Vacuoles as storage compartments for nitrate in barley leaves

Abstract

Nitrate, the principal nitrogen source of most plants, can accumulate in large quantities in certain crop plants, notably members of the Chenopodiaceae (spinach and beet), Gramineae, Cruciferae (radish and kale) and Compositae (lettuce). Concentrations may exceed 2% fresh weight (17–24% dry weight) in extreme physiological conditions1. This is alarming because nitrate is readily reduced in organisms to the toxic nitrite, which may react with amines to form very potent carcinogenic nitrosamines2. Large stores of nitrate can be maintained in plant cells even in the presence of high nitrate reductase activity3–8. Incoming nitrate does not seem to mix with existing stores4–6,9 and a steady influx of nitrate into the cells was found to be necessary to keep nitrate reductase stably induced3,6. Such observations have been explained by postulating a small ‘metabolic’ pool of nitrate accessible to nitrate reductase4 and presumably also responsible for the induction of the enzyme3, and a large ‘storage’ pool separate from the sites of metabolism3–9,14. Repeated attempts to measure these pools by an indirect method4 have given equivocal results1,10, but storage pools of nitrate have been generally thought to be located in vacuoles, which are difficult to isolate and analyse. However, we were able to isolate and purify the large central vacuoles of barley mesophyll cells and found most of the accumulated nitrate in the vacuoles.