Overexpression of sucrose-phosphate synthase in tomato plants grown with CO2 enrichment leads to decreased foliar carbohydrate accumulation relative to untransformed controls

Tomato plants expressing the maize sucrose-phosphate synthase (SPS) cDNA under the control of the promoterof the small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase (rbcS) promoter were grown 5 weeks in air (450 μmol.m^–2.s^–1 irradiance, 350 ppm CO₂) and then either maintained in air or exposed to CO₂ enrichment (1 000 ppm CO₂) for 8 d. A linear relationship between the foliar sucrose to starch ratio and maximal extractable SPS activity was found both in air and high CO₂. Starch accumulation was dramatically increased in all plants subjected to CO₂ enrichment but the CO₂-dependent increase in foliar starch accumulation was much lower in the leaves of the SPS transformants than in those of the untransformed controls in the same conditions. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase activity was reduced by growth at high CO₂ to a similar extent in both plant types. The carbon/nitrogen ratios were similar in both plant lines in both growth conditions after 20 d exposure to high CO₂. A small (5 %) increase in carbon export capacity was observed at high CO₂ in the leaves of transformed plants compared to leaves from untransformed controls. Increased foliar SPS activity did not, however, prevent acclimation of photosynthesis in plants grown with long-term CO₂ enrichment.     

Overexpression of sucrose phosphate synthase increases sucrose unloading in transformed tomato fruit

Sucrose unloading and sink activity were examined in tomato plants (Lycopersicon esculentum) overexpression sucrose phosphate synthase (SPS; EC 2.3.1.14). Like the leaves, the fruit of the transformed tomato plants had elevated (2.4-fold) SPS activity. SPS over-expression in tomato fruit did not significantly change acid invertase, and only slightly reduced ADPglc ppase activity, but enhanced sucrose synthase activity by 27%. More importantly, the amount of sucrose unloaded into the fruit was considerably increased. Using [³H]- (fructosyl)-sucrose in in vitro unloading experiments with harvested 20-d-old fruit, 70% more sucrose was unloaded into the transformed fruits compared to the untransformed controls. Furthermore, the turnover of the sucrose unloaded into the fruit of transformed plants was 60% higher than that observed in the untransformed controls. Taken together, these results demonstrate that SPS overexpression increases the sink strength of transformed tomato fruit.     

Modulation of amino acid metabolism in transformed tobacco plants deficient in Fd-GOGAT

Tobacco (Nicotiana tabacum) plants expressing a partial ferredoxin-dependent glutamine-2-oxoglutarate aminotransferase (Fd-GOGAT) cDNA in the antisense orientation under the control of the 35S promoter, were used to study the metabolism of amino acids, 2-oxoglutarate and ammonium following the transition from CO₂ enrichment (where photorespiration is inhibited) to air (where photorespiration is a major process of ammonium production in leaves). The leaves of the lowest Fd-GOGAT expressors accumulated more foliar glutamine (Gln) and α-ketoglutarate (α-KG) than the untransformed controls in both growth conditions. Photorespiration-dependent increases in foliar ammonium, glutamine, α-KG and total amino acids were proportional to the decreases in foliar Fd-GOGAT activity. No change in endoprotease activity was observed following transfer to air in the Fd-GOGAT transformants or the untransformed controls which has similar activities over a broad range of pH values. We conclude that several pathways of amino acid biosynthesis are modified when NH₃ ⁺ and Gln accumulate in leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modulation of carbon and nitrogen metabolism, and of nitrate reductase, in untransformed and transformed Nicotiana plumbaginifolia during CO2 enrichment of plants grown in pots and in hydroponic culture

