Although nitrate reductase (NR. EC 1.6.6.1) is thought to control the rate of nitrate assimilation, mutants with 40-45% of wildtype (WT) NR activity (NRA) grow as fast as the WT. We have investigated how tobacco (Nicotiana tabacum L. cv. Gatersleben) mutants with one or two instead of four functional nia genes compensate. (i) The nia transcript was higher in the leaves of the mutants. However, the diurnal rhythm was retained in the mutants, with a maximum at the end of the night and a strong decline during the photoperiod. (ii) Nitrate reductase protein and NRA rose to a maximum after 3-4 h light in WT leaves, and then decreased by 50-60% during the second part of the photoperiod and the first part of the night. Leaves of mutants contained 40-60% less NR protein and NRA after 3-4 h illumination, but NR did not decrease during the photoperiod. At the end of the photoperiod the WT and the mutants contained similar levels of NR protein and NRA. (iii) Darkening led to a rapid inactivation of NR in the WT and the mutants. However, in the mutants, this inactivation was reversed after 1-3 h darkness. Calyculin A prevented this reversal. When magnesium was included in the assay to distinguish between the active and inactive forms of NR, mutants contained 50% more activity than the WT during the night. Conversion of [15N]-nitrate to organic compounds in leaves in the first 6 h of the night was 60% faster in the mutants than in the WT. (iv) Growth of WT plants in enhanced carbon dioxide prevented the decline of NRA during the second part of the photoperiod, and led to reactivation of NR in the dark. (v) Increased stability of NR in the light and reversal of dark-inactivation correlated with decreased levels of glutamine in the leaves. When glutamine was supplied to detached leaves it accelerated the breakdown of NR, and led to inactivation of NR, even in the light. (vi) Diurnal changes were also investigated in roots. In the WT, the amount of nia transcript rose to a maximum after 4 h illumination and then gradually decreased. The amplitude of the changes in transcript amount was smaller in roots than in leaves, and there were no diurnal changes in NRA. In mutants, nia transcript levels were high through the photoperiod and the first part of the night. The NRA was 50% lower during the day but rose during the night to an activity almost as high as in the WT. The rate of [15N]-nitrate assimilation in the roots of the mutants resembled that in the WT during the first 6 h of the night. (vii) Diurnal changes were also compared in Nia30(145) transformants with very low NRA, and in nitrate-deficient WT plants. Both sets of plants had similar low growth rates. Nitrate reductase did not show a diurnal rhythm in leaves or roots of Nia30(145), the leaves contained very low glutamine, and NR did not inactivate in the dark. Nitrate-deficient WT plants were watered each day with 0.2 mM nitrate. After watering, there was a small peak of nia transcript NR protein and NRA and, slightly later, a transient increase of glutamine and other ami...

