Characterization of the epidermis from barley primary leaves. 2. The role of the epidermis in ion compartmentation

The cation and anion distribution between the epidermis and mesophyll of primary leaves of 10-d-old barley (Hordeum vulgare L.) seedlings was studied in relation to growth conditions. A new method was employed to isolate epidermal protoplasts. The following observations were made: (i) Under standard hydroponic growth conditions, K+ was the dominant cation and NO3- the predominant anion, both in epidermal and mesophyll protoplasts. (ii) Levels of Cl- and particularly of Ca2+ were specifically increased in the epidermis when growth conditions stimulated uptake of these ions into the plants. (iii) Epidermal PO4(3-) was maintained at a very low level even in the presence of 50 mM phosphate in the rooting medium, whereas the phosphate concentration in the mesophyll was high, with phosphate being accumulated in the mesophyll vacuoles. (iv) Sulphate and NO3- accumulated to a similar degree in the mesophyll and the epidermis when K2SO4 or KNO3 in the rooting medium caused salt stress to the plants. (v) Epidermal protoplasts took up (SO4(2-))-S-35 and Cl-36- from the surrounding medium. A comparison of the uptake rates indicates that specific ion deposition into the epidermis may partially be the consequence of differential uptake of ions from the transpiration stream. The results indicate an important function of the epidermis for ion compartmentation in barley leaves.