Abstract:
Legumes have the unique ability to fix atmospheric nitrogen (N2) via symbiotic bacteria in their nodules
but depend heavily on phosphorus (P), which affects nodulation, and the carbon costs and energy
costs of N2 fixation. Consequently, legumes growing in nutrient-poor ecosystems (e.g., sandstone-derived
soils) have to enhance P recycling and/or acquisition in order to maintain N2 fixation. In this study,
we investigated the flexibility of P recycling and distribution within the nodules and their effect on N
nutrition in Virgilia divaricata Adamson, Fabaceae, an indigenous legume in the Cape Floristic Region
of South Africa. Specifically, we assessed tissue elemental localization using micro-particle-induced
X-ray emission (PIXE), measured N fixation using nutrient concentrations derived from inductively coupled
mass-spectrometry (ICP-MS), calculated nutrient costs, and determined P recycling from enzyme
activity assays. Morphological and physiological features characteristic of adaptation to P deprivation
were observed for V. divaricata. Decreased plant growth and nodule production with parallel increased
root:shoot ratios are some of the plastic features exhibited in response to P deficiency. Plants resupplied
with P resembled those supplied with optimal P levels in terms of growth and nutrient acquisition. Under
low P conditions, plants maintained an increase in N2-fixing efficiency despite lower levels of orthophosphate
(Pi) in the nodules. This can be attributed to two factors: (i) an increase in Fe concentration under
low P, and (ii) greater APase activity in both the roots and nodules under low P. These findings suggest
that V. divaricata is well adapted to acquire N under P deficiency, owing to the plasticity of its nodule
physiology
