Photosynthesis in relation to leaf nitrogen and phosphorus content in Zimbabwean trees.
CO2assimilation in relation to light intensity and the relationship between leaf nitrogen and phosphorus concentrations and CO2assimilation in 14 species of ecologically important Zimbabwean trees were examined. Eight of the species are members of the Fabaceae (Leguminosae). In the majority of Zimbabwean climax woodlands, the dominant trees are non-nodulating members of the sub-family Caesalpinioideae. The species examined have higher light saturation points (>700 μmol m-2s-1) than woody species from temperate areas; one species, Acacia nigrescens, did not reach saturation at photon fluxes greater than 1500 μmol m-2sec-1. Higher leaf nitrogen content was found to correlate positively with higher CO2assimilation rates (r=0.85; P≦0.0003); there was no correlation between leaf phosphorus content and CO2uptake rates. There were no significant differences between sites in terms of leaf nitrogen or phosphorus content, but the mean photosynthetic rate at one of the sites (Chizedzi) was lower. Taxa from the nodulating legumes were found to have higher leaf nitrogen contents (309.1±SD 22 mmol m-2) than those of the non-nodulating species (239±33); the lowest nitrogen contents were found in nonleguminous trees (179±42), with the exception of Ziziphus mucronata. This species may form an association with an N2-fixing actinomycete.
Department of Biological Sciences, University of Zimbabwe, Mount Pleasant, P.O. Box MP 167, Harare, Zimbabwe.
Janet M
JM
Tuohy
Plant Technology Section, Department of Biology, Imperial College, Prince Consort Road, SW7 2BB, London, UK.
Juliet A B
JA
Prior
Biology Department (Darwin Building), University College London, Gower Street, WC1 6BT, London, UK.
George R
GR
Stewart
Leaf nitrogen; Leaf phosphorus; Photosynthesis; Trees; Zimbabwe