Nitrate reduction and compartmentation in tomato leaves. II. Nitrate loss by leaf sections in a gaseous atmosphere and the size of the metabolic nitrate pool

Nitrate disappearance in tomato ( (ycopersicon esculentum Mill. cv. Azes) leaf sections kept under a stream of gas (nitrogen or air) has been studied, using leaf sections from plants supplied with low (7.5 mM) or high (17.5 mM) nitrate levels in their nutrient solution. Cessation of nitrate loss occurred in leaf sections taken from plants irrigated with low (7.5 mM) nitrate-containing nutrient solution. Resumption of nitrate disappearance occurred upon addition of exogenous nitrate by vacuum infiltration to leaf sections, suggesting that cessation of nitrate loss was due to exhaustion of the metabolic pool. We estimated that 53% of the total nitrate in leaf sections from low nitrate plants was located in a storage pool, probably the vacuole. The remainder was located in a pool, readily available for reduction (the metabolic pool). This pool is composed of nitrate in the free space as well as in the cytoplasm which was estimated to contain about 20% of the total nitrate.
Either under air or nitrogen, less nitrite was accumulated than nitrate assimilated suggesting that nitrite accumulation was not an adequate parameter for the estimation of nitrate utilization. Anaerobic conditions inhibited nitrite reduction whereas nitrate assimilation was not blocked. Nitrate loss from endogenous pool in leaf sections placed under aerobic conditions is suggested as an adequate method for the estimation of the metabolic pool of nitrate.