Water relations of the epidermal bladder cells ofOxalis carnosa Molina

All of the cells of the upper (adaxial) epidermis of the leaves ofOxalis carnosa are transformed into large bladders, while in the lower epidermis the bladder cells are interrupted by “normal” cells with stomata. The epidermal bladders contain a high concentration of free oxalic acid (pH approx. 1). Water-relations parameters of these epidermal bladder cells have been determined using the pressure probe. Original cell turgor (P₀) of the closely packed bladders of theupper epidermis was P₀=0.7 to 2.9 bar ( (overline {P_0 } = 1.7 pm 0.5 bar) ; mean±SD;N=25 cells) and lower than that in the club-shaped bladders of thelower epidermis (P₀=1.3 to 3.7 bar; (overline {P_0 } = 2.5 pm 0.7 bar) ;N=25 cells). Large differences in the elastic modulus (ε) and the hydraulic conductivity (Lp) of the two different types of cells were observed. For the lower epidermal bladders, ε=18 to 166 bar and was similar to that of other higher plant cells. Also, for these cells it was found that ε was increasing with both, cell turgor and cell volume. By contrast, ε of the cells of the upper epidermis was by one order of magnitude smaller (ε=1.9 to 17.0 bar) and no dependence of ε on cell volume could be detected. The Lp values of the cell membranes were also different (lower epidermis: (overline {Lp} = (2.3 pm 1.6) cdot 10^{ - 5} cm s^{ - 1} bar^{ - 1}) ; upper epidermis: (overline {Lp} = (3.8 pm 2.4) cdot 10^{ - 6} cm s^{ - 1} bar^{ - 1}) ). These differences seem to be too large to be caused by errors in determining the exchange area for water (A) between cells and adjacent tissue. The half-times of water exchange between bladders and leaf (T_1/2) were, on average, somewhat longer for the upper than for the lower epidermis (lower epidermis: T_1/2=7 to 38 s; upper epidermis: T_1/2=22 to 213 s), but the differences in the T_1/2 values were not as distinct as for ε and Lp. This is because of the compensatory effects of ε, Lp and the different ratios of volume to exchange area. Since the bladders make up about 75% of the entire volume of the leaf, it is assumed that the rate of response of the leaf to changes in the water potential should be similar to that of the bladder cells. The results are discussed in terms of a possible function of the bladders in the leaf.