EFFECTS OF SOLUTION PH AND BICARBONATE ON THE GROWTH AND NODULATION OF A RANGE OF GRAIN LEGUME SPECIES

Defining plant adaptation to soil conditions is critical for the succe ssful introduction of grain legume species into farming systems. This nutrient solution study examined the effects of pH (4, 5, 6, 7 and 8) and bicarbonate (5 mM KHCO3) on the growth and nodulation of 14 grain legume species supplied with N or reliant on N-2-fixation. Species inc luded Pisum sativum L., Cicer arietinum L., Lens culinaris Med., and a range of Lupinus, Vicia and Lathyrus species. Species differed greatl y in response to solution pH. For both N-fertilized and N-2-fixing pla nts, shoot growth of L. culinaris was very sensitive to low pH (pH < 7 ), whereas shoot growth of Lupinus angustifolius L. and Lupinus albus L. was sensitive to higher pH (pH greater than or equal to 6). Other s pecies had a broader optimal pH range for growth when supplied with N, but were generally sensitive to low pH (pH < 7 for C. arietinum and V icia sativa L., pH < 6 for P. sativum, Vicia faba L., Lathyrus sativus L. and Lathyrus cicera L., and pH < 5 for Vicia benghalensis L. and V icia narbonensis L.) when reliant on N-2-fixation. For these other spe cies, symbiotic N-2-fixation appeared to be more sensitive than host p lant growth to low pH. This finding was supported by lower nodule numb ers and mass, and lower N concentrations in shoots of sensitive specie s at low pH relative to higher pH. For L. culinaris, nodule numbers an d mass were relatively unaffected by pH 5-8, N concentrations in shoot s were high at low pH and plants developed symptoms relating to H+ tox icity at pH as high as 7. These results indicate that host plant growt h of L. culinaris is more sensitive to low pH than the Rhizobium symbi osis. For L. albus and L. angustifolius, both host plant growth and sy mbiotic N-2-fixation appeared to be equally sensitive to pH greater th an or equal to 6. Lupinus pilosus Murr. was more tolerant of high pH t han the other Lupinus species. At pH 4, two genotypes of C. arietinum had better early nodulation than other species. Vicia ervilia L. nodul ated poorly at all levels of solution pH, indicating that the commerci al Group E inoculum (Rhizobium leguminosurum by, viceae SU303) may not be effective for this species in solution culture. Addition of bicarb onate decreased shoot growth, nodulation and N concentrations in shoot s of most species. Early nodulation (nodule number) of Lathyrus ochrus (L.) DC was not affected by the bicarbonate treatment.