventing peas from inoculation by native Rhizobium (3). In summer the experiments were performed in the field, in autumn and winter in the greenhouse. Before sowing the seeds were inoculated with water suspension of 245a or A-1 stock cultures.

Crosses were carried out with preliminary emasculation. One hundredflowers were crossed in each combination. The lines and hybrids were evaluated at the bud stage for modulation and nitrogenase activity traits. Each 5 plants were tested. Nitrogenase activity of roots with nodules was measured by the acetylene reduction method. After harvesting the seed productivity of pea plants was determined. General combining ability (GCA) and specific combining ability (SCA) were calculated using equations in Mather and Jinks (2).

Evaluation of the cultivars combining ability is one of the most important criterion when selecting the parental forms for hybridization. Crossing of the cultivars with high GCA provides the high productive hybrid populations. The cultivars investigated differ in GCA and SCA with nonsignificant reciprocal effect in all variants. Influence of GCA was higher than that of SCA in terms of the number of nodules per plant (20 to 25 times) and plant weight (10-15 times) while the influence of SCA was 3 times higher as compared to GCA with respect to nitrogen-fixing activity and 7 times higher with respect to symbiotic effectiveness. The most valuable contributors to the number of nodules were the T-111 line and Orlus cultivar; to the plant weight, Skif, l.12/38, A-3-93-1995; to nitrogenase activity, T-111, l.12/38; to symbiotic effectiveness, OP-91-829, T-111, Skif.

The hybrids of the first generation displayed heterosis in terms of symbiotic traits (from 9.6 to 32.7%) depending on the hybrid combination. Maximum heterosis was obtained in combinations where one of the parents was OP-91-829. In the second hybrid generation the heterosis effect had decreased and was 3.5-9.7%. Therefore, the analysis of combining ability of the pea symbiotic traits demonstrated additivity character in their inheritance. Additivity should permit the improvement of symbiosis by plant breeding. The expression of heterosis of symbiotic traits in peas depends on the crossing combination. It can be expressed either on the majority or few traits. The level and frequency of its manifestation with respect to different traits is variable. More often, the hybrids exceed the parental forms in nodule number, plant weight and seed productivity and, rarely, in nitrogenase activity.