PLANT SYMBIOTIC MUTANTS AS A TOOL TO ANALYZE NITROGEN NUTRITION AND YIELD RELATIONSHIP IN FIELD-GROWN PEAS (PISUM-SATIVUM L)

Pisum sativum L. is known for high seed and protein yields but also fo r yield instability. Because legumes utilize two sources of nitrogen ( atmospheric N, fixed in nodules and assimilation of Soil mineral N), s tudies on their nitrogen nutrition is more complex than in other plant s. In this work, pea symbiotic mutants (with no nodules at all ([Nod-] ), with inefficient nodules ([Nod+Fix-]) or showing an hypernodulating and a 'nitrate-tolerant symbiosis' character ([Nod++Nts]), their semi -leafless isogenic homologues and the parental control line cv Frisson were fertilized with three levels of mineral nitrogen (0, 25 or 50 g N m-2) to generate a range of mineral nitrogen regimes in the same gen etic background. Impact of the source and level of nitrogen nutrition was measured on reproductive development, growth, nitrogen accumulatio n and seed yield. It was shown that a N deficiency induced flowering t ermination. It also led to a large decrease in the number of seeds pro duced and the amount of N accumulated in forage and in seeds, when lit tle effect was observed on the progression rates of reproductive stage s along the stem. The single seed weight and the amount of dry matter accumulated in forage neither responded strongly to N deficiency. The source of nitrogen was shown to be of little importance to yield but t he application of about 50 g N m-2 was necessary to reach the yield of the control cv Frisson when exclusive assimilation was ensuring the N requirements of the plant. Despite the fact that the nitrate-tolerant and hypernodulating mutant P64 used in this study did not yield as we ll as the parent cv Frisson, it is proposed that [Nod++Nts] characters could act as a yield regulating factor.