Acclimation of photosynthesis to irradiance in Arabidopsis thaliana
The work contained within this thesis describes the acclimation of photosynthesis in A.thaliana to growth over a broad range of irradiance. Using Pmax and Chl a/b as an index, the ability of A.thaliana to modulate the composition of the photosynthetic apparatus across a range of irradiance has been demonstrated. A non-linear response to growth irradiance was seen for both parameters, the shape of which was similar. In addition the overall magnitude of the response was large compared to other plant species. These changes took place with no alteration in overall chlorophyll content per unit leaf area. A detailed analysis of the changes in chloroplast composition followed. This revealed a good correlation between changes in Chl a/b and bulk LHCII as well as Pmax and Rubisco content. However measurement of the reaction centre content along with immunoblot analysis of all ten Chl a/b binding light harvesting polypeptides established that changes in other thylakoid components were also responsible for changes in Chl a/b. Significant changes in reaction centre content were seen at both the low and high extremes of growth irradiance with PSI content doubling at 35 umol m-2 s-1 and PSII content rising significantly at 600 umol m-2 S-I. The extent of the changes in reaction content is well beyond those previously reported for other plant species. Other previously unreported changes in thylakoid composition are also observed for A.thaliana. For example there was a doubling in the minor LHCII complexes, Lhca5 and 6, at very low growth irradiance. In addition a complex pattern of change was observed for all 4 LHCI polypeptides. The functional consequence of photosynthetic acclimation was also investigated using room temperature chlorophyll fluorescence. Measurements of the maximum rate of electron transport (ETR) for plants grown at all irradiance revealed a higher rate for plants grown at 400 umol m-2 S-I than would have been predicted from the maximum photosynthetic rate (Pmax). This discrepancy suggested an alternative fate for electrons (other than CO2 fixation) for plants grown at this irradiance. Since this electron sink must involve molecular oxygen it is suggested that enhanced Mehler reaction accounts for the increased ETR. Relaxation kinetics of chlorophyll a fluorescence quenching was used to resolve qN into two components, qE and ql. The maximum capacity for qE clearly increased withincreasing growth irradiance and correlated well with Chl a/b and the xanthophyll cycle pool size establishing that the capacity for short term photosynthetic regulation is itself subject to acclimation. Finally the dynamic nature of photosynthetic acclimation was demonstrated following transfers between high and low irradiance.