Variable P supply affects N metabolism in a legume tree, Virgilia divaricata, from nutrient-poor Mediterranean-type ecosystems

Abstract

Virgilia divaricata Adamson is a forest margin legume that is known to invade the N- and P-poor soils of the mature fynbos, implying that it tolerates variable soil N and P levels. It is not known how the legume uses inorganic N from soil and atmospheric sources under variable P supply. Little is known about how P deficiency affects the root nodule metabolic functioning of V. divaricata and the associated energy costs of N assimilation. This study aimed to determine whether P deficiency affects the metabolic status of roots and nodules, and the impact on the routes of N assimilation in V. divaricata.V. divaricata had reduced biomass, plant P concentration and biological nitrogen fixation during P deficiency. Based on adenylate data, P-stressed nodules maintained their P status better than P-stressed roots. V. divaricata was able to alter C and N metabolism differently in roots and nodules under P stress. This was achieved via internal P cycling by possible replacement of membrane phospholipids with sulfolipids and galactolipids, and increased reliance on the pyrophosphate (PPi)-dependent metabolism of sucrose via UDP-glucose (UDPG) and to fructose-6-phosphate (Fru-6-P). P-stressed roots mostly exported ureides as organic N and recycled amino acids via deaminating glutamate dehydrogenase. In contrast, P-stressed nodules largely exported amino acids. Compared with roots, nodules showed more P conservation during low P supply. The roots and nodules of V. divaricata metabolised N differently during P stress, meaning that these organs may contribute differently to the success of this plant in soils from forest to fynbos.