The epidermis of barley leaves is a dynamic intermediary storage compartment of carbohydrates, amino acids and nitrate

Abstract
Sugars, amino acids and nitrate were measured in the epidermis of barley seedlings and compared with whole leaf levels. I. Under all conditions, concentrations of glucose and fructose were lower in the epidermis than in the remaining leaf tissues; levels were lowest at the end of the dark period (0.2–1.4 mM) and increased under conditions of inhibited assimilate export (3–6 mM). 2. Epidermal sucrose concentrations were very low (« 0.2 mM) even in excised leaves which had accumulated assimilates. 3. Similar to the sugars, amino acids were also less concentrated in the epidermis than in whole leaf extracts. However, the amino acid profiles showed cell type‐specific differences. 4. Nitrate was accumulated in the epidermis. The epidermal pool decreased during nitrate starvation. Since no nitrate reductase activity was associated with the epidermis, nitrate was mobilised from the epidermis and metabolized presumably in the mesophyll. These results suggest that the epidermis functions as a regulated intermediary storage compartment of the leaves and that stored substances are readily available for remobilisation.