The non-osmotic volume of Commelina guard cells
Abstract. The non-osmotic volume (NOV) of Connnelina communis L. guard cells was estimated by observing the volumes of guard cell protoplasts incubated in mannitol solutions of different solute potential, and applying the Boyle-van't Hoff relation to the results. NOV values of between 517 and 1782 μm3 were obtained for different batches of protoplasts. There was a negative correlation between NOV and apparent protoplast solute contents, and the NOV and solute content were observed to alter when pretreatments affecting stomatal aperture were given. H is hypothesized that changes in guard cell chloroplast starch levels could account for variation in NOV and solute content. 
 
For closed stomata, it is calculated that the NOV could reduce the proportion of the total guard cell volume which is osmotically active by over 40%. Serious inaccuracy may thus result if the NOV is not taken into account in the estimation of guard cell solute potential or solute concentration from measurements of solute levels per cell. The error is maximal at low stomatal apertures.