Critical examination of the quantitative evidence for and against photosynthetic CO2 fixation by guard cells

Publisher Website
Google Scholar
Abstract
The evidence for and against photosynthetic CO2 fixation by guard cells is described and critically evaluated. There is a large body of literature on this subject, including enzyme activity assays, immunological assays for ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in extracts, in situ immunological indicators at the light‐and electron‐microscope levels. products of 14CO2 fixation, and indirect indications such as the effect of light on the P‐glycerate pool size, chlorophyll a (Chi a) flourescence transients, and alkalinization of medium. Although it is not possible to reconcile all the literature, I show that most reports indicate that the photosynthetic carbon‐reduction pathway is absent in guard cells or, at most, does not exceed about 5% of that in mesophyll cells on a Chl basis. Because a mesophyll cell contains approximately 30 times more Chl than a guard cell does, the reported level of the pathway is equivalent to ?0.1 to 0.2% mesophyll contamination, of which it is difficult to ensure the absence. Even if present at these levels, the pathway would not contribute significantly to carbon metabolism.