BEHAVIOR OF PEA NODULATION MUTANTS AS AFFECTED BY INCREASING NITRATE LEVEL

Nodule number and size, acetylene-reducing activity, leghemoglobin con tent in nodules, and nitrogen content in the shoot were measured in pe a symbiotic mutants inoculated with Rhizobium leguminosarum bv. viceae strain 128C30 and cultivated on a range of nitrate levels in rooting medium. The abundantly nodulating mutants RisfixC and RisfixV, and the ineffective mutant RisfixO were compared to the parental cultivar 'Fi nale' RisfixO displayed nodule initiation and nodule growth suppressio n similar to the wild-type plants, indicating that nodule effectivenes s is not substantial for both endogenous and nitrate regulation of nod ulation. RisfixC was seen to produce effective and nitrate-resistant n odulation in abundance, confirming its classification as a new superno dulating pea mutant. The nitrate resistance was apparent not only in n odule initiation, but also in nodule growth and specific nitrogenase a ctivity. Shoot nitrogen content was markedly increased in RisfixC, ind icating carbohydrate depletion or a regulatory fault. Nodulation in Ri sfixV was abundant like in RisfixC, but the nodules possesed only trac es of nitrogenase activity, and their number sharply diminished with n itrate level increase. The small and nitrate-independent size of Risfi xV nodules suggests that their development had been arrested before ni trate level affected nodule growth. The hypernodulation of RisfixV is supposed to result from the early deterioration of nodules of this mut ant, which releases the suppression of further nodule initiation by th e established nodules.