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 successful 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 included Pisum sativum L., Cicer arietinum L., Lens culinaris Med., and a range of Lupinus, Vicia and Lathyrus species. Species differed greatly in response to solution pH. For both N-fertilized and N-2-fixing plants, 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 species 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 Vicia 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 Vicia narbonensis L.) when reliant on N-2-fixation. For these other species, symbiotic N-2-fixation appeared to be more sensitive than host plant growth to low pH. This finding was supported by lower nodule numbers and mass, and lower N concentrations in shoots of sensitive species at low pH relative to higher pH. For L. culinaris, nodule numbers and mass were relatively unaffected by pH 5-8, N concentrations in shoots were high at low pH and plants developed symptoms relating to H+ toxicity at pH as high as 7. These results indicate that host plant growth of L. culinaris is more sensitive to low pH than the Rhizobium symbiosis. For L. albus and L. angustifolius, both host plant growth and symbiotic N-2-fixation appeared to be equally sensitive to pH greater than or equal to 6. Lupinus pilosus Murr. was more tolerant of high pH than the other Lupinus species. At pH 4, two genotypes of C. arietinum had better early nodulation than other species. Vicia ervilia L. nodulated poorly at all levels of solution pH, indicating that the commercial Group E inoculum (Rhizobium leguminosurum by, viceae SU303) may not be effective for this species in solution culture. Addition of bicarbonate decreased shoot growth, nodulation and N concentrations in shoots of most species. Early nodulation (nodule number) of Lathyrus ochrus (L.) DC was not affected by the bicarbonate treatment.