Responses of sugarcane to aluminium toxicity

Rowland, Ronelle (2006-03)

Thesis (MSc)--University of Stellenbosch, 2006.

Thesis

ENGLISH ABSTRACT: The aims of this study were (1) to determine whether N03- orNH4+ nutrition could influence the effect of Al on N12 and N19 sugarcane plants grown in sand or hydroponic medium and (2) to assess whether the different root environments of sand and hydroponically cultured plants could influence the effect of Al on Nl2 and N19 sugarcane plants supplied with N03- or NH/ nutrition. N12 and N19 sugarcane was grown in sand and hydroponic culture with and without Al and with either N03- or NH4+as N source. Biomass accumulation, tissue N, P and Al and root assimilation of 14C02 supplied to the root system were measured. Both N12 and N19 sugarcane plants were found to be relatively Al tolerant (tolerating up to 1 mM Al). This lack of effect of Al on plant growth might be due to amelioration of Al-toxicity in sugarcane by the adequate supply of carbon skeletons from the C4 photosynthetic pathways of sugarcane. The supply of carbon skeletons may enable both cultivars to exude large amounts of organic acids into the rhizosphere, which confers a dual advantage to these plants. Organic acids can form soluble complexes with Al thus preventing its entry into the roots and can form soluble complexes with nutrients (cations), which makes some nutrients (e.g. P) more available for plant uptake. The availability of carbon in the root system is dependent on the N source. Increased growth of Al treated plants supplied with NH4 +, relative to those grown on N03-, might be due to the capacity of C4 photosynthesis to meet the needs for both the assimilation of NH4 + into amino acids and the synthesis/excretion of organic acids for Al-detoxification. The fact that growth was improved with NNH4 + and Al may indicate that NH4 + and Al cations compete for access to the root tissue resulting in an increase in root activity, organic acid exudation and nutrient uptake. It was postulated that sand-grown plants would be less influenced by Al than hydroponic plants because organic acids can form a protective sheath that shields the root apex from the toxic Al cations in sand due to the relative lack of mobility of the soil solution. However, Al increased the growth of NH4 +-fed hydroponically grown plants more than that of NH4 +-fed sand grown plants. Thus we did not find evidence to support our expectation that the roots of the hydroponically grown plants would be more exposed to Al due to nutrients and organic acids being uniformly distributed in the growth solution compared to sand grown plants.

AFRIKAANSE OPSOMMING: Die doelwitte van die studie was om te bepaal of ( 1) N03- of NH4+ voeding die effek van Al beinvloed op N12 en N19 suikerriet plante wat gegroei is in sand of hidroponiese medium en (2) te bepaal of die verskillende wortel omgewings van sand en hidroponiese gekultureerde plante die effek van Al op N12 en N19 suikerriet plante voorsien van N03- of NH4 + voeding. N12 and N19 suikerriet plante was gegroei in sand en hidroponiese kulture met en sonder Al en met eerder N03- of NH4+ as N bron. Biomasse ophoping, weefsel N, P en Al, wortel assimilasie van 14C02 gevoorsien aan die wortel was gemeet. Beide N12 en N19 suikerriet plante was bevind om relatief Al tolerant (tolerantsie tot 1 mM Al) te wees. Die geringe effek van Al op plant groei mag toegeskryf word aan die verbetering van Al-toksisiteit in suikerriet plante deur die genoegsame voorsiening van koolstof geraamtes van die C4 fotosintetiese paaie van suikerriet. Die voorsiening van koolstof geraamtes mag beide kultivaars toelaat om groot hoeveelhede organiese sure uit te skei in die rhisosfere, wat 'n tweedoelige voordeel gee aan die plante. Organiese sure kan oplosbare komplekse met Al vorm wat toegang tot wortel voorkom en kan ook oplosbare komplekse met voedingstowwe (katione) vorm wat party voedingstowwe (bv. P) meer geredelik beskikbaar maak vir opname. Die beskikbaarheid van koolstowwe in die wortel sisteem is athanklik van die N broil. Verhoogde groei van Al behandelde plante voorsien van NH/, relatief tot die gegroei in N03- mag te wyte wees aan die vermoe van die C4 fotosintese om die behoeftes te voorsien van beide die assimilasie van NH4+ in die aminosure en die sintese/uitskeiding van organiese sure vir Al-detoksifisering. Die feit dat groei verbeter het met NH4 +en Al mag 'n indikasie wees dat NHi +en Al katione kompeteer vir toegang tot die wortel weefsel wat meebring 'n verhoging in wortel aktiwiteit, organiese suur uitskeiding en voedingstof opname. Dit was gepostuleer dat sand-gegroeide plante minder beMoed word deur Al as hidroponiese plante omdat organiese sure vorm 'n beskermende skede wat die wortel punt beskerm teen die giftige Al katione in sand as gevolg van die relatiewe gebrek aan beweging van die sand oplossing. Alhoewel, Al die groei verhoog van NH4 + - gevoerde hidroponiese gegroeide plante meer as die van NH4 + -gevoerde sand gegroeide plante. Ons vind dus bewyse om ons verwagting te ondersteun dat die wortels van hidroponies gegroeide plante sal meer blootgestel wees aan Al as gevolg van voedingstowwe en organiese sure wat eenvormig verspreid is in die groei oplossing vergelyke met die van sand gegroeide plante.

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