Nitrate Accumulation and Reduction in Spinach and Their Relations to Plant Growth

Previous research indicates that nitrate accumulation in plants changed with plant growth,but mechanisms behind those changes are unknown.Understanding these mechanisms will benefit adopting appropriate strategies to decrease the nitrate content in plants since high nitrate accumulation in leafy vegetables is potentially toxic to human health.A pot experiment was carried out to investigate the dry weight,water content,and nitrate-N concentrations of petioles and leaf blades,and in vivo and in vitro nitrate reductase activity(NRA) in leaf blades of two spinach cultivars at different growth stages.The objective was to study the dynamic changes of nitrate-N concentrations and NRA of two spinach cultivars,and their relations to plant growth.The results showed that the dry weights and water contents of petioles,leaf blades,and shoot for both cultivars increased with the increase of plant biomass and then declined with the decrease of plant growth.However,the nitrate-N concentrations of both cultivars decreased with plant growth,and the decrease for cultivar S9 with lower nitrate-N concentration was more significantly than that for cultivar S4 with higher nitrate-N concentration.During the early growth stage,in vivo and in vitro NRA increased with plant growth,and these for cultivar S4 increased by 379% and 199% which were significantly higher than that for cultivar S9,but they were decreased for cultivar S4 and increased for cultivar S9 at later growth stage.The increase of dry weight and water content,nitrate-N concentrations,in vivo and in vitro NRA of cultivar S4 were significantly higher,but the ratio of in vivo /in vitro NRA was lower than those of cultivar S9 on days 40 and 52 after germination.These differences between spinach cultivars were not significant at later growth stage.This indicated that the nitrate decrease of cultivar S4 was more slowly than that of cultivar S9 should be due to its lower ratio of in vivo /in vitro NRA,and this hypothesis was proved by the fact that the nitrate-N concentration of cultivar S4 decreased significantly with the increase of in vivo /in vitro NRA ratio at late stage.This suggested that the variations of nitrate-N concentration for both cultivars be closely related to the ratio of in vivo /in vitro NRA at different growth stages.