Cluster roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) take up glycine intact:: An adaptive strategy to low mineral nitrogen in soils?

Background and Aims South African soils are not only low in phosphorus (P) but most nitrogen (N) is in organic form, and soil amino acid concentrations can reach 2.6 g kg(-1) soil. The Proteaceae (a main component of the South African Fynbos vegetation) and some Fabaceae produce cluster roots in response to low soil phosphorus. The ability of these roots to acquire the amino acid glycine (Gly) was assessed. Methods Uptake of organic N as C-13-N-15-Gly was determined in cluster roots and non-cluster roots of Leucadendron laureolum (Proteaceae) and Lupinus albus (Fabaceae) in hydroponic culture, taking account of respiratory loss of (CO2)-C-13. Key Results Both plant species acquired doubly labelled (intact) Gly, and respiratory losses of (CO2)-C-13 were small. Lupin (but not leucadendron) acquired more intact Gly when cluster roots were supplied with C-13-N-15-Gly than when non-cluster roots were supplied. After treatment with labelled Gly (C-13 : N-15 ratio = 1), lupin cluster roots had a C-13 : N-15 ratio of about 0.85 compared with 0.59 in labelled non-cluster roots. Rates of uptake of label from Gly did not differ between cluster and non-cluster roots of either species. The ratio of C : N and C-13 : N-15 in the plant increased in the order: labelled roots < rest of the root < shoot in both species, owing to an increasing proportion of C-13 translocation. Conclusions Cluster roots of lupin specifically acquired more intact Gly than non-cluster roots, whereas Gly uptake by the cluster and non-cluster roots of leucadendron was comparable. The uptake capacities of cluster roots are discussed in relation to spatial and morphological characteristics in the natural environment.