The effect of an elevated atmospheric CO(_2) concentration on secondary metabolism and resource allocation in Plantago maritima and Armeria maritima
There are relatively few reports concerning carbon and nitrogen partitioning into secondary metabolism in plants grown in elevated atmospheric CO(_2). This thesis investigates the effects of changes in atmospheric CO(_2) and soil nitrogen availability on compatible solute accumulation and resource allocation in Plantago maritima and Armeria maritima. It was hypothesised that contrasting metabolic responses to abiotic stress cause a species-specific response in P. maritima and A. maritima in resource allocation. In response to drought, P. maritima accumulates the carbon based compatible Solute sorbitol, whilst A. maritima accumulates the nitrogen based solutes betaine and proline. Over ten weeks, A. maritima was more responsive to changes in C and N availability than P. maritima, especially in the amount of C fixed into leaf matter. After elevated CO(_2) exposure for one year, the biomass of P. maritima increased and of A. maritima decreased, compared to the respective plants at ambient CO(_2). Compatible solute concentrations were increased by drought at ambient CO(_2) but decreased in droughted plants at elevated CO(_2). This was hypothesised to be due to lower transpiration rates. Rates of transpiration in P. maritima decreased in response to elevated CO(_2) but A. maritima did not. Tissue water-potential was also lowered in response to elevated CO(_2). This thesis also examines resource allocation to other major C and N sinks. In P. maritima grown at elevated CO(_2) for one year, total soluble phenolic concentrations increased and the concentration of soluble protein decreased. In A. maritima, phenolic concentrations decreased and protein concentrations increased. The effect of enhanced CO(_2) on the secondary metabolism of P. maritima was investigated in greater detail. Six phenolics were identified using metabolite profiling, namely p-coumaric acid, ferulic acid, caffeic acid, verbascoside, plantamajoside and luteolin. P. maritima exposed to elevated CO(_2) had higher concentrations of some of these individual phenolic compounds and histochemical analysis identified increases in the number of lignified vessels and a decrease of lignified vessel-wall thickness.