To investigate the effect of water stress on carbon metabolism in growing potato tubers (Solanum tuberosum L.), freshly cut and washed discs were incubated in a range of mannitol concentrations corresponding to external water potential between 0 and )1.2 MPa. (i) Incorporation of [ 14 C]glucose into starch was inhibited in water-stressed discs, and labeling of sucrose was increased. High glucose overrode the changes at low water stress (up to )0.5 MPa) but not at high water stress. (ii) Although [ 14 C]sucrose uptake increased in water-stressed discs, less of the absorbed [ 14 C]sucrose was metabolised. (iii) Analysis of the sucrose content of the discs confirmed that increasing water deficit leads to a switch, from net sucrose degradation to net sucrose synthesis. (iv) In parallel incubations containing identical concentrations of sugars but differing in which sugar was labeled, degradation of [ 14 C]sucrose and labeling of sucrose from [ 14 C]glucose and fructose was found at each mannitol concentration. This shows that there is a cycle of sucrose degradation and resynthesis in these tuber discs. Increasing the extent of water stress changed the relation between sucrose breakdown and sucrose synthesis, in favour of synthesis. (v) Analysis of metabolites showed a biphasic response to increasing water deficit. Moderate water stress (0-200 mM mannitol) led to a decrease of the phosphorylated intermediates, especially 3-phosphoglycerate (3PGA). The decrease of metabolites at moderate water stress was not seen when high concentrations of glucose were supplied to the discs. More extreme water stress (300-500 mM mannitol) was accompanied by an accumulation of metabolites at low and high glucose. (vi) Moderate water stress led to an activation of sucrose phosphate synthase (SPS) in discs, and in intact tubers. The stimulation involved a change in the kinetic properties of SPS, and was blocked by protein phosphatase inhibitors. (vii) The amount of ADP-glucose (ADPGlc) decreased when discs were incubated on 100 or 200 mM mannitol. There was a strong correlation between the in vivo levels of ADPGlc and 3PGA when discs were subjected to moderate water stress, and when the sugar supply was varied. (viii) The level of ADPGlc increased and starch synthesis was further inhibited when discs were incubated in 300-500 mM mannitol. (ix) It is proposed that moderate water stress leads to an activation of SPS and stimulates sucrose synthesis. The resulting decline of 3PGA leads to a partial inhibition of ADP-glucose pyrophosphorylase and starch synthesis. More-extreme water stress leads to a further alteration of partitioning, because it inhibits the activities of one or more of the enzymes involved in the terminal reactions of starch synthesis. Abbreviations: ADPGlc = ADP-glucose; AGPase = ADP-glucose pyrophosphorylase; Fru6P = fructose-6-phosphate; Glc6P = glucose-6-phosphate; 3PGA = 3-phosphoglycerate; P i = inorganic phosphate; SPS = sucrose-phosphate synthase; UDPGlc = uridine-5′-diphosphoglucose; V max = activity of SPS assaye...

The nitrogen concentration usually decreases in elevated [CO 2 ] (Wong 1979;Hocking & Meyer 1985;Hocking & Meyer 1991a, 1991bColeman et al. 1993;Pettersson, MacDonald & Stadenburg 1993;Rogers et al. 1993;McKee & Woodward 1994;Jacob, Greitner & Drake 1995;Nie et al. 1995;Poorter et al. 1997), indicating that nitrogen uptake lags behind carbohydrate synthesis and growth in elevated [CO 2 ]. The effect of elevated [CO 2 ] on the nitrate uptake rates per unit root weight is rather variable and apparently depends on the nitrogen concentration supplied (Larigauderie, Reynolds & Strain 1994) and the species. Whereas elevated [CO 2 ] increased the rate of nitrate uptake per unit root weight in loblolly pines (Bassirirad et al. 1996) and Prosopis glandulosa (Bassirirad et al. 1997), it did not alter nitrate uptake in Nardus agrostis (Bassirirad et al. 1997), and it decreased nitrate uptake in a mixed field community (Jackson & Reynolds 1996). Although nitrate uptake might be improved in elevated [CO 2 ] because plants possess more roots and exploit a larger soil volume (see, e.g. Stulen & den Hertog 1993;Pettersson et al. 1993;Jackson & Reynolds 1996), the decreased water flow in elevated [CO 2 ] will tend to decrease the root surface concentrations of nitrate (Van Vuuren et al. 1997).The organic nitrogen concentration decreases in elevated [CO 2 ] (Wong 1979;Curtis, Drake & Whigham 1989;Garbutt, Williams & Bazzaz 1990;Coleman et al. 1991;Hocking & Meyer 1991a, 1991bColeman & Bazzaz 1992;Gries, Kimball & Idso 1993;Pettersson et al. 1993;Körner & Miglietta 1994;Pettersson & MacDonald 1994; FerrarioMery et al. 1997;Poorter et al. 1997), indicating that nitrate assimilation fails to keep pace with growth. There is conflicting evidence with respect to the effect of elevated [CO 2 ] on nitrate reductase (NR) activity. Although elevated [CO 2 ] led to a small increase of NR activity in mustard (Maeskaya et al. 1990) and Vigna radiata (Sharma & Sen Gupta 1990), it produced a two-fold decrease of NR activity in wheat (Hocking & Meyer 1991a), maize (Purvis, Peters & Hageman 1974), and a 15-25% decrease in Nicotiana plumbaginifolia (Ferrario-Mery et al. 1997). It also led to a decrease of nitrite reductase activity in lettuce (Besford & Hand 1989). In Plantago major, elevated [CO 2 ] led to a transient increase in NR activity, that was reversed after a few days (Fonseca, Bowsher & Stulen 1997). These reports of a decrease of NR activity in elevated [CO 2 ] are rather surprising, because exogenous sugars lead to increased expression of Nia (Cheng et al. 1992;Vincentz et al. 1993;Krapp et al. 1993;Krapp & Stitt 1995;Morcuende et al. 1998) and post-translational activation of NR (Kaiser & Huber 1994; Huber, Bachman & Huber 1996) in detached leaves. Recently Geiger et al. (1998) showed that although elevated [CO 2 ] does not markedly increase the maximum NR activity in tobacco the diurnal regulation of NR is modified, allowing higher activity in the later part of the light period and during the night.Even less is kno...

