Nitrogen metabolism in the seagrass Zostera noltii

Tese de dout., Ciências do Mar (Ecologia Marinha), Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2011Nitrogen metabolism is a vital component in plant’s life. The main purpose of this thesis was to investigate the ecophysiology of nitrogen metabolism of the seagrass Zostera noltii in Ria Formosa lagoon. In the global change scenario, the effects of CO2 enrichment on photosynthesis, growth and nitrogen metabolism of Z. noltii were also investigated. Ammonium was identified as the preferential Ni source for Z. noltii, although nitrate uptake rates were considerable in the absence of ammonium. The ammonium uptake rates through the leaves were one order of magnitude higher than through the roots. Leaves were also identified as the main site for the reduction of ammonium and nitrate, as revealed by the much higher activity of the enzymes nitrate reductase and glutamine synthetase in the leaves than in the roots. The simultaneous supply of both Ni forms to Z. noltii enhanced the ammonium uptake rates and decreased the rates of nitrate uptake comparatively to the rates obtained when Ni forms were supplied separately. The uptake of ammonium or nitrate by one plant part (e.g. leaves) did not affect the uptake rate of the other plant part (e.g. roots), and no internal translocation of incorporated nitrogen was detected. The estimated whole-plant nitrogen budget of Z. noltii in the peak production season (spring) was slightly lower than the total nitrogen requirement for growth, which indicates that the growth of Z. noltii in the lagoon is only slightly limited by nitrogen. Z. noltii took up ammonium and nitrate at similar rates in the light and in the dark. In both light conditions, the nitrogen uptake displayed a temporal pattern of enhanced initial rates followed by lower but relatively constant rates. The uptake of nitrogen in the dark represented an additional use and mobilization of carbohydrate reserves. Z. noltii plants exposed to CO2-enriched conditions enhanced the photosynthetic rates while growth and ammonium uptake rates were not affected, suggesting that Z. noltii may benefit from future increases in seawater CO2 concentrations.Fundação para a Ciência e Tecnologia (FCT