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, r esulting in stunted plants that often develop lime-induced chlorosis. In co ntrast, some genotypes of L. pilosus Murr. occur naturally in calcareous so ils without developing any visible symptoms of stress. Some genotypic varia tion for tolerance to calcareous soil does exist in L. albus and the tolera nce mechanisms need to be determined. The adaptation through root system mo rphological plasticity of L. albus and L. pilosus, to heterogeneous limed s oil profiles (pH 7.8) containing either patches of acid (non-limed) soil, o r vertically split between acid and limed soil, was investigated. When grow n 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 ho mogeneous limed soil. Total root dry matter in the acid-soil patches was gr eater than in the control limed-soil patches. This was due to a four-fold i ncrease 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 citr ic 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. pilo sus responded to the patches of acid soil in a manner comparable with L. al bus. When grown in the homogeneous limed soil, L. pilosus had a greater max imum net CO2 assimilation rate (P-max) than L. albus, however, the P-max of both species increased after they had accessed a patch of acid soil. Diffe rences were apparent between the L. albus genotypes grown in soil profiles split vertically into limed and acid soil. A genotype by soil interaction o ccurred in the partitioning between soils of the cluster roots. The genotyp e La 674 was comparable with L. pilosus and produced over 11% of its cluste r roots in the limed soil, whereas the other genotypes produced only 1-3% o f 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 re spond to a heterogeneous soil by producing mainly cluster roots in an acid- soil patch.