Wheat (C3) and maize (C4) leaves
were exposed to light treatments that were limiting for
CO2 assimilation and which excite preferentially
photosystem I (PSI) or photosystem II (PSII) and induce State 1 or State 2,
respectively. In order to examine the relationships between linear electron
transport and CO2 in leaves during State transitions,
simultaneous measurements of CO2 assimilation,
chlorophyll fluorescence and absorbance at 820 nm were used to estimate the
quantum efficiencies of CO2 assimilation and PSII and
PSI photochemistry. In wheat leaves with photorespiratory activity, no
significant change in quantum efficiency of
CO2assimilation was observed during State transitions.
This was not the case when photorespiration was inhibited with either
2% O2 or 1000 ppm CO2 and
transition from State 2 to State 1 was accompanied by a large decrease
(c. 20%) in the quantum efficiency of
CO2 assimilation which was not associated with a
decrease in the quantum efficiency of electron transport through PSII.
Photorespiration appears to buffer the quantum efficiency of
CO2 assimilation from changes associated with decreases
in the rate of CO2 fixation resulting from imbalances in
PPFD absorption by PSI and PSII.
When maize leaves were subjected to similar State transitions, no significant
change in the quantum efficiency of CO2 assimilation was
observed on transition from State 2 to State 1, but on switching back to State
2 a very large decrease (c. 40%) was observed.
This decrease could be prevented if leaves were maintained in either 2%
O2 or 593 ppm CO2. The possible
occurrence of photorespiration in maize leaves on transition from State 1 to
State 2, which could result from an inhibition of the
CO2 concentrating mechanism, cannot account for the
decrease in the quantum efficiency of CO2 assimilation
since the relationship between PSII electron transport and
CO2 assimilation remained similar throughout the State
transitions. Also changes in the phosphorylation status of the
light-harvesting chlorophyll a/b protein associated with
PSII cannot be implicated in this phenomenon.
The relationship between CO2 assimilation, photosynthetic electron transport and water-water cycle in chill-exposed cucumber leaves under low light and subsequent recovery
