Root of edaphically controlled Proteaceae turnover on the Agulhas Plain, South Africa: phosphate uptake regulation and growth

The influence of phosphorus (P) availability on growth and P uptake was investigated in South African Proteaceae: (1) Protea compacta R.Br., endemic on severely nutrient-impoverished colluvial sands; (2) Protea obtusifolia Bueck ex Meissner; and (3) Leucadendron meridianum I. J. Williams, the latter both endemic on comparatively fertile limestone-derived soils. Plants were grown hydroponically in 1000 L tanks at 0.01, 0.1 or 1.0 microm P for 14 weeks. Biomass accumulation was influenced by P availability, doubling as [P] increased from 0.1 to 1.0 microm. Total biomass was greatest for P. compacta, but L. meridianum and P. obtusifolia had two to four times greater relative biomass accumulation at 0.1 and 1.0 microm [P]. Proteoid root clusters developed at both 0.01 and 0.1 microm[P], but were suppressed at 1.0 microm [P]; this was a 10-fold lower [P] than previously reported to inhibit cluster root formation. Rates of net P uptake at 5 microm P decreased in response to increased P availability from 0.01 to 1.0 microm P. Significant between-species differences in rates of P uptake and capacity to down-regulate P uptake were observed: P. compacta < P. obtusifolia < L. meridianum. The species responses are discussed in terms of adaptation to mosaics in soil P availability and the high beta diversity in the natural habitat.