The Relationships Between Nitrate Uptake, Reduction And Nitrate Accumulation Of Different Rapeseed Cultivars

Luxury absorption of nitrogen by leafy vegetables often causes high accumulation ofnitrate in vegetables, thus not only decreases nitrogen use efficiency, but also affects thequality of vegetables and influences on human health. There is a close liner relationshipbetween nitrate reduction and carbon metabolism in plant, and nitrate uptake and reductiondirectly depend on the photosynthetic capacity. Root is an another important place wherenitrate is reducted, and nitrate reductase activity of root can directly regulate nitrateaccumulation in shoot. In order to makes a thorough study of the relationships among nitrateuptake, nitrate reduction and accumulation, the hydroponics, pot and field experiment wereconducted to screen two cultivars of oilseed rape, which had significant difference on nitrateaccumulation (high nitrate accumulated cultivar and low nitrate accumulated cultivar), andthey were used as the testing cultivars to study the relationship between nitrogen metabolismand photosynthetic metabolism, the effect of reduction capacity of root on shoot nitrateaccumulation, and the effect of nitrate reductase activity inhibitor on nitrate in root, in and toreveal the causes of nitrate accumulation in the differences between different varieties ofvegetables. The main results were as follows.1. A field experiment with different nitrogen application rates was conducted to study thedifference of nitrate content in27oilseed rape cultivars. Results showed that there was asignificant difference on nitrate content among27rape cultivars, and petiole had highestnitrate content in all cultivars, which accounted for76.4%. There was a significant positivecorrelation relationship between nitrate in petiole and that in the whole plant. With the growthof rape, nitrate content in petiole of all cultivars decreased, but there was aslo significantdifference among different cultivars, therefore, petiole could be used as a screening indicatorfor selecting the different nitrate accumulate rape cultivars. Lastly, two low nitrateaccumulation cultivars (Za631dali, Hongyou3, with average nitrate content of1600mg·kg^-1)and two high nitrate accumulation cultivars (Zheshuang6and Zhognshuang6, with averagenitrate content of3400mg·kg^-1) were screened.2. Based on the two selected oilseed rape cultivars, the relationship between nitrateaccumulation and nitrate reduction, nitrate accumulation and photosynthetic capacity were analyzed. Results showed that low nitrate accumulation varieties had higher nitrate reductaseactivity in lateral root, higher net photosynthetic rate, transpiration rate and SPAD valuescompared with those of high nitrate accumulation varieties. There was a negative correlation(p<0.05)between nitrate concentration in petiole and nitrate reductase activity in lateralroot, between nitrate concentration in petiole and SPAD values, between nitrateconcentration in petiole and net photosynthetic rate, between nitrate concentration in petioleand transpiration rate. However, there was no relationship between nitrate concentration inpetiole and intercellular CO2concentration, nitrate concentration in petiole and stomatalconductance, between nitrate reductase activity in leaf blade and nitrate content in petiole.These indicated that highly effective photosynthetic metabolism and higher lateral rootnitrate reduction capacity may be another important reason for low nitrate accumulationvarieties with low nitrate content.3. A exprement with nitrate reductase activity inhibition by inhibitor (Na₂WO₄) in rootwas conduct to study the effect of nitrate reduction capacity on shoot nitrate accumulation.Results showed that nitrate reductase inhibitor (Na₂WO₄) significantly reduced nitratereductase activity and the rate of nitrate-N absorption in the root of rape comparing with thecontrol (CK). However, nitrate reductase inhibitor significantly increased nitrate-N content.Root nitrate reductase activity not only decided the nitrate reductase ability in root, but alsoaffected nitrate uptake rate by root.4,After inhibiting the nitrate reductase activity of root, nitrate content in root wassignificantly increased, but nitrate content decreased in shoot. Nitrate concentration in petiolesignificantly decreased and nitrogen deficiency occured in shoot, and nitrate content of petioledecreased by11.9%for Hongyou3and5.5%for Zhongshuang6, respectively, but nitrateconcentration in leaf remained relatively stable. These results indicated that nitrate in petioleof Hongyou3(low nitrate accumulation types) was more easily transported to shootcomparing with that of Zhongshuang6(high nitrate accumulation types).