Short-term effects of nitrate,nitrite and ammonium assimilation on

Introduction
The hypothesis that the pathways of carbon and nitrogen assimilation compete for carbon skeletons and energy is widely accepted but the mechanisms that co-regulate these pathways are only poorly understood in higher plants. Several lines of evidence suggest that there is "cross-talk" between nitrogen and carbon metabolism which may involve several steps of reciprocal control. Short-term inhibition of NO 3- reduction was observed under conditions of mild water-stress or when turgid spinach leaves were kept in CO2-free air (Kaiser and Forster 1989). The availability of CO2 can thus determine
Abstract. Maize (Zea mays L. cv. Contessa) was grown with a nitrogen supply that was just sufficient to support maximal biomass production. The third leaves from 14to 21-d-old plants were harvested and net photosynthesis allowed to attain steady-state rates at an irradiance of either 250 or 700 gmol.m 2.s 1. Nitrogen in the form of either KNO3, K N O 2 or NH4C1 was then supplied to the leaves through the transpiration stream. In all cases the addition of the nitrogen source resulted in an approximate doubling of the total amino-acid content of the leaves within 1 h. The glutamine pool increased to ten times the level found in control leaves in the light in the absence of added nitrogen. Glutamine accounted for about 21-24% of the total amino-acid content in leaves fed with 40 m M NH4C1. Nitrate caused a rapid, but transient inhibition of the rate of net CO2 assimilation, accompanied by an increase in the activity of phosphoenolpyruvate carboxylase and a decrease in the maximum extractable activity of sucrose-phosphate synthase. This demonstrates that the activities of phosphoenolpyruvate carboxylase and sucrose-phosphate synthase are modulated by N O 3 in the Ca plant maize, in a similar manner to that observed in C3 plants. Nitrite or ammonium feeding resulted in decreased rates of CO2 assimilation for as long as the nitrogen source was supplied. In all cases the degree of inhibition was greatest at high irradiance and least at low irradiance, even though the total amino-acid contents of the leaves were comparable at the time when maximum inhibition of CO2 assimilation occurred. Measurements of chlorophyll-a fluorescence showed that the quantum efficiency of PSII decreased and non-radiative dissipation of excitation energy increased as CO 2 assimilation was inhibited by niAbbreviations and symbol: Chl = chlorophyll; FBPase = fructose-l,6-bisphosphatase; Gln = glutamine; Glu = gluramie acid; KD = index of the rate of thermal energy dissipation within the PSII antenna; LHCII = light-harvesting chlorophyll-a/ b-binding protein; PEPCase = phosphoenolpyruvate carboxylase; PFD = photon flux density; SPS = sucrose-phosphate synthase; ~psn = relative quantum efficiencyfor electron transport by PSII Correspondence to: Christine H. Foyer
Planta (1994)192:2 Nhomakorabea1-220
P l a n t ~ 9 Springer-Verlag 1994
Short-term effects of nitrate, nitrite and ammonium assimilation on photosynthesis, carbon partitioning and protein phosphorylation in maize
212 the rate of N O 3- reduction in C3 leaves (Kaiser and Forster 1989). This co-regulation is facilitated by the reversible inactivation of NO 3 reductase (Kaiser and Forster 1989; Kaiser and Spill 1991; Huber et al. 1992a,b). In addition, when C3 leaves were supplied with NO3, CO2 assimilation was rapidly inhibited in the short-term but CO2-dependent 02 evolution was much less affected (Champigny et al. 1991a) or even enhanced (De LaTorre et al. 1991). This transitory response to high-NO 3 treatment precedes the adaptive changes in enzyme activities that enable the plant to support the increased demand for carbon residues associated with N assimilation (Sugiharto et al. 1990; Sugiharto and Sugiyama 1992). Short-term changes in metabolism precede the stimulatory effect of N availability on growth that is observed when plants are provided with high NO3-. Nitrate-induced regulation of metabolism in wheat leaves involves inhibition of sucrose synthesis. It is related to the covalent modification of specific cytosolic enzyme proteins in response to the presence of NO 3- (Van Quy et al. 1991b; Champigny and Foyer 1992). In the presence of N O 3- the phosphorylation states of wheat leaf sucrose-phosphate synthase (SPS) and phosphoenolpyruvate carboxylase (PEPCase) were increased (Van Quy et al. 1991b; Champigny and Foyer 1992), causing activation of the latter enzyme and inhibition of the former. Glutamine (Gln) and glutamic acid (Glu) are important regulators of PEPCase in leave (Sugiharto and Sugiyama 1992; Sugiharto et al. 1992; Manh et al. 1993). Glutamine acts at the level of gene expression and the Gln: Glu ratio modulates the PEPCase protein kinase (Sugiharto et al. 1992; Manh et al. 1993). In the short term the Gin: Glu ratio determines the phosphorylation level of the PEPCase protein, thereby regulating the flow of carbon through the anapleurotic pathway providing carbon skeletons for the tricarboxylic-acid cycle and for amino-acid synthesis. The mechanism by which NO3modulates phosphorylation of SPS is as yet unknown (Champigny and Foyer 1992). Carbon flow is thus directed away from sucrose synthesis toward amino-acid synthesis (Van Quy et al. 1991a,b; Champigny and Foyer 1992). Growth of the alga Selenastrum minutum under conditions of N limitation leads to a situation where the capacity for N assimilation is potentially greater than that for carbon assimilation (Elrifi and Turpin 1986, 1987; Smith et al. 1989). The addition of NO 3 or NH4 + in the light has been shown to inhibit CO 2 assimilation by 70 and 40%, respectively (Elrifi and Turpin 1986). Thus, the relative fluxes for carbon and N assimilation can be rapidly and drastically altered. The extent to which carbon and N assimilation compete for the products of photosynthetic electron flow, i.e. ATP and reducing equivalents, is unclear. De LaTorre et al. (1991) observed increased light-dependent 02 evolution in both barley and pea leaves fed N O 3 prior to illumination. Light-dependent 02 evolution was increased in leaves pre-fed NO 3 but unchanged in leaves pre-fed NH4 + (Bloom et al. 1989). The extent of the stimulation of 02 evolution by N O 3- was greatest at higher light intensities (Bloom et al. 1989; De LaTorre et al. 1991).