Growth and partitioning of C and fixed N in the shrub legumeAcacia littoreain the presence or absence of the root hemiparasiteOlax phyllanthi

Abstract

Nodulated plants of Acacia littorea were pot cultured singly in minus nitrogen sand culture in the presence or absence of a transplanted seedling of the root hemiparasite Olax phyllanthi and harvests of cultures made 4 and 8 months after introducing the parasite. Parasitism decreased host shoot growth while increasing root growth to a similar extent. Final shoot:root dry weight ratio was 2.2 for parasitized versus 4.3 for unparasitized Acacia. Partitioning of fixed N showed 4-fold larger N increments in shoots than roots of unparasitized plants, whereas parasitized plants lost a small amount of shoot N, made a root gain of N double that of unparasitized plants and lost over half of their N to Olax. The increment of fixed N in the host:parasite association was similar to that of unparasitized Acacia. Data on dry matter gain per unit foliage area and mean CO 2 assimilation rates per shoot of Olax and Acacia (parasitized or unparasitized) were discussed in relation to an estimated heterotrophic gain of xylem C from the host equivalent to 40% of the increment of dry matter C made by the parasite. Growth of Olax was accompanied by large increases in numbers of haustoria, 9% of which were attached to root nodules as opposed to roots. Structural and nutritional features of direct parasitism of nodules are described. Models of flow and utilization of C and N in the Acacia : Olax association and unparasitized Acacia are discussed in relation to published data for other host:parasite associations.