The Quantum Yield of CO₂ Fixation is Reduced for Several Minutes after Prior Exposure to Darkness. Exploration of the Underlying Causes

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 darkness. In the work reported here, we investigated this phenomenon more fully and have deduced information about the underlying processes. This was done mainly by concurrent measurements of O₂ and CO₂ exchange in an oxygen-free atmosphere. Measurements of O₂ evolution indicated that photochemical efficiency was not lost through dark adaptation, and that O₂ 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 measurement 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 being 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 a 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.

Key words

Helianthus annuus - induction - lightfleck - photosynthesis - quantum yield

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Editor: R. Monson