Nutritional And Physiological Basis Of The Difference Of Nitrate Accumulation In Petiole Of Spinach Cultivars

Nitrate accumulated in leafy vegetables has posed detrimental threats to human's health.Petiole is known as the main part of plant in nitrate accumulation, but now most studies isfocused on the leaf blade. Why does the petiole accumulate nitrate in so high concentration,and why is it so difficult for the petiole to reduce the nitrate in its cell? Information on thisfield is still lacking. Pot experiments were carried out, using three spinach cultivars with obviously differentnitrate concentration as test crops and at N rates of 0.1 and 0.3 g N kg-1, to study the effects ofthe nitrate concentration of incubation solution on the induced nitrate reductase activity (NRA)of the blade of spinach; nitrate accumulation in different organs and its relationship tobiomass, moisture content and total N concentration; if the NRA of the blade can not be fullyexert is the reason for nitrate accumulation in the blade, and why it is so difficult for nitrate tobe reduced in the petiole. The main conclusions are as follows: 1 Experiments with blade tissue of spinach incubated in nitrate solution at differentconcentration, and that incubated for different time in nitrate solution of same concentration,showed that nitrate concentration in the blade tissue did not increase with the nitrateconcentration of the incubation solution. The nitrate concentration of 0.04 mol N L-1and 0.06mol N L-1, and the incubation time of 1 hour were the optimum option for determination ofthe induced NRA in blade tissue. 2 Of the different organs and parts of spinach, the nitrate concentration in the petiole wasthe highest one, which was significantly higher than that in the blade and root. The differenceof nitrate concentration in the petiole could reflect the difference of nitrate accumulationamong cultivars. Results over the 3 spinach cultivars indicated that nitrate concentration in thepetiole was positively correlated with the plant biomass, and the negative correlation-shipbetween nitrate concentration and contents of dry matter and total N was only found at high Nrate (0.3g N kg-1 Soil). 3 Among the three different varieties, the in vivo NRA, the ratio of the in vivo to in vitroNRA, the nitrate metabolic pool size (NMPS), the ratio of NMPS to nitrate storage pool size(NSPS) of the blade were all negatively correlated with the nitrate concentration in thepetioles. Induced by external nitrate solution, the increases of NRA in the blade werepositively correlated with the nitrate concentration in the petiole of the three spinach cultivars.Therefore, the NRA of the blade and its exerting degree, and the NMPS and the distribution ofnitrate between metabolic pool and storage pool in the blade cell were the main reasons forthe difference of nitrate concentration in the petiole of spinach cultivars. 4 The in vivo and in vitro NRA and the NMPS of petiole were obviously lower andsmaller than that of the blade, especially the NMPS. Nitrate concentration in the petiole wasnegatively correlated with its in vivo NRA, and positively correlated with its storage pool sizeover different cultivars. However, no certain relationship was found between the ratio ofNMPS to NSPS and the nitrate concentration in the petiole. The nitrite concentration wasincreased by the added nitrate reductase from the blade, while the increase showed no obviousrelationship to the nitrate concentration in the petiole of different cultivars. This indicated thatthe too small NMPS was the reason why the petiole has much higher nitrate concentrationthan that in the blade, and the exerting degree of the potential NRA and the NMPS were themain reasons for the difference of nitrate concentration in the petiole among spinach cultivars.