Phosphorus mobilization and localisation in the roots and nodules of a Cape Floristic Region legume, and its impact on nitrogen fixation

Vardien, Waafeka (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: During phosphorus (P) deficiency, plants can exhibit a wide array of morphological, physiological and biochemical responses. Legume plants are vulnerable to P deficiency, because it affects their ability to fix atmospheric nitrogen (N2) via their symbiotic interaction with rhizobial bacteria. In particular, legumes from nutrient poor ecosystems, such as the fynbos in the Cape Floristic Region (CFR) would have evolved on P deficient soils and may therefore display unique modifications. Moreover, since P distribution in soils is heterogenous, even less is known about the recovery from P deficiency responses in nodules. The aim of this research was to investigate P recycling and distribution in the nodules of the fynbos legume Virgilia divaricata, during low P supply and its recovery from P deficiency. The legume species was inoculated with a locally compatible N2 fixing bacterial strain, Burkholderia, isolated from V. divaricata nodules grown in fynbos soil. Plants were grown under glasshouse conditions, using a modified Long Ashton Nutrient Solution (LANS) to simulate the low nutrient conditions of the fynbos ecosystem. Plants were subsequently analysed for growth kinetics, nutrient acquisition and distribution, nodule anatomy, P recycling and P metabolite composition. The results indicated that V. divaricata can experience P deficiency during exposure to low P supply. Under low P conditions, plants experienced lower biomass and nodule production. Although biological N2 fixation (BNF) was lower during P deficiency as compared to during conditions of optimal P supply, the nodules of plants grown under P deficient conditions had a greater BNF per nodule mass and unit P. In addition, low P nodules also showed homogenous P tissue localisation and a greater concentration of Fe. The total P level was lower in nodule tissues, and the activities of phosphohydrolases (APase, RNase and phytase) higher. In addition, there was also a possible remobilization of membrane phospholipids, in order to release additional Pi. Although V. divaricata experiences P deficiency in its biomass and P nutrition, it also has a remarkable physiological ability to recover from P deficiency during P resupply. In contrast to the observed perturbations in biomass and nutrition during P stress, the impact on the nodules was different to that of the roots. The underlying mechanisms for functional maintenance of the nodules during low P seems to be associated with an internal mechanism, related to P mobilization from organic sources, metabolic bypass mechanisms to conserve P and a re-allocation of Fe to the infected cells. The higher enzyme activity of the internal phosphohydrolases (APase, RNase and phytase) favours the liberation of cellular P for metabolic reactions and internal P turnover. This research has generated knowledge regarding the physiological impact and flexibility of mechanisms involving below-ground P recycling in legumes. It has demonstrated that a legume from a nutrient poor ecosystem, favours internal mechanisms of P recycling and conservation, in order to maintain the efficient functioning of nodules under P stress rather than improve acquisition from external sources.

AFRIKAANSE OPSOMMING: Gedurende fosfor ( P) korting, kan plante 'n wye verskeidenheid van morfologiese, fisiologiese en biochemiese reaksies uit stal. Peulplante is sensitief vir P vermindering, deur hul vermoë om atmosferiese stikstof ( N2) te bind saam hul simbiotiese interaksie met rhizobiale bakterieë. In die besonder, sou peulplante van voedingstowwe swak ekosisteme, soos die fynbos in die Kaapse Floristiese Streek ontwikkel het op P gebrekkige grond en kan dus unieke wysigings vertoon. Verder, aangesien P verspreiding in gronde heterogene is, is nog minder bekend oor die herstel van P vermindering in wortel-knoppies . Die doel van hierdie navorsing was om P herwinning en verspreiding in die wortel-knoppies van Virgilia divaricata tydens lae P aanbod en sy herstel van P –tekort te bepaal. Virgilia divaricata is ingeënt met 'n lokale versoenbaar bakterië, Burkholderia. Plante is gekweek in gesteriliseerde sand, onder glasshuis voorwaardes, met 'n aangepaste Long Ashton voedingsoplossing om die lae voedingstowwe van die fynbos ekosisteem te simuleer. Plante is daarna ontleed vir groei kinetika, voedingstof verkryging en verspreiding, wortelknoppies anatomie, P herwinning en P metaboliet samestelling. Die resultate dui dat V. divaricata P-tekort kan ervaar tydens blootstelling aan lae P aanbod. Onder lae P voorwaardes, ervaar plante laer biomassa en wortel-knoppie produksie. Hoewel biologiese N2 fiksasie laer was in P-tekort plante, is biologiese N2 fiksasie per wortel-knoppie massa en eenheid P, meer. Die totale P vlak was laer in wortel-knoppie weefsel, en die aktiwiteite van phosphohydrolases (APase, RNase en fytase) hoër. Daarbenewens was daar ook 'n moontlike remobilization van membraan fosfolipiede, ten einde bykomende Pi vry te stel. Hoewel V. divaricata P-tekort ervaar in biomassa en P voeding, het dit ook 'n merkwaardige fisiologiese vermoë om te herstel van P-tekort. In teenstelling met die waargeneem versteurings in biomassa en voeding tydens P stres, was die impak op die wortel-knoppies anders as dié van die wortels. Die onderliggende meganismes vir funksionele instandhouding van die wortel-knoppie tydens lae P is in verband met 'n interne meganisme. Die hoër ensiemaktiwiteit van die interne phosphohydrolases bevoordeel die bevryding van sellulêre P vir metaboliese reaksies en interne P omset. Hierdie navorsing het kennis van die fisiologiese impak en buigsaamheid van meganismes wat onder-grond P herwinning gegenereer. Dit het getoon dat V. divaricata interne meganismes van P herwinning en bewaring, ten einde die doeltreffende funksionering van wortel-knoppies onder P stres te handhaaf eerder as verbeter verkryging van eksterne bronne.

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