ANATOMY, PHYSIOLOGY AND BIOCHEMISTRY OF ROOT-NODULES OF SPRINT-2 FIX(-), A SYMBIOTICALLY DEFECTIVE MUTANT OF PEA (PISUM-SATIVUM L)

A plant-determined pea mutant Sprint-2 Fix(-) and the parental line Sp rint-2 were compared for selected physiological and biochemical parame ters. The Fix(-) mutation prevented differentiation of Rhizobium legum inosarum bacteria into bacteroids and produced large, white, non-fixin g nodules. These lacked nitrogenase-linked respiration, but had a back ground rate of CO2 evolution similar to the normal Fix(+) nodules, The cortical structure of the ineffective nodules suggests the existence of an oxygen diffusion barrier and this was supported by a low oxygen concentration in the central region (0.5-3.0%), measured using an O-2- sensitive micro-electrode. Sucrose and starch contents were similar in normal and ineffective nodules while ononitol content was about 15 ti mes lower in the Fix- nodules, The distribution of starch was also dif ferent in the two nodule types. The activities of glutamine synthetase (GS), sucrose synthase (SS), phosphoenolpyruvate carboxylase (PEPC) a nd alanine pyruvate aminotransferase (APAT) were markedly higher in Fi x(+) nodules while the activities of pyruvate decarboxylase (PDC), alc ohol dehydrogenase (ADH) and glutamate dehydrogenase (GDH) were higher in Fix(-) nodules. The data from immunodetection of host nodule prote ins confirmed the reduced levels of sucrose synthase and the almost co mplete absence of glutamine synthetase and leghaemoglobin in mutant no dules. There was no significant difference in the amount of nitrogenas e component 1 extracted from the microsymbiont of normal and ineffecti ve nodules, but component 2 was hardly detectable in the Fix(-) mutant .