School of Botany - Theses

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Type: Masters Research thesis × Subject: Photosynthesis. ×

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    Role of Ribulose-1,5-Bisphosphate Carboxylase
    Kelly, Michael (Michael Ernest) (University of Melbourne, 1998)
    Physiological adaptations by plants in order to better utilise sunflecks were studied using plants from the rainforest understorey. Transgenic Rubisco and wild-type Nicotiana tabacum L., from the Solanaceae family, were used to model these adaptations and to better characterise the light induced activation kinetics. Tobacco plants with varying Rubisco concentrations were used to obtain plants with differing Activase to Rubisco stoichiometry. An increase in the stoichiometry caused proportional changes in the rate of Rubisco activation. This was reflected in a decrease in the relaxation times from 2.5 to 3 minutes down to 35 to 40 seconds. This faster rate of activation lead to a reduction in the amount of C02 assimilation effectively forgone due to the activation process. In wild-type tobacco this amount of forgone photosynthesis was found to be, on average, 23.5 ?mol C02 m-2 compared to 6.0 ?mol C02 m-2 for Rubisco mutant tobacco plants. Understorey plants from the rainforest of Cape Tribulation National Park (QLD) were examined to see if they adjusted their stoichiometry of Activase and Rubisco in order to increase C02 assimilation in sunflecks. Gas exchange analysis of the rainforest plants showed them to behaved in a similar fashion to the Rubisco mutant tobacco. Examination of the kinetics of Rubisco activation in rainforest and Rubisco mutant tobacco plants found, on average, comparable relaxation times (58 and 67 seconds, respectively) and similar initial activation rates (46.6 and 46.1 nmol active sites m-2 s-1, respectively). When compared to wild-type tobacco, the rainforest plants on average were found to have over 5.5 times the ratio of Activase to Rubisco. This leads to a reduction in the average amount of forgone photosynthesis from 23.5 ?mol C02 m-2 for wild-type to 4.2 ?mol C02 m-2 for rainforest plants. These results suggest that plants grown under fluctuating light environments alter their stoichiometry in order to better utilise sunflecks for carbon gain.