You are worrying unnecessarily. Virtually all of the CO2 that is fixed
in photosynthesis is taken up in exchange for water vapor through the
stomata of the leaf. Both water vapor and CO2 are gasses and there is
no barrier to the diffusion of either except that imposed by the degree
of opening of the stomatal pore. As the pore size increases the amount
of transpired water vapor increases as long as the vapor pressure
inside the leaf is greater than the vapor pressure outside the leaf and
there is enough water available to the roots. The CO2 enters the open
stomate as the water vapor moves out and the two do not interact
directly. As far as I know, there is no CO2 taken up from bicarbonate
dissolved in the liquid water of the xylem that comes from the roots.
This is an interesting concept because bicarbonate is an available CO2
source for submerged aquatic vegetation and algae. I just do not know
how much bicarbonate is dissolved in the xylem. I hope that Bill
Williams and Holly Gorton will comment on this issue as well. They are
the real world experts on this subject.
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