Nodules from Fynbos legume Virgilia divaricata have high functional plasticity under variable P supply levels

Abstract

Legumes have the unique ability to fix atmospheric nitrogen (N 2 ) via symbiotic bacteria in their nodules but depend heavily on phosphorus (P), which affects nodulation, and the carbon costs and energy costs of N 2 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 N 2 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 1 (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 N 2 -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.

Legume nodules from nutrient-poor soils exhibit high plasticity of cellular phosphorus recycling and conservation during variable phosphorus supply

“…Virgilia divaricata seeds (Silverhill Seeds, Kenilworth, South Africa) were germinated as described in Vardien et al (2014). Following the initial leaf emergence (Figure 1 a), seedlings were transferred to and inoculated with a locally sourced strain of Burkholderia.…”

“…Inoculum was prepared as previously documented (Vardien et al 2014). Three treatment categories, based on P concentration, were used: low P, high P (control), and resupplied P (four weeks of low P followed by three weeks of high P).…”

“…These equations were however modified to include nodules instead of roots for SPAR and whole plants for SPUR, in accordance with previous work by Vardien et al (2014).…”

“…The δ 15 N analyses were carried out as described in Vardien et al (2014). The values obtained were subsequently used to determine the percentage N derived from the atmosphere (NDFA) (Vardien et al 2014).…”

“…The values obtained were subsequently used to determine the percentage N derived from the atmosphere (NDFA) (Vardien et al 2014).…”