Solute concentration and osmotic pressure patterns were studied in epidermal cells of Tradescantia virginiana at open and closed stomata conditions, using single cell sampling and analysis techniques. The studies were carried out in intact plants that were responding to light conditions under normal physiological situation. Only charged solutes were found to participate in stomatal movement and each cell type of the epidermis showed distinct patterns of solute changes both quantitatively and qualitatively. Individual solutes showed characteristic patterns. Two major patterns were observed when the stomata opened. K+, Cl-, tartrate and succinate moved into the guard cells. Malate, NO3- and PO43- migrated to lateral subsidiary cells. A third group of solutes, citrate and SO42-, were redistributed in all cells, suggesting a charge-balancing function for these solutes during stomatal opening. These movements reversed on stomatal closure. Patterns of solute changes were similar on both (right and left) sides of the midrib. Each cell type of the epidermis also showed predictable patterns of osmotic pressure changes when stomata opened. Guard cell osmotic pressure increased (1.75 ± 0.06 to 2.26 ± 0.12 MPa) during stomatal opening. Osmotic pressure remained unchanged in lateral subsidiary cells but, in contrast, decreased in apical subsidiary and juxta apical cells. A physiologically-distinct subset of the pavement cells, juxta apical cells, is being proposed.
These qualitative and quantitative differences found in the distribution of solutes between cell types, suggest that each cell type of the epidermis contributes a definite part in the orchestration of stomatal responses. Working together, these cell types are linked by solute concentration gradients that drive the stomatal movements.