Plant growth and N2 fixation in Cyclopia longifolia (Vogel L.) supplied with mineral nutrients in pot and field experiments

Abstract

Cyclopia longifolia Vogel L. is indigenous to the Cape fynbos, and widely grown for the production of Honeybush tea, a beverage with health benefits and economic potential in the cosmetic and nutraceutical industries. The aim of this study was to assess plant growth and symbiotic performance of Cyclopia longifolia in field and pot experiments. Field plants were supplemented with different levels of P, Mg and Ca at Kanetberg mountains using 0, 5, 25 and 50mM of K2HPO4, MgCl2·6H2O and CaCl2·2H2O in split application, while P, Ca and Mg were applied as Supergrow (20.3% P), CaMg·(CO3)2, and MgSO4·7H2O) at 0, 10, 20 and 50kg·ha−1 to potted plants. Shoots were harvested at 240 and 300days after last fertilisation for pot and field experiments respectively, and analyzed for nodulation and N2 fixation. The data revealed significantly increased nodule number, nodule dry weight, root and shoot biomass in the pot experiment with P, Ca and Mg supply. Shoot dry matter, N content, amount of N-fixed, soil N uptake and tea yield of C. longifolia was significantly increased with increasing supply of P, Mg and Ca to field plants, and was highest at 50mM level of each element. Applying different levels of Mn, Cu, Zn and Mo to field plants of C. longifolia also markedly increased shoot biomass, N content, δ15N, amount of N-fixed, soil N uptake and tea yield. The increase in δ15N values, and hence reduced %Ndfa, of Cyclopia longifolia plants supplied with P, Mg, Ca, Mn, Cu, Zn and Mo suggests that the endogenous soil concentration of these nutrients were more limiting for plant growth than for nodule functioning. As a result, supplying P, Mg, Ca, Mn, Cu, Zn and Mo increased plant growth and biomass, but not %N derived from fixation.