Investigations into the exploitation of heterogeneous soils by Lupinus albus L. and L. pilosus Murr. and the effect upon plant growth

In calcareous soils, genotypes of Lupinus albus L. generally grow poorly, resulting in stunted plants that often develop lime-induced chlorosis. In contrast, some genotypes of L. pilosus Murr. occur naturally in calcareous soils without developing any visible symptoms of stress. Some genotypic variation for tolerance to calcareous soil does exist in L. albus and the tolerance mechanisms need to be determined. The adaptation through root system morphological plasticity of L. albus and L. pilosus, to heterogeneous limed soil profiles (pH 7.8) containing either patches of acid (non-limed) soil, or vertically split between acid and limed soil, was investigated. When grown in the presence of patches of acid soil, L. albus had a 52% greater shoot dry weight and visibly greener leaves compared with plants grown in the homogeneous limed soil. Total root dry matter in the acid-soil patches was greater than in the control limed-soil patches. This was due to a four-fold increase in the cluster root mass, accounting for 95% of the root dry matter in the acid-soil patch. Although these cluster roots secreted no more citric acid per unit mass than those in the limed soil did, their greater mass resulted in a higher citrate concentration in the surrounding soil. L. pilosus responded to the patches of acid soil in a manner comparable with L. albus. When grown in the homogeneous limed soil, L. pilosus had a greater maximum net CO2 assimilation rate (Pmax) than L. albus, however, the Pmax of both species increased after they had accessed a patch of acid soil. Differences were apparent between the L. albus genotypes grown in soil profiles split vertically into limed and acid soil. A genotype by soil interaction occurred in the partitioning between soils of the cluster roots. The genotype La 674 was comparable with L. pilosus and produced over 11% of its cluster roots in the limed soil, whereas the other genotypes produced only 1–3% of their cluster roots in the limed soil. These results indicate L. pilosus is better adapted to the limed soil than L. albus, but that both species respond to a heterogeneous soil by producing mainly cluster roots in an acid-soil patch.