Kirschbaum, M.U.F., Oja, V. and Laisk, A. (2005). The quantum yield of CO2 fixation is reduced for several minutes after prior exposure to darkness. Exploration of the underlying causes. Plant Biology 7: 58-66.

Abstract. Previous work has shown that the apparent quantum yield of CO2 fixation can be reduced for up to several minutes after prior exposure to very low light levels. In the work reported here, we investigated this phenomenon more fully and deduced information about the underlying processes. This was done mainly by concurrent measurements of oxygen and CO2 exchange in an oxygen-free atmosphere.

Measurements of oxygen evolution indicated that photochemical efficiency was not lost through dark adaptation, and that oxygen evolution could proceed immediately at high rates provided that there were reducible pools of photosynthetic intermediates. Part of the delay in reaching the full quantum yield of CO2 fixation could be attributed to the need to build up pools of photosynthetic intermediates to high enough levels to support steady rates of CO2 fixation. There was no evidence that Rubisco inactivation contributed towards delayed CO2 uptake (under conditions of low light).

However, we obtained evidence that an enzyme in the reaction path between triose phosphates and RuBP must become completely inactivated in the dark. As a consequence, in dark-adapted leaves, a large amount of triose phosphates were exported from the chloroplast over the first minute of light rather than converted to RuBP for CO2 fixation. That pattern was not observed if the pre-incubation light level was increased to just 3-5 µmol quanta m-2 s-1.

The findings from this work underscore that there are fundamental differences in enzyme activation between complete darkness and even the very low light level of only 3-5 µmol quanta m-2 s‑1 which predispose leaves to different gas-exchange patterns once leaves are transferred to higher light levels.


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