Do cluster roots contribute to the costs of carbon and nitrogen metabolism during variations in phosphate supply in the legume Lupinus albus

Thuynsma, Rochelle (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: The generally low concentrations of P and N in the soil, causes most plants to experience nutrient deficiency during their life cycle. Lupins can rely on both cluster roots and nodules for P acquisition and biological nitrogen fixation (BNF) respectively. The legume Lupinus albus is able to survive under low nutrient conditions, because it has two specialized belowground organs for the acquisition of N and P. In this regard, cluster roots increase P uptake and root nodules acquire atmospheric N2 via biological nitrogen fixation (BNF). Although these organs normally tolerates low P conditions, very little is known about their physiological and metabolic flexibility during variations in P supply. Furthermore, the resource allocation (C, N and P) between cluster roots and nodules has also been largely understudied. The aim of this investigation was therefore to determine the resource allocation, physiological and metabolic flexibility of these organs during variations in P supply. Although variation on P supply had no effect on the total biomass, there were significant differences in specialised below-ground organ allocation to cluster roots and nodule formation. Cluster root formation and the associated C-costs increased during low P supply. In contrast to the cluster root decline at high P supply, there was an increase in nodule growth allocation and corresponding C-costs. Since cluster roots were able to increase P acquisition under low P conditions, this below-ground investment may also have benefited the P nutrition of nodules. These findings provide evidence that when lupins acquire N via BNF in their nodules, there may be a trade-off in resource allocation between cluster roots and nodules. The short-term elevated P supply, caused an increased allocation of C and respiratory costs to nodules, at the expense of cluster roots. This alteration was also reflected in the increase in nodule enzyme activities related to organic acid synthesis, such as Phosphoenol-pyruvate Carboxylase (PEPC), Pyruvate Kinase (PK), Malate Dehydrogenase (NADH-MDH) and Malic Enzyme (ME). In cluster roots, the elevated P conditions, caused a decline in these organic acid synthesizing enzymes. This suggests that during short-term elevated P supply, there is a great degree of physiological and metabolic flexibility in the lupin nutrient acquiring structures.

AFRIKAANSE OPSOMMING: Die algemeen lae konsentrasies van fosfaat en stikstof in die grond , veroorsaak dat die meeste plante voedingstekorte ervaar tydens hul lewensiklus. Lupiene kan staatmaak op beide groep-wortels en wortel-knoppies vir P verkryging en biologiese stikstofbinding onderskeidelik. Die peulplant Lupinus albus is in staat om te oorleef onder lae voedings toestande , as gevolg van die twee gespesialiseerde ondergrondse organe vir die verkryging van stikstof en fosfaat. In hierdie verband verhoog groep-wortels fosfaat opname en wortel-knoppies verkry atmosferiese stikstof via biologiese stikstofbinding. Alhoewel hierdie organe normaalweg lae fosfaat toestande verdra , is baie min bekend oor hul fisiologiese en metaboliese buigsaamheid tydens variasies in fosfaat aanwending. Daar is verder ‘n tekort aan die studie van hulpbron toekenning tussen groep-wortels en wortel-knoppies. Die doel van hierdie ondersoek was dus om die toekenning van hulpbronne , fisiologiese en metaboliese buigsaamheid van hierdie organe tydens variasies in fosfaat aanwending te bepaal. Variesie in fosfaat verskaffing het geen invloed op die totale plant biomassa gehad nie, maar daar was wel ‘n beduidende verskil in gespesialiseerde ondergrondse toekenning tussen groep- wortels en wortel-knoppies. Groep-wortel vorming en die gepaardgaande koolstof koste het toegeneem met lae fosfaat verskaffing. In teenstelling met die groep-wortel daling met hoë fosfaat verskaffing, was daar 'n toename in groei van wortel-knoppies en die ooreenstemmende koolstof koste daarvan. Aangesien groep-wortels in staat was om fosfaat verkryging te verhoog onder lae fosfaat toestande, mag hierdie ondergrondse belegging bygedra het tot die voeding van wortel-knoppies . Hierdie bevindings bewys dat lupiene afhanklik van wortel-knoppies ‘n wisselwerking in toekenning van hulpbronne, tussen groep-wrotels en wortel-knoppies handaaf. Kort termyn verhoogde fosfaat aanwending veroorsaak 'n verhoogde toekenning van koolstof en respiratoriese energie na wortel-knoppiess, ten koste van groep-wortels . Hierdie verandering is ook weerspieël in die toename in wortel-knoppie ensiem aktiwiteit in verband met organiese suur sintese (PEPC PK,MDH,ME) . In groep-wortels, het die verhoogde P toestande verder 'n afname in die organiese suur produserende ensieme veroorsaak. Dit dui aan dat tydens kort termyn verhoogde P aanwending, daar 'n groot mate van fisiologiese en metaboliese buigsaamheid in die lupiene voedingstowwe verkryging strukture plaasvind.

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