Abstract: Nitrate accumulation in vegetables has proposed detrimental impact on human heath, and understanding how nitrate can accumulate in plant is the key to solve the problem. Pot experiment was carried out, using three spinach (Spinacia oleracea L.) cultivars obviously different in nitrate concentration as test plants and at two N rates of 0.1 and 0.3 g N kg-1 on the basal application of 0.3 kg P2O5 kg-1 soil, to study the nitrate accumulation, distribution and allocation, and its reduction in the vegetable petioles. The petiole nitrate concentration, in vivo and the in vitro nitrate reductase activity (NRA) in the tissue of petiole, the nitrate metabolic pool size (NMPS) and storage pool size (NSPS) of the petiole cell, and the formation rate of nitrite by petiole tissue when addition of blade nitrate reductase were determined. The obtained results showed that the nitrate concentration of the petiole had no direct relationship with its NRA over different cultivars, but was negatively correlated with the ratio of in vivo to in vitro NRA. The NMPS of the petiole cell showed no certain relation with the nitrate concentration, but the NSPS was positively correlated with the nitrate concentration in petiole of different cultivars. When the nitrate reductase extracted from leaf blades was added to the culture solution of petiole tissue, the formation rate of nitrite by petiole tissue was all increased for the three spinach cultivars, and the formation rate was obviously higher for the cultivars with relatively lower petiole nitrate concentration. This indicated that the expressing degrees of the potential NRA in the petiole tissue, the NSPS of the petiole cell, and the capacity for the cell transporting nitrate from its storage pool (the vacuole) to metabolic pool (the plasma) was the main reason why it is difficult for nitrate to be reduced in petiole and nitrate concentrations in the petiole were different over spinach cultivars.

Key words: Petiole, Nitrate N, Nitrate reductase activity, Nitrate metabolic pool