Water relation parameters of leaf cells of the aquatic plant Elodea densa have been measured using the pressure probe. For cells in both the upper and lower epidermis it was found that the elastic modulus (ε) and the hydraulic conductivity (Lp) were dependent on cell turgor (P). Lp was (7.8±5.5)·10(-7) cm s(-1) bar(-1) (mean±SD; n=22 cells) for P>4 bar in cells of the upper epidermis and was increasing by a factor of up to three for P→0 bar. No polarity of water movement or concentration dependence of Lp was observed. For cells of the lower epidermis the Lp-values were similar and the hydraulic conductivity also showed a similar dependence on turgor. No wall ingrowth or wall labyrinths (as in transfer cells) could be found in the cells of the lower epidermis. The elastic modulus (ε) of cells of the upper epidermis could be measured over the whole pressure range (P=0-7 bar) by changing the osmotic pressure of the medium. ε increased linearly with increasing turgor and ranged between 10 and 150 bar. For cells of the lower epidermis the dependence of ε on P was similar, although the pressure dependence could not be measured on single cells. The Lp-values are compared with literature data obtained for Elodea by a nuclear magnetic resonance (NMR)-technique. The dependence of Lp on P is discussed in terms of pressure dependent structural changes of the cell membranes and interactions between solute and water transport.