Foliar flavonol concentration in Sclerocarya birrea saplings responds to nutrient fertilisation according to growth-differentiation balance hypothesis

The growth-differentiation balance hypothesis (GDBH) postulates a nonlinear, 'hump-shaped' response of plant secondary metabolites (PSMs), such as flavonoids, in relation to nitrogen (N) availability. The response is most easily detected when there are at least five levels of N availability, but very few studies that directly or indirectly tested the GDBH have used three or more levels. Fifty Sclerocarya birrea (marula) saplings (mean basal diameter=35.5 mm, SEM=0.52; mean height=80.9 cm, SEM=1.02), were randomly assigned to five fertiliser application rates in an orchard. Fertilizer [N:P:K (3: 1: 5) (26)] containing slow release nitrate was applied to each sapling as a top dressing every two months during the growing season at rates equivalent to 0, 20, 40, 80 and 120 g Nm(-2). Average flavonoid content of 30-35 recently matured leaves per sapling was indirectly measured every two months during the growing season using the Dualex (R) Series 4 instrument. The nonlinear, 'hump-shaped' response predicted by GDBH was observed, indicating that allocation of carbon to PSMs was more limited at both high and low nutrient availability than at intermediate nutrient availability. Successful detection of the quadratic response was probably facilitated by only measuring flavonols, which are the main PSMs in S. birrea, and by conducting the experiment on soils of very low fertility.