The effects of NO 2 on the uptake and assimilation of nitrate by soybean plants

Abstract

Twelve-day-old soybean plants were supplied with low (1 mM) or high (5 mM) nitrate to the roots and exposed to 0 (control), 0.22 (low), or 1.1 (high) μl l −1 NO 2 for 7 days. The low NO 2 treatment had no significant effects on nitrate uptake and dry weight of plants. The high NO 2 treatment decreased the amount of nitrate uptake and dry weight per plant. This inhibition of nitrate uptake and growth by high NO 2 was more significant for the plants growing at low nitrate than the plants growing at high nitrate. The acidity of both leaves and growth medium of the plants exposed to high NO 2 was higher than that of non-exposed plants. High NO 2 treatment increased the ammonium concentration in roots and decreased the amount of organic N per plant. These results suggested that exposure to high-level NO 2 caused an increase in H + in the leaves of exposed plants. The H + could be partially neutralized by OH − produced from the reduction of nitrate, or exuded into the growth medium through roots. The accumulation of H + in exposed plants may inhibit nitrate uptake, ammonium assimilation, and plant growth.