Transformed plants of Nicotiana plumbaginifolia Viv. constitutively expressing nitrate reductase (35S-NR) or β-glucuronidase (35S-GUS) and untransformed controls were grown for two weeks in a CO₂-enriched atmosphere. Whereas CO₂ enrichment (1000 μl · l⁻¹) resulted in an increase in the carbon (C) to nitrogen (N) ratio of both the tobacco lines grown in pots with vermiculite, the C/N ratio was only slightly modified when plants were grown in hydroponic culture in high CO₂ compared to those grown in air. Constitutive nitrate reductase (NR) expression per se did not change the C/N ratio of the shoots or roots. Biomass accumulation was similar in both types of plant when hydroponic or pot-grown material, grown in air or high CO₂, were compared. Shoot dry matter accumulation was primarily related to the presence of stored carbohydrate (starch and sucrose) in the leaves. In the pot-grown tobacco, growth at elevated CO₂ levels caused a concomitant decrease in the N content of the leaves involving losses in NO⁻ ₃ and amino acid levels. In contrast, the N content and composition were similar in all plants grown in hydroponic culture. The 35S-NR plants grown in air had higher foliar maximum extractable NR activities and increased glutamine levels (on a chlorophyll or protein basis) than the untransformed controls. These increases were maintained following CO₂ enrichment when the plants were grown in hydroponic culture, suggesting that an increased flux through nitrogen assimilation was possible in the 35S-NR plants. Under CO₂ enrichment the NR activation state in the leaves was similar in all plants. When the 35S-NR plants were grown in pots, however, foliar NR activity and glutamine content fell in the 35S-NR transformants to levels similar to those of the untransformed controls. The differences in NR activity between untransformed and 35S-NR leaves were much less pronounced in the hydroponic than in the pot-grown material but the difference in total extractable NR activity was more marked following CO₂ enrichment. Foliar NR message levels were decreased by CO₂ enrichment in all growth conditions but this was much more pronounced in pot-grown material than in that grown hydroponically. Since β-glucuronidase (GUS) activity and message levels in 35S-GUS plants grown under the same conditions of CO₂ enrichment (to test the effects of CO₂ enrichment on the activity of the 35S promoter) were found to be constant, we conclude that NR message turnover was specifically accelerated in the 35S-NR plants as well as in the untransformed controls as a result of CO₂ enrichment. The molecular and metabolic signals involved in increased NR message and protein turnover are not known but possible effectors include NO₃ ⁻, glutamine and asparagine. We conclude that plants grown in hydroponic culture have greater access to N than those grown in pots. Regardless of the culture method, CO₂ enrichment has a direct effect on NR mRNA stability. Received: 17 October 1996 / Accepted: 11 February 1997     

Effects of CO(2) Enrichment and Carbohydrate Content on the Dark Respiration of Soybeans

During the period of most active leaf expansion, the foliar dark respiration rate of soybeans (Glycine max cv Williams), grown for 2 weeks in 1000 microliters CO₂ per liter air, was 1.45 milligrams CO₂ evolved per hour leaf density thickness, and this was twice the rate displayed by leaves of control plants (350 microliters CO₂ per liter air). There was a higher foliar nonstructural carbohydrate level (e.g. sucrose and starch) in the CO₂ enriched compared with CO₂ normal plants. For example, leaves of enriched plants displayed levels of nonstructural carbohydrate equivalent to 174 milligrams glucose per gram dry weight compared to the 84 milligrams glucose per gram dry weight found in control plant leaves. As the leaves of CO₂ enriched plants approached full expansion, both the foliar respiration rate and carbohydrate content of the CO₂ enriched leaves decreased until they were equivalent with those same parameters in the leaves of control plants. A strong positive correlation between respiration rate and carbohydrate content was seen in high CO₂ adapted plants, but not in the control plants.

Mitochondria, isolated simultaneously from the leaves of CO₂ enriched and control plants, showed no difference in NADH or malate-glutamate dependent O₂ uptake, and there were no observed differences in the specific activities of NAD⁺ linked isocitrate dehydrogenase and cytochrome c oxidase. Since the mitochondrial O₂ uptake and total enzyme activities were not greater in young enriched leaves, the increase in leaf respiration rate was not caused by metabolic adaptations in the leaf mitochondria as a response to long term CO₂ enrichment. It was concluded, that the higher respiration rate in the enriched plant's foliage was attributable, in part, to a higher carbohydrate status.

     

Decrease in leaf sucrose synthesis leads to increased leaf starch turnover and decreased RuBP regeneration-limited photosynthesis but not Rubisco-limited photosynthesis in Arabidopsis null mutants of SPSA1

We investigated the individual effect of null mutations of each of the four sucrose‐phosphate synthase (SPS) genes in Arabidopsis (SPSA1, SPSA2, SPSB and SPSC) on photosynthesis and carbon partitioning. Null mutants spsa1 and spsc led to decreases in maximum SPS activity in leaves by 80 and 13%, respectively, whereas null mutants spsa2 and spsb had no significant effect. Consistently, isoform‐specific antibodies detected only the SPSA1 and SPSC proteins in leaf extracts. Leaf photosynthesis at ambient [CO₂] was not different among the genotypes but was 20% lower in spsa1 mutants when measured under saturating [CO₂] levels. Carbon partitioning at ambient [CO₂] was altered only in the spsa1 null mutant. Cold treatment of plants (4 °C for 96 h) increased leaf soluble sugars and starch and increased the leaf content of SPSA1 and SPSC proteins twofold to threefold, and of the four null mutants, only spsa1 reduced leaf non‐structural carbohydrate accumulation in response to cold treatment. It is concluded that SPSA1 plays a major role in photosynthetic sucrose synthesis in Arabidopsis leaves, and decreases in leaf SPS activity lead to increased starch synthesis and starch turnover and decreased Ribulose 1,5‐bisphosphate regeneration‐limited photosynthesis but not ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco)‐limited photosynthesis, indicating a limitation of triose‐phosphate utilization (TPU).     

Direct and Indirect Effects of Atmospheric Carbon Dioxide Enrichment on Leaf Respiration of Glycine max (L.) Merr

Long-term and short-term effects of CO2 enrichment on dark respiration were investigated using soybean (Glycine max [L.] Merr.) plants grown at either 35.5 or 71.0 Pa CO2. Indirect effects, or effects of growth in elevated CO2, were examined using a functional model that partitioned respiration into growth and maintenance components. Direct effects, or immediate effects of a short-term change in CO2, were examined by measuring dark respiration, first, at the CO2 partial pressure at which plants were grown, and second, after equilibration in the reciprocal CO2 partial pressure. The functional component model indicated that the maintenance coefficient of respiration increased 34% with elevated CO2, whereas the growth coefficient was not significantly affected. Changes in maintenance respiration were correlated with a 33% increase in leaf total nonstructural carbohydrate concentration, but leaf nitrogen content of soybean leaves was not affected by CO2 enrichment. Thus, increased maintenance respiration may be a consequence of increased nonstructural carbohydrate accumulation. When whole soybean plants were switched from low CO2 to high CO2 for a brief period, leaf respiration was always reduced. However, this direct effect of CO2 partial pressure was approximately 50% less in plants grown in elevated CO2. We conclude from this study that there are potentially important effects of CO2 enrichment on plant respiration but that the effects are different for plants given a short-term increase in CO2 partial pressure versus plants grown in elevated CO2.     

Analysis of carbon metabolism in transgenic<Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> transformed with the cyanobacterial sucrose phosphate synthase gene

To investigate the effects of sucrose-phosphate synthase (SPS) on carbon partitioning, transgenicArabidopsis plants transformed withSynechocystis SPS were constructed. The integration, copy number and expression level were confirmed by Southern and Northern blot analyses. SPS activity in leaves from the transgenic and wild type plants was not significantly different. The level of sucrose and starch in the leaves of transgenic plant were slightly decreased compared to wild type. The glucose and fructose contents were increased up to two-fold compared to wild type during the light period. It is our speculation that the decreased sucrose level of the transgenic plant might be caused by the high acid invertase. These authors contributed equally to this work     

Responses of carboxylating enzymes,sucrose metabolizing enzymes and plant hormones in a tropical epiphytic CAM orchid to CO2 enrichment

After exposure to a doubled CO₂ concentration of 750 µL L^?1 for 2 months, average relative growth rate (RGR) of Mokara Yellow increased 25%. The two carboxylating enzymes, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPCase), responded differently to CO₂ enrichment. There was a significant daytime down‐regulation in Rubisco activity in the leaves of CO₂‐enriched plants. However, PEPCase activity in CO₂‐enriched plants was much higher in the dark period, although it was slightly lower during the daytime than that at ambient CO₂. Leaf sucrose–phosphate synthase (SPS) and sucrose synthase (SS) activities in CO₂‐enriched plants increased markedly, along with a night‐time increase in total titratable acidity and malate accumulation. There was a remarkable increase in the levels of indole‐3‐acetic acid (IAA), gibberellins A₁ and A₃ (GA₁₊₃), isopentenyladenosine (iPA) and zeatin riboside (ZR) in the expanding leaves of plants grown at elevated CO₂. It is suggested that (1) the down‐regulation of Rubisco and up‐regulation of SPS and SS are two important acclimation processes that are beneficial because it enhanced both photosynthetic capacity at high CO₂ and reduced resource investment in excessive Rubisco capacity; (2) the increased levels of plant hormones in CO₂‐enriched M. Yellow might play an important role in controlling its growth and development.     

Altered photosynthesis,flowering, and fruiting in transgenic tomato plants that have an increased capacity for sucrose synthesis

Photosynthesis, leaf assimilate partitioning, flowering, and fruiting were examined in two lines of Lycopersicon esculentum Mill. transformed with a gene coding for sucrose-phosphate synthase (SPS) (EC 2.3.1.14) from Zea mays L. expressed from a tobacco ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit promoter. Plants were grown at either 35 or 65 Pa CO₂ and high light (1000 mol photons·m^–2·s^–1). Limiting and maximum SPS activities were significantly greater (up to 12 times) in the leaves of SPS-transformed lines for all treatments. Partitioning of carbon into sucrose increased 50% for the SPS transformants. Intact leaves of the control lines exhibited CO₂-insensitivity of photosynthesis at high CO₂ levels, whereas the SPS transformants did not exhibit CO₂-insensitivity. The O₂-sensitivity of photosynthesis was also greater for the SPS-transformed lines compared to the untransformed control when measured at 65 Pa CO₂. These data indicate that the SPS transformants had a reduced limitation on photosynthesis imposed by end-product synthesis. Growth at 65 Pa CO₂ resulted in reduced photosynthetic capacity for control lines but not for SPS-transformed lines. When grown at 65 Pa CO₂, SPS transformed lines had a 20% greater photosynthetic rate than controls when measured at 65 Pa CO₂ and a 35% greater rate when measured at 105 Pa CO₂. Photosynthetic rates were not different between lines when grown at 35 Pa CO₂. The time to 50% blossoming was reduced and the total number of inflorescences was significantly greater for the SPS transformants when grown at either 35 or 65 Pa CO₂. At 35 Pa CO₂, the total fruit number of the SPS transformants was up to 1.5 times that of the controls, the fruit matured earlier, and there was up to a 32% increase in total fruit dry weight. Fruit yield was not significantly different between the lines when grown at 65 Pa CO₂. Therefore, there was not a strict relationship between yield and leaf photosynthesis rate. Flowering and fruit development of the SPS-transformed lines grown at 35 Pa CO₂ showed similar trends to the controls grown at 65 Pa CO₂. Incidences of blossom-end rot were also reduced in the SPS-transformed lines. These data indicate that altering starch/sucrose partitioning by increasing the capacity for sucrose synthesis can affect acclimation to elevated CO₂ partial pressure and flowering and fruiting in tomato.Abbreviations DAS days after seeding - nptII neomycin phos-photransferase - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuBP ribulose-1,5-bisphosphate - SPS sucrose-phosphate synthase - SSU Rubisco small subunit This research was supported by U.S. Department of Energy grant FG02-87ER13785. B.J.M. thanks the Natural Sciences and Engineering Research Council of Canada for financial support. We are grateful to Toni A. Voelker (Calgene Inc.) for supplying tomato seeds and valuable advice.     

Up-regulation of sucrose metabolizing enzymes in Oncidium goldiana grown under elevated carbon dioxide

Experiments were conducted in controlled growth chambers to evaluate how increase in CO₂ concentration affected sucrose metabolizing enzymes, especially sucrose phosphate synthase (SPS; EC 2.4.1.14) and sucrose synthase (SS; EC 2.4.1.13), as well as carbon metabolism and partitioning in a tropical epiphytic orchid species (Oncidium goldiana). Response of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) to elevated CO₂ was determined along with dry mass production, photosynthesis rate, chlorophyll content, total nitrogen and total soluble protein content. After 60 days of growth, there was a 80% and 150% increase in dry mass production in plants grown at 750 and 1 100 μl l^?1 CO₂, respectively, compared with those grown at ambient CO