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

Abstract

Light dependency of nitrate and nitrite assimilation to reduced-N in leaves remains a controversial issue in the literature. With the objective of resoling this controversy, the lght requirement for nitrate and nitrite assimilation was investigated in several plant species. Dark and light assimilation of I'5Nlnitrate and 1"Nlnitrite to a ium and am_io-N was determined with leaves of wheat, corn, soybean, sunflower, and tobacco. In dark aerobic conditions, assimilation of I1Nlnitrate as a percentage of the lght rate was 16 to 34% for wheat, 9 to 16% for tobacco, 26% for corn, 35 to 76% for soybean, and 55 to 63% for sunflower. In dark aerobic conditions, assimlation of I'5Ninitrite as a percentage of the lght rate was 11% for wheat, 7% for tobacco, 13% for corn, 28 to 36% for soybeans, and 12% for sunflower. It Is concluded that variation among plant species in the light requirement for nitrate and nitrite assimilation explain some of the contradictory results in the literature, but ad i explanations must be sought to fuliy resolve the controversy.In dark anaerobic conditons, the assimilation of [l'Nlnitrate to ammonium and amino-N in leaves of wheat, corn, and soybean was 43 to 58% of the dark aerobic rate while dark anaerobic assimilation of l"Nlnitrite for the same species was 31 to 41% of the dark aerobic rate. In contrast, 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 conditons. Oxygen apparently inhibits nitrate reduction in the dark even in leaves of plant species that exhibit a relatively high dark rate of I15Nlnitrite assimilation.In the in vivo NR3 assay (12) leaf tissue accumulates N02-when incubated under dark anaerobic conditions but N02-accumulation is strongly suppressed by low concentrations of 02 (2,3,6,13,17 In contrast to the results of these studies, substantial rates of 15N03-and 15N02-assimilation to NH4' and amino-N in the dark have been reported for leaves of barley (1), sunflower (10), citrus (11), and several plant species (21). Such results have led to uncertainty of the light dependency of N03 and N02 assimilation and of the interpretation of 02 inhibition of N02 accumulation in the dark. With the objective of resolving these contradictions in the literature, the present study examines the light and 02 requirements for 15N03-and '5N02-assimilation in leaves of several plant species. Similar experiments were conducted independently in two separate laboratories to both confirm the results and to eliminate experimental procedure as a causal factor in the controversies. MATERL4,S AND METHODSThe experiments presented in this paper were conducted at Queen's University, Kingston, Ontario (Experiment A) and at the University of Illinois, Urbana (Experiment B).Plant Material. At Kingston, plant material comprised wheat (Triticum aestivum L. cv Freder...

Posttranslational Regulation of Nitrate Reductase Strongly Affects the Levels of Free Amino Acids and Nitrate, whereas Transcriptional Regulation Has Only Minor Influence

“…shown for various lines of wild-type tobacco that nitrate as well as nitrite reduction can take place in the dark in leaves, although at a reduced rate (Reed et al, 1983;Lejay et al, 1997). The values obtained in this work are very similar to those obtained previously for the wild-type and Del lines using detached leaves (Lejay et al, 1997).…”

An analytical model of non‐photorespiratory CO2 release in the light and dark in leaves of C3 species based on stoichiometric flux balance

“…Nitrogen assimilation is also suppressed by a reduction in carbohydrate status (Fritz et al 2006), and therefore by the duration of the dark period. Nevertheless, nitrate reduction often continues in the dark, albeit often at reduced rates (9-76% of diurnal rates in a range of species [Reed et al 1983];~50% [Aslam et al 1979] or 88% [Bloom et al 1989] in Hordeum vulgare L.). In species that perform little or no nitrate reduction in leaves, amino acid synthesis in leaves does not, in principle, require light.…”

Assimilation of 15NO3− Taken Up by Plants in the Light and in the Dark

“…Plants can assimilate NO3-during both the light and dark portions of the photoperiod, although rates of assimilation may differ. In experiments using '"NO3-and excised leaves or leaf disks, it has been amply demonstrated that N03 reduction can occur in darkness (8,11,24,32), and the rate of reduction is associated with availability ofcarbohydrate (2, 15). Nevertheless, it was found consistently that reduction rates were lower in darkness.…”