Assimilation of [15N]Nitrate and [15N]Nitrite in Leaves of Five Plant Species under Light and Dark Conditions

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Abstract
Light dependency of nitrate and nitrite assimilation to reduced-N in leaves remains a controversial issue in the literature. The light requirement for nitrate and nitrite assimilation was investigated in several plant species. Dark and light assimilation of [15N]nitrate and [15N]nitrite to ammonium and amino-N was determined with leaves of wheat, corn, soybean, sunflower and tobacco [Triticum aestivum cultivars Frederick and Centurk, Corn cv. XL81, Glycine max cultivars Maple Presto and Williams, Helianthus annuus cv. CM90RR and Nicotiana tabacum cv. Havana Connecticut, respectively]. In dark aerobic conditions, assimilation of [15N]nitrate as a percentage of the light rate was 16-34% for wheat, 9-16% for tobacco, 26% for corn, 35-76% for soybean and 55-63% for sunflower. In dark aerobic conditions, assimilation of [15N]nitrite as a percentage of the light rate was 11% for wheat, 7% for tobacco, 13% for corn, 28-36% for soybeans and 12% for sunflower. Evidently, variation among plant species in the light requirement for nitrate and nitrite assimilation explains some of the contradictory results in the literature, but additional explanations must be sought to fully resolve the controversy. In dark anaerobic conditions, the assimilation of [15N]nitrate to ammonium and amino-N in leaves of wheat, corn and soybean was 43-58% of the dark aerobic rate while dark anaerobic assimilation of [15N]nitrite for the same species was 31-41% of the dark aerobic rate. Accumulation of nitrite in leaves of the same species in the dark was 2.5-to 20-fold higher under anaerobic than aerobic conditions. Therefore, dark assimilation of nitrite cannot alone account for the absence of nitrite accumulation in the in vivo nitrate reductase assay under aerobic conditions. O2 apparently inhibits nitrate reduction in the dark even in leaves of plant species that exhibit a relatively high dark rate of [15N]nitrite assimilation.