Higher rates of nitrate assimilation are required to support faster growth in enhanced carbon dioxide. To investigate how this is achieved, tobacco plants were grown on high nitrate and high light in ambient and enhanced (700 µmol mol ). Enhanced carbon dioxide only led to small changes of NR activity, nitrate decreased, and overall amino acids were not significantly increased. (h) Young seedlings had a high growth rate (0·5 g -1 d-1

Tocopherols (vitamin E) are lipophilic antioxidants presumed to play a key role in protecting chloroplast membranes and the photosynthetic apparatus from photooxidative damage. Additional nonantioxidant functions of tocopherols have been proposed after the recent finding that the Suc export defective1 maize (Zea mays) mutant (sxd1) carries a defect in tocopherol cyclase (TC) and thus is devoid of tocopherols. However, the corresponding vitamin E deficient1 Arabidopsis mutant (vte1) lacks a phenotype analogous to sxd1, suggesting differences in tocopherol function between C4 and C3 plants. Therefore, in this study, the potato (Solanum tuberosum) ortholog of SXD1 was isolated and functionally characterized. StSXD1 encoded a protein with high TC activity in vitro, and chloroplastic localization was demonstrated by transient expression of green fluorescent protein-tagged fusion constructs. RNAi-mediated silencing of StSXD1 in transgenic potato plants resulted in the disruption of TC activity and severe tocopherol deficiency similar to the orthologous sxd1 and vte1 mutants. The nearly complete absence of tocopherols caused a characteristic photoassimilate export-defective phenotype comparable to sxd1, which appeared to be a consequence of vascular-specific callose deposition observed in source leaves. CO 2 assimilation rates and photosynthetic gene expression were decreased in source leaves in close correlation with excess sugar accumulation, suggesting a carbohydrate-mediated feedback inhibition rather than a direct impact of tocopherol deficiency on photosynthetic capacity. This conclusion is further supported by an increased photosynthetic capacity of young leaves regardless of decreased tocopherol levels. Our data provide evidence that tocopherol deficiency leads to impaired photoassimilate export from source leaves in both monocot and dicot plant species and suggest significant differences among C3 plants in response to tocopherol reduction.

