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

Sugar beet provides around one third of the sugar consumed worldwide and serves as a significant source of bioenergy in the form of ethanol. Sucrose accounts for up to 18% of plant fresh weight in sugar beet. Most of the sucrose is concentrated in the taproot, where it accumulates in the vacuoles. Despite 30 years of intensive research, the transporter that facilitates taproot sucrose accumulation has escaped identification. Here, we combine proteomic analyses of the taproot vacuolar membrane, the tonoplast, with electrophysiological analyses to show that the transporter BvTST2.1 is responsible for vacuolar sucrose uptake in sugar beet taproots. We show that BvTST2.1 is a sucrose-specific transporter, and present evidence to suggest that it operates as a proton antiporter, coupling the import of sucrose into the vacuole to the export of protons. BvTST2.1 exhibits a high amino acid sequence similarity to members of the tonoplast monosaccharide transporter family in Arabidopsis, prompting us to rename this group of proteins 'tonoplast sugar transporters'. The identification of BvTST2.1 could help to increase sugar yields from sugar beet and other sugar-storing plants in future breeding programs.

The sessile lifestyle of higher plants is accompanied by their remarkable ability to tolerate unfavorable environmental conditions. This is because, during evolution, plants developed a sophisticated repertoire of molecular and metabolic reactions to cope with changing biotic and abiotic challenges. In particular, the abiotic factors light intensity and ambient temperature are characterized by altering their amplitude within comparably short periods of time and are causative for onset of dynamic plant responses. These rapid responses in plants are also classified as 'acclimation reactions' which differ, due to their reversibility and duration, from non-reversible 'adaptation reactions'. In this review, we demonstrate the remarkable importance of stress-induced changes in carbohydrate homeostasis of plants exposed to high light or low temperatures. These changes represent a co-ordinated process comprising modifications of (i) the concentrations of selected sugars; (ii) starch turnover; (iii) intracellular sugar compartmentation; and (iv) corresponding gene expression patterns. The critical importance of these individual processes has been underlined in the recent past by the analyses of a large number of mutant plants. The outcome of these analyses raised our understanding of acclimation processes in plants per se but might even become instrumental to develop new concepts for directed breeding approaches with the aim to increase abiotic stress tolerance of crop species, which in most cases have high stress sensitivity. The latter direction of plant research is of special importance since abiotic stress stimuli strongly impact on crop productivity and are expected to become even more pronounced because of human activities which alter environmental conditions rapidly.

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The energy status of plant cells strongly depends on the energy metabolism in chloroplasts and mitochondria, which are capable of generating ATP either by photosynthetic or oxidative phosphorylation, respectively. Another energy-rich metabolite inside plastids is the glycolytic intermediate phosphoenolpyruvate (PEP). However, chloroplasts and most non-green plastids lack the ability to generate PEP via a complete glycolytic pathway. Hence, PEP import mediated by the plastidic PEP/phosphate translocator or PEP provided by the plastidic enolase are vital for plant growth and development. In contrast to chloroplasts, metabolism in non-green plastids (amyloplasts) of starch-storing tissues strongly depends on both the import of ATP mediated by the plastidic nucleotide transporter NTT and of carbon (glucose 6-phosphate, Glc6P) mediated by the plastidic Glc6P/phosphate translocator (GPT). Both transporters have been shown to co-limit starch biosynthesis in potato plants. In addition, non-photosynthetic plastids as well as chloroplasts during the night rely on the import of energy in the form of ATP via the NTT. During energy starvation such as prolonged darkness, chloroplasts strongly depend on the supply of ATP which can be provided by lipid respiration, a process involving chloroplasts, peroxisomes, and mitochondria and the transport of intermediates, i.e. fatty acids, ATP, citrate, and oxaloacetate across their membranes. The role of transporters involved in the provision of energy-rich metabolites and in pathways supplying plastids with metabolic energy is summarized here.

Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g., roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types developed according to their respective function, and cells of either tissue type are highly compartmentalized. Photoassimilates are allocated to distinct compartments of these tissues in all organs, requiring a set of metabolite transporters mediating this intercompartmental transfer. The general route of photoassimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triose phosphates either enter the cytosol for mainly sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil in oil bodies, or – in combination with nitrogen – as protein in protein storage vacuoles and protein bodies. Here, we focus on transport proteins known for either of these steps, and discuss the implications for yield increase in plants upon genetic engineering of respective transporters.

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