Nitrate-Dependent O₂ Evolution in Intact Leaves

Abstract

Evolution of 02 by illuminated intact detached leaves from barley (Hordwum vulgare L. cv Athos) and pea (Pisum sativum L. cv Lincoln) in a C02-saturating atmosphere was enhanced when KNO (1-2.5 millimolar) had been previously supplied through the transpiration stream. The extra 02 evolution observed after feeding KNO increased with the light intensity, being maximal at near saturating photon flux densities and resulting in no changes in the initial slope of the 02 vers light-intensity curve. No stimulation of 02 evolution was otherwise observed after feeding KCI or NH4CI. The data indicate that nitrate assimilation uses photosynthetically generated reductant and stimulates the rate of noncyclic electron flow by acting as a second electron-accepting assimilatory process in addition to CO2 fixation.Nitrate is the primary nitrogen source for green cells. In plant leaves, nitrate-N is reduced to ammonium through the sequencial operation ofNAD(P)H-nitrate reductase (NR) and ferredoxin-nitrite reductase (NiR), the resulting ammonium being incorporated to carbon skeletons via the glutamine synthetase (GS)-glutamate synthase (GOGAT) cycle (1, 9, 1 1), according to the reactions: MATERIALS AND METHODSBarley seedlings (Hordeum vulgare cv "Athos") and pea (Pisum sativum cv "Lincoln") were grown on vermiculite beds supplied daily with nutrient solution containing 10 mm nitrate as the nitrogen source (6), at 26C and 65% RH during the photoperiod (370 gE m-2 s-' PPFD, 12 h) and at 20TC during darkness.Salt solutions were fed to the detached leaves through the transpiration stream. Barley leaves of about 12 cm length (from 1-week-old seedlings), with their cut end immersed in distilled water or the different salt solutions were placed in a temperature controlled chamber with a glass window of 3.5 cm diameter to provide illumination perpendicular to the leaf surface. Transpiration, induced by illumination (370 ,E m-2 s-' PPFD) under a continuous flow of humidified air (100 mL min-') at 25TC, was monitored by a RH and temperature sensor (Vaisala)

Effects of nitrogen form on growth, CO2 assimilation, chlorophyll fluorescence, and photosynthetic electron allocation in cucumber and rice plants

“…However, the N source had no significant effect on the photosynthesis parameters in rice plants in other studies (Guo et al, 2008;Li et al, 2009). Similar to the changes observed with gas exchange analysis, a higher Ф PSII value was found in cucumber plants grown with the (Bloom et al, 1989;de la Torre et al, 1991). It is worthwhile to note that cucumber plants experienced photo-inhibition when they were grown with 4 NH + as the N source, as has been observed in the 4 NH + -grown French bean plants (Zhu et al, 2000).…”

Role of light and CO2 fixation in the control of nitrate-reductase activity in barley leaves

“…Huber et al (1992) have also reported lack of effect of nitrate supply on extractable NR activity from illuminated or darkened spinach leaves. It, thus, appears that nitrate supply, would accelerate in-vivo nitrate reduction, as indicated by nitrate-dependent O2 evolution previously observed under the same experimental conditions (De la Torre et al 1991), without affecting significantly the in-vivo activation state of NR.…”

Nitrogen assimilation and growth of wheat under elevated carbon dioxide

“…1B and 2B). For example (27), oxygen evolution monitored with a polarographic O 2 electrode at saturating CO 2 did not differ between detached barley leaves given NH 4 ϩ and those given NO 3 Ϫ at levels similar to the xylem NO 3 Ϫ concentrations measured here (15.4 Ϯ 0.7 mM; mean Ϯ SE, n ϭ 55). Another technique, chlorophyll fluorescence, in contrast with O 2 fluxes, does not provide an accurate measure for electron transport rates of an entire wheat shoot (28).…”