Reduction of nitrate and oxalate content in spinach (Spinacia oleracea L.) has become the major concern in terms of their toxicity to human health. The primary objectives of this study were 1) to determine the seasonal change in nitrate and oxalate concentrations and 2) to elucidate the relationship between growth rate and concentration of nitrate and oxalate in spinach. In a replicated field experiment in Hiratsuka, Japan, the authors grew 182 cultivars of spinach over four growing seasons (winter, spring, summer, and fall) under the nitrogen application rate of 100 kg N·ha–1. The average number of days required for harvest was the shortest in summer (32.7days) and was longest in the winter (85.7 days). Mean nitrate concentration in spinach was significantly low in the winter (3797 mg·kg–1 fresh weight) compared with the other three seasons (4122–4328 mg·kg–1 fresh weight) in which no significant differences were found in mean nitrate levels. In contrast, oxalate concentrations showed a distinct seasonal variation, being the lowest in the fall (6149 mg·kg–1 fresh weight), followed by the summer (7525 mg·kg–1 fresh weight) and the spring (8903 mg·kg–1 fresh weight), and was the highest in the winter (10,929 mg·kg–1 fresh weight). Relative nitrate concentration showed a moderate negative correlation with relative days required for harvest (r = 0.411, P < 0.001), whereas relative oxalate concentration showed a strong positive correlation with relative days required for harvest (r = 0.566, P < 0.001). Accordingly, a moderate negative correlation (r = 0.325, P < 0.001) was detected between nitrate and oxalate concentrations. Moreover, fast-growing cultivars contained higher nitrate and lower oxalate, whereas slow-growing cultivars contained lower nitrate and higher oxalate. These results indicate that the growth rate primarily accounts for the nitrate and oxalate concentration in spinach, and nitrate and oxalate might play a counterrole to each other.
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