Abstract:

The role of tissue acid phosphatase (APase) activity of legumes and non-legumes in their P nutrition and adaptation to low-P soils is not well understood. To better understand this, a relationship between APase activity and P concentration in leaves, stems, roots and nodules of legumes, Cyclopia and Aspalathus and a non-legume, Leucadendron strictum, all native to the P-poor soils of the Cape fynbos biome, was assessed.

Plants were collected and each separated into leaves, stems and roots. Phosphatase enzyme activity was assayed in soil using the p-nitrophenol method, while soil P and shoot P were measured using ICP-MS. To measure tissue APase activity, an acetate buffer was added into ground plant material and contents filtered. An acetate buffer and a p-nitrophenyl solution were added to the supernatant and contents incubated. After incubation, NaOH (0.5 M) was added and absorbance read at 405 nm.

At Koksrivier, Cyclopia genistoides exhibited the highest leaf enzyme activity whilst Aspalathus aspalathoides showed the highest enzyme activity in the stems. At both Kleinberg and Kanetberg, Cyclopia subternata and Cyclopia longifolia showed the highest APase activity in leaves, followed by stems and lowest in roots. P concentration closely mirrored enzyme activity in organs of all test species from each site. APase activity positively correlated with P concentration in organs of all the test Cyclopia and Leucadendron species, indicating that intracellular APase activity is directly linked to P mobilization and translocation in these species. Percentage of N derived from fixation was positively correlated with tissue APase activity in C. genistoides (r = 0.911*), A. aspalathoides (r = 0.868*) and Aspalathus caledonensis (r = 0.957*), suggesting that APase activity could be directly or indirectly linked to symbiotic functioning in these fynbos legumes, possibly via increased P supply to sites of N₂ fixation.

Key words: Cape fynbos, δ¹⁵N, Leucadendron strictum, percent N derived from fixation, P nutrition