The role of the apoplast as an osmotic compartment in Suaeda maritima L. Dum. and Beta vulgaris L.
The plant cell wall is a living and dynamic compartment of the plant cell. Its many diverse functions range from cell expansion and differentiation to defence and signalling. Furthermore, there is currently a growing body of evidence which suggests that the cell wall/apoplast also plays an important role in cell water relations. The aim of this study was to highlight the importance of apoplastic solutes in plant cell water relations, particularly in turgor regulation. The water relations parameters of two members of the family Chenopodiaceae, Suaeda maritima L. Dum. and Beta vulgaris L., were studied at single cell resolution using the cell pressure probe, single cell sampling and analysis techniques, and the xylem pressure probe. These species share a common peculiarity, in that certain cell types, namely the leaf epidermal cells in Suaeda maritima and the taproot storage parenchyma cells in Beta vulgaris, maintain cell turgor pressure (Pau) at a level which is dramatically lower than the respective cell osmotic pressures (III�). This phenomenon is attributed to the properties of the cell wall/apoplast. The hydrostatic component of the apoplast (P, uau) accounts for only a small fraction of the difference between P. u and H.,, in these species. In light of this the discrepancy between P. u and H can only be due to the presence of osmotically active solutes in the adjacent apoplast Suaeda maritima leaf epidermal cells accumulate NaCl in response to an increase in external NaCl concentration. This accumulation of solutes leads to an increase in leaf epidermal osmotic pressure, which exactly mirrors the increase in the osmotic pressure of the external medium (ITI). Leaf epidermal turgor pressure (P, -. u), however, is maintained at a constant level over a range of external salinities. In the short term the leaf epidermal cells are shielded from abrupt changes in flea by the properties of the root system, and a root reflection coefficient which is close to 0. In the longer term, as NaCl accumulates in the protoplast, Pcen is apparently maintained by the parallel adjustment of solutes in the protoplast and apoplast. Page III Changes in Suaeda maritima leaf epidermal turgor pressure (P. u), induced by modulating the solute content of the apoplast (11. u) in excised leaves, initiated a mechanism which regulated P., u back to in vivo levels within 40 minutes. Turgor regulation was not accompanied by equivalent changes in cell osmotic pressure (H n), suggesting that osmotic adjustment leading to turgor regulation is apoplastic rather than protoplastic in nature. This apoplastic osmotic adjustment mechanism was dependent on the permeant nature of the apoplastic solutes and on the volume of the apoplast. A comparable upward turgor regulation mechanism was observed in excised Beta vulgaris taproot tissue, within 40 - 80 minutes. The presence of apoplastic KK apparently facilitated the turgor regulation mechanism in this case. Proton efflux studies on Beta vulgaris taproot tissue revealed that the driving force behind this osmotic adjustment mechanism is likely to be turgor/external osmotic pressure (P,. u/H, ) dependent modulation of plasma membrane proton ATPase activity. It was concluded that the apoplast should be regarded as a true osmotic compartment in higher plants.