Excessive NO3
accumulation is often detected in vegetables
from soilless cultivation due to frequent use of nitrate fertilizers.
Nitrate itself is not toxic, but it could be converted to metabolites
such as nitrite and N-nitroso compounds, which have harmful effects
on human health (Santamaria, 2006). Therefore, a high dietary
NO3
intake is considered to be undesirable, as it poses a
health risk to humans. Decreasing nitrate accumulation in vegetables
should be an effective way for humans to reduce dietary NO3
intake. Here we found that the 1 lMFeEDTA treatment had a lower
nitrate concentration in the edible parts of spinach plants than the
other Fe treatments, suggesting that mild Fe-deficiency facilitates
the decrease of NO3
in vegetables (Fig. 3). Previous studies have
shown that short-term Fe-deficient treatments could stimulate
the activity of nitrate reductase (NR) in plants (Jin, Du, Shamsi,
Luo, & Lin, 2011). Reduction of nitrate to nitrite by NR is the limiting
step in the nitrate assimilation pathway in plants. Consequently,
the decrease of nitrate accumulation during mild Fedeficiency
may be attributed to the stimulation of NR activity.
However, because Fe is a cofactor of the NR enzyme, extreme Fedeficiency
may result in inhibition of NR activity, which is probably
the reason for high nitrate accumulation in the blades from the
0 lM FeEDTA treatment.