Identification of the leaf vacuole as a major nitrate storage pool.

Highly purified vacuoles were isolated from protoplasts derived from green barley (Hordeum vulgare var. Numar) leaves, in order to determine their role as a NO(3) (-) storage sink. alpha-Mannosidase and acid phosphatase activities were used as markers to identify vacuoles, alpha-mannosidase being the more suitable. Nitrate and alpha-mannosidase, which were released from vacuoles destroyed during lysis of protoplasts, moved at unequal rates in the density gradient used for vacuole isolation. Purified vacuoles retained less NO(3) (-) than alpha-mannosidase during a single washing. Empirically determined corrections were used to account for NO(3) (-) movement in estimating the percentage of total cellular nitrate found in the vacuole. Vacuoles from plants grown in the presence of NO(3) (-) contained 58% of the total cellular NO(3) (-) and therefore represent a major NO(3) (-) storage pool.