Browsing by Author "Groenewald, Lida-Mari"

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    The development of phosphate stress and its functional consequences in the model legume plant Medicago truncatula
    (Stellenbosch : Stellenbosch University, 2016-12) Groenewald, Lida-Mari; Valentine, Alexander J.; Kleinert, Aleysia; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Phosphate is an abundant nutrient in the soil; however it is mostly bound to other elements that make phosphate unavailable for plant uptake. This bound state makes phosphate the second most limiting nutrient for plant growth. Phosphate is also a nonrenewable mined resource that forms a major constituent of fertiliser given to crops grown in nutrient poor soils. The second most important crop family in agriculture is Leguminosae. In an attempt to to reduse possible nitrogen stress, legumes can form a symbiosis with nitrogen-fixing soil bacteria. This symbiosis, found in the nodules, exchanges fixed nitrogen with host photosynthate and phosphate. The nodules are thus a phosphate sink that place stress on the rest of the plant. Legumes have adapted different ways to optimise the limited available phosphate to continue their own growth while maintaining the adenosine-triphosphate expensive nitrogen-fixing reaction. In this study, we looked at how the genetic model legume, Medicago truncatula Gaertner, has adapted to phosphate stressed conditions as it relied solely on biological nitrogen fixation as a source of nitrogen. In the first treatment, Medicago truncatula seedlings were infected with Sinorhizobium meliloti and received a low concentration of phosphate throughout the growth period. This was done to simulate Medicago truncatula growing in already phosphate deprived soils. The comparisons of biomass and growth, internal free phosphate concentrations, and organic acid and acid phosphatases enzyme activities were done on the above versus below ground tissues. Photosynthesis parameters were also recorded. Above ground tissues responded to phosphate stress with increased activity of bypass enzymes at the steps that required adenosine-triphosphate. While the below ground tissues focused on using acid phosphatases to recycle phosphate. Although the rate of photosynthesis had decreased in the phosphate stressed plants, the efficiency of photosynthesis with the phosphate that was available in the leaves had increased. The second treatment involved the growth of nodulated Medicago truncatula with an optimal phosphate concentration, followed by an induced phosphate stress period. In this manner, soil that had been depleted of phosphate during plant growth was simulated. With the addition of determining differences in activities of nitrogen assimilating enzymes, the above-mentioned comparisons were made on the nodules and roots of the sample plants. Under the induced stress condition, available phosphate was concentrated to the nodules. A possible cause for this was the increase in activity of the organic acid synthesising enzymes present in the nodule. The nitrogen assimilating enzyme activities indicated that stressed nodules may export glutamine rather than asparagine to the roots. Root nitrogen assimilating enzyme activities remained relatively constant during phosphate stress. Reduced nitrogen and carbon content of stressed plants indicated that phosphate had a direct impact on nitrogen fixation. From this study, we deduced that above ground tissues adapted metabolically for improved photosynthesis phosphate use efficiency; while below ground tissues recycle the available phosphate to be used for nitrogen-fixation. After the induction of phosphate stress it was found that the nodules relied on saving available phosphate for nitrogenfixation, while the roots recyled assimilated glutamine to maintain function.