Low CO₂ Prevents Nitrate Reduction in Leaves

Abstract

The correlation between CO2 assimilation and nitrate reduction in detached spinach (Spinacia oleracea L.) leaves was examined by measuring light-dependent changes in leaf nitrate levels in response to mild water stress and to artificially imposed CO2 deficiency. The level of extractable nitrate reductase (NR) activity was also measured. The results are: (a) In the light, detached turgid spinach leaves reduced nitrate stored in the vacuoles of mesophyll cells at rates between 3 and 10 micromoles per milligram of chlorophyll per hour. Nitrate fed through the petiole was reduced at similar rates as storage nitrate. Nitrate reduction was accompanied by malate accumulation. (b) Under mild water stress which caused stomatal closure, nitrate reduction was prevented. The inhibition of nitrate reduction observed in water stressed leaves was reversed by external CO2 concentrations (10-15%) high enough to overcome stomatal resistance. (c) Nitrate reduction was also inhibited when turgid leaves were kept in C02-free air or at the C02-compensation point or in nitrogen. (d) When leaves were illuminated in C02-free air, activity of NR decreased rapidly. It increased again, when CO2 was added back to the system. The half-time for a 50% change in activity was about 30 min. It thus appears that there is a rapid inactivation/activation mechanism of NR in leaves which couples nitrate reductase to net photosynthesis. (10,20,22, 27) 14, 23, 28, 29). This might lead to the suspicion that nitrate reduction in leaves should respond to intercellular CO2 concentrations, and thereby to stomatal aperture. Therefore, we examined the response of nitrate reduction to water stress and to CO2 availability in detached spinach leaves. It is shown that nitrate reduction and net CO2 assimilation in leaves are tightly coupled, and that under mild water stress nitrate reduction decreases as a result of stomatal closure. MATERIALS AND METHODS Plant MaterialSpinach (Spinacia oleracea cv 'Yates') was grown in a greenhouse under additional artificial illumination providing an average photon flux density of 400 ,uE m-2 s-' (PAR), with a day length of 11 h at about 22°C. Nitrate Reduction in Whole Leaves and Leaf SegmentsFor measuring nitrate reduction, four to eight freshly harvested detached leaves (with the petiole in distilled water or in various salt solutions) were kept in a large glass cuvette (about 4 L gas volume) and were supplied with humidified air, C02-free air or N2 at a flow rate of about 4 L min-'. The cuvette was mounted in a temperature-controlled growth chamber at a photon flux density of 400 ,uE m-2 s-' PAR (HQi-lamps 400 W, Schreder, Winterbach, FRG) and a temperature of 20°C. Determination of Nitrate LevelsFor determination of nitrate levels (and levels of other anions), leaf samples were taken at the appropriate time intervals over an 8 h period. The cuvette was opened without stopping the gas flow, and leaf discs were cut out with a cork borer (10 mm diameter). Discs from four leaves of the same treatment were usually pooled...

Influence of malate and 2-oxoglutarate on the NIA transcript level and nitrate reductase activity in tobacco leaves

“…Third, malate led to a marked decrease of the NIA transcript level and NIA activity but did not markedly affect NIA activation (Figs 2,3 & 6). This contrasts with the response to stomatal closure or low CO 2 , which lead to post-translational inactivation of NIA (Kaiser & Förster 1989;Kaiser & Huber 1994). Fourth, a strikingly different response was found when 2-oxoglutarate was supplied (Figs 2, 4 & 6) even though both organic acids lead to a decrease in the transpiration rate.…”

“…The increase of the NIA transcript and NIA activity was antagonized by malate, and synthesized and exported via the phloem to the roots where it is decarboxylated (Raven 1986(Raven , 1988Martinoia & Rentsch 1994). During the day, nitrate assimilation and the accompanying synthesis of malate proceed more rapidly than malate export, and malate accumulates to high levels that are equivalent to 25-30% of the amount of assimilated nitrate (Touraine, Grignon & Grignon 1988;Deng, Moureaux & Lamaze 1989;Kaiser & Förster 1989;Scheible et al 1997b).…”

Interaction of Sulfate Assimilation with Carbon and Nitrogen Metabolism in Lemna minor

“…In contrast, nitrate assimilation was shown to be dependent on the supply of carbon skeletons produced during CO 2 assimilation (Oaks, 1994). NR, as the key enzyme of this pathway, is inactivated upon exposure of plants to air without CO 2 (Kaiser and Fö rster, 1989). NR activity and expression is induced by light, but also by sugars in the dark (Cheng et al, 1992;Campbell, 1999).…”

A model of isoprene emission based on energetic requirements for isoprene synthesis and leaf photosynthetic properties for Liquidambar and Quercus

“…7a) as well as on leaf photosynthetic electron transport. Similarly, nitrate photo-assimilation was low in conditions of low C i , which resulted from moderate leaf water stress (Kaiser & Förster 1989).…”