Association between water and carbon dioxide transport in leaf plasma membranes: Assessing the role of aquaporins

The role of some aquaporins as CO₂ permeable channels has been controversial. Low CO₂ permeability of plant membranes has been criticized because of unstirred layers and other limitations. Here we measured both water and CO₂ permeability (<em>P_os</em>, <em>P_CO2</em>) using stopped flow on plasma membrane vesicles (pmv) isolated from <em>Pisum sativum</em> (pea) and <em>Arabidopsis thaliana</em> leaves. We excluded the chemical limitation of carbonic anhydrase (CA) in the vesicle acidification technique for <em>P_CO2</em> using different temperatures and CA concentrations. Unstirred layers were excluded based on small vesicle size and the positive correlation between vesicle diameter and <em>P_CO2</em>. We observed high aquaporin activity (<em>P_os</em> 0.06 to 0.22 cm s^-1) for pea pmv based on all the criteria for their function using inhibitors and temperature dependence. Inhibitors of <em>P_os</em> did not alter <em>P_CO2</em>. <em>P_CO2</em> ranged from 0.001 to 0.012cm s^-1 (mean 0.0079 <span class="underlined ">+</span>0.0007 cm s^-1) with activation energy of 30.2 kJ mol^-1. Intrinsic variation between pmv batches from normally grown or stressed plants revealed a weak (<em>R</em>²= 0.27) positive linear correlation between <em>P_os</em> and <em>P_CO2</em>. Despite the low <em>P_CO2</em>, aquaporins may facilitate CO₂ transport across plasma membranes, but probably via a different pathway than for water