The influence of elevated [CO 2 ] on the uptake and assimilation of nitrate and ammonium was investigated by growing tobacco plants in hydroponic culture with 2 m M nitrate or 1 m M ammonium nitrate and ambient or 800 p.p.m.[CO 2 ]. Leaves and roots were harvested at several times during the diurnal cycle to investigate the levels of the transcripts for a high-affinity nitrate transporter ( NRT2 ), nitrate reductase ( NIA ), cytosolic and plastidic glutamine synthetase ( GLN1 , GLN2 ), the activity of NIA and glutamine synthetase, the rate of 15 N-nitrate and 15 N-ammonium uptake, and the levels of nitrate, ammonium, amino acids, 2-oxoglutarate and carbohydrates. (i) In source leaves of plants growing on 2 m M nitrate in ambient [CO 2 ], NIA transcript is high at the end of the night and NIA activity increases three-fold after illumination. The rate of nitrate reduction during the first part of the light period is two-fold higher than the rate of nitrate uptake and exceeds the rate of ammonium metabolism in the glutamate: oxoglutarate aminotransferase (GOGAT) pathway, resulting in a rapid decrease of nitrate and the accumulation of ammonium, glutamine and the photorespiratory intermediates glycine and serine. This imbalance is reversed later in the diurnal cycle. The level of the NIA transcript falls dramatically after illumination, and NIA activity and the rate of nitrate reduction decline during the second part of the light period and are low at night. NRT2 transcript increases during the day and remains high for the first part of the night and nitrate uptake remains high in the second part of the light period and decreases by only 30% at night. The nitrate absorbed at night is used to replenish the leaf nitrate pool. GLN2 transcript and glutamine synthetase activity rise to a maximum at the end of the day and decline only gradually after darkening, and ammonium and amino acids decrease during the night. (ii) In plants growing on ammonium nitrate, about 30% of the nitrogen is derived from ammonium. More ammonium accumulates in leaves during the day, and glutamine synthetase activity and glutamine levels remain high through the night. There is a corresponding 30% inhibition of nitrate uptake, a decrease of the absolute nitrate level, and a 15-30% decrease of NIA activity in the leaves and roots. The diurnal changes of leaf nitrate and the absolute level and diurnal changes of the NIA transcript are, however, similar to those in nitrate-grown plants. (iii) Plants growing on nitrate adjust to elevated [CO 2 ] by a coordinate change in the diurnal regulation of NRT2 and NIA , which allows maximum rates of nitrate uptake and maximum NIA activity to be maintained for a larger part of the 24 h diurnal cycle. In contrast, tobacco growing on ammonium nitrate adjusts by selectively increasing the rate of ammonium uptake, and decreasing the expression of NRT2 and NIA and the rate of nitrate assimilation. In both conditions, the overall rate of inorganic nitrogen utilization is increased in elevated [CO 2 ] due to...

-This paper gives an overview of adaptive discretization methods for linear second-order hyperbolic problems such as the acoustic or the elastic wave equation. The emphasis is on Galerkin-type methods for spatial as well as temporal discretization, which also include variants of the Crank-Nicolson and the Newmark finite difference schemes. The adaptive choice of space and time meshes follows the principle of "goaloriented" adaptivity which is based on a posteriori error estimation employing the solutions of auxiliary dual problems.2000 Mathematics Subject Classification: 35L05, 65M50, 65M60, 74S05.

SummaryEven though plastid aldolase catalyses a reversible reaction, does not possess properties allowing it to contribute to 'fine' regulation, and would therefore be considered unimportant for the control of metabolism and growth, antisense transformants with a 50-70% decrease in aldolase activity showed an inhibition of photosynthesis and growth. We now show that acclimation of photosynthesis to growth conditions includes and requires changes in plastid aldolase activity. Wild-type potato plants and transformants were grown at low irradiance (70 µmol m -2 sec -1 ), and at high irradiance (390 µmol m -2 sec -1 ) at 400 or 800 p.p.m. carbon dioxide. (i) Ambient photosynthesis was always inhibited by a 30-40% decrease of aldolase activity, the strongest inhibition being observed when plants were growing in high irradiance and elevated carbon dioxide. (ii) The inhibition was due to a low rate of ribulose-1,5-bisphosphate regeneration in low light, exacerbated by an inadequate rate of starch synthesis in high light and elevated carbon dioxide. Decreased expression of aldolase in antisense transformants was also accompanied by a Received 14 August 1998; revised 14 December 1998; accepted 17 December 1998. *For correspondence (fax ϩ49 6221 54 5859; e-mail mstitt@botanik1.bot.uni-heidelberg.de). † Volker Haake and Michael Geiger contributed equally to this paper. Abbreviations: A, photosynthesis; c i , intracellular CO 2 concentration; FBPase, plastid fructose-1,6-bisphosphatase; Fru6P, fructose-6-phosphate; FW, fresh weight; Glc6P, glucose-6-phosphate; g H2O , stomatal conductance for water; NADP-GAPDH, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase; PGK, glycerate-3-phosphate kinase (3-phosphoglycerate kinase); PRK, ribulose-5-phosphate kinase (5-phosphoribulokinase); SBPase, sedoheptulose-1,7-bisphosphatase; WUE, water use efficiency; pAld, transcript for plastidic aldolase; pFbp, transcript for plastidic FBPase; RbcS, transcript for the small subunit of Rubisco; Tkt, transcript for transketolase; 3PGA, glycerate-3-phosphate; Ru1,5bisP, ribulose-1,5-bisphosphate.