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 ChI alb 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 ChI alb and bulk LHCn as well as Pmax and
Rubisco content. However measurement of the reaction centre content along with
immunoblot analysis of all ten ChI alb binding light harvesting polypeptides established
that changes in other thylakoid components were also responsible for changes in ChI
alb. 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 Lllf'Il 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 with
increasing growth irradiance and correlated well with ChI alb 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.