Concentrations of inorganic and organic solutes in extracts from individual epidermal, mesophyll and bundle-sheath cells of barley leaves

Abstract

The distribution of solutes between epidermal, mesophyll and bundle-sheath cells in barley (Hordeum vulgare L. cv. Klaxon) leaves was studied by analysing extracts obtained from single cells with a modified pressure probe. Activity of the cytoplasmic marker enzyme, malate dehydrogenase, revealed that epidermal cell extracts were completely vacuolar in origin, but extracts from mesophyll cells also contained cytoplasmic constituents. The extracts were analysed for osmolality and the concentrations of K, Na, Ca, Cl, P, S, NO 3 − , sugars and total amino acids. Epidermal and mesophyll cell extracts had similar osmolalities but these varied between 420 and 565 mosmol, kg 1 depending on the leaf developmental stage; the osmolality of bundle-sheath extracts was approximately 100 mosmol, kg−1 lower. Under the growth conditions used, K and NO 3 − were found in all three cell types and their concentrations generally ranged between 180 and 230 mM. In contrast, Ca was almost restricted to epidermal cells, where it increased to 70 mM during leaf ageing. Phosphorus was only detectable (≥ 5 mM) in extracts from mesophyll and bundle-sheath cells, while Cl concentrations were highest in epidermal and lowest in mesophyll cell extracts. The concentrations of sugars and amino acids were close to the detection limit (approx. 2 mM) in epidermal cells but mesophyll cells contained total sugar (glucose, fructose and sucrose) of up to 78 mM and total amino-acid concentrations of up to 13.5 mM. Concentrations in bundle-sheath cells were intermediate between those in the epidermis and mesophyll.