Water permeability differs between growing and non-growing barley leaf tissues

A pressure probe technique and an osmotic swelling assay were used to compare water transport properties between growing and non-growing tissues of leaf three of barley. The epidermis was analysed in planta by pressure probe, whereas (predominantly) mesophyll protoplasts were analysed by osmotic swelling. Hydraulic conductivity (Lp) and, by implication, water permeability (P _f) of epidermal cells was 31% higher in the leaf elongation zone (Lp=0.5±0.2 μm s^-1 MPa^-1; P_f= 65±25 μm s^-1; means ±SD of n=17 cells) than in the, non-growing, emerged leaf zone (Lp=0.4±0.1 μm s^-1 MPa ^-1; P_f=50±15 μm s^-1; n=24; P <0.05). Similarly, water permeability of mesophyll protoplasts was by 55% higher in the elongation compared with emerged leaf zone (P_f= 13±1 μm s^-1 compared with 8±1 μm s^-1; n=57 and 36 protoplasts, respectively; P <0.01). Within the leaf elongation zone, a small population of larger-sized protoplasts could be distinguished. These protoplasts, which originated most likely from parenchymateous bundle sheath or midrib parenchyma cells, had a three-fold higher water permeability (P <0.001) as mesophyll protoplasts. The effect on Lp and P_f of known aquaporin inhibitors was tested with the pressure probe (Au⁺, Ag⁺, Hg²⁺, phloretin) and the osmotic swelling assay (phloretin). Only phloretin, when applied to protoplasts in the swelling assay caused an average decrease in P_f, but the effect varied between isolations. Technical approaches and cell-type and growth-specific differences in water transport properties are discussed.