Effects of Nitrogen on Oxalate Accumulation Mechanisms in Different Spinach Genotypes

Over application N fertilizer is quite common, especially for vegetable in China. Therefore, supported by IPNI China program, the project cooperators in Zhejiang University carried out this research project in recent years to study the negative effect of over application N fertilizer on vegetable quality and yield,

IPNI-2013-CHN-ZJ26

24 Mar 2015

2014 Annual Interpretive Summary


Spinach is widely cultivated and offers better nourishment than many vegetables. However, spinach also contains large amounts of oxalate, which is considered to be not beneficial, or a toxin for humans. Oxalate content differs among genotypes for many crops, but mechanisms behind the genotypic differences of oxalate are far from being understood. Nitrogen is one of the most important factors affecting oxalate accumulation. In this research, two spinach genotypes (HZZ and WL), which differed in oxalate accumulation, were used to investigate the genotypic differences in oxalate accumulation as affected by N supply.. The relationship between N and oxalate metabolism in the two genotypes was investigated by using N uptake kinetics, precursors of oxalate biosynthesis, inhibitors of N uptake, and reduction.

The results showed that ammonium reduces oxalate accumulation by inhibiting root uptake of nitrate. Nitrate exposure induced oxalate accumulation in vegetables, while mixed nitrate and ammonium exposure usually have negative effects. Correlation analysis confirmed that blade oxalate accumulation was positively associated with root uptake of nitrate but not ammonium. Treatments of mixed nitrate and ammonium (1:1) solution reduced oxalate accumulation. Compared with the controls, the values of Vmax for nitrate were reduced by 55% and 67% with addition of ammonium for HZZ and WL. The high-oxalate accumulating cultivar HZZ showed a greater nitrate uptake rate compared with the low-oxalate accumulating cultivar (WL). This suggests that oxalate synthesis in spinach is associated with root uptake of nitrate. Ammonium is able to minimize oxalate synthesis by inhibiting nitrate uptake.

Oxalate synthesis in leaves is associated with root nitrate uptake and its assimilation in plants. We investigated the possible association of nitrate assimilation in spinach plant by using two different genotypes. Increasing nitrate levels enhanced synthesis of oxalate and increased root uptake of nitrate and leaf activities of nitrate reductase (NR) and glutamine synthetase (GS). HZZ greatly enhanced root uptake rate of nitrate, leaf NR and GS activities compared with WL. Oxalate accumulation in spinach leaves was positively related to rate of nitrate uptake by roots and leaf activities of NR and GS. Addition of plasmalemma H+-ATPase-inhibitor sodium vanadate (Na3VO4) reduced leaf oxalate accumulation. Presence of NR or GS inhibitors led to reduction of leaf oxalate contents, GS/NR activities, and reduced nitrate uptake rate. Higher levels of nitrate root uptake, leaf NR and GS activities were observed in HZZ rather than WL. This study provides clear evidence that oxalate accumulation in spinach is positively associated with root uptake of nitrate, nitrate reduction, and its assimilation.