Differences in nitrogen metabolism between cultivars of tomato and pepper during exposure to glasshouse atmosphere containing oxides of nitrogen

Abstract

Metabolism of the products of atmospheric oxides of nitrogen (NO x) once they enter the plant cell involves normal nitrate assimilation pathways. Activities of two enzymes involved in this pathway have been investigated in two important horticultural plants (tomato and sweet pepper) exposed to atmospheric levels of nitric oxide (NO) or nitrogen dioxide (NO 2) commonly found during commercial cultivation under glass. In most cultivars of tomato, the response to NO x has been shown to be a significant induction of nitrite reductase (NiR) but in leaves of sweet pepper, exposure to either of the two gases reduced the level of in vitro NiR activities even though an accumulation of amine compounds in the tissue suggested that the rates of nitrate reduction had increased in vivo. This decrease in activity in pepper was not due to storage of nitrate or nitrite ions, contrasting sharply with the effect of NO x upon the levels of NiR activity from cultivars of tomato. No effect on nitrate reductase (NaR) activity of either of the two gases was detected in pepper although an increase in NaR activity in the presence of NO 2 was shown by one tomato cultivar. This increase, which was dependent on both the photon flux density and the NO 3 − supply, involved synthesis of NaR. On the other hand nitrate reduction in the roots of both species appears to be unaffected by exposure to NO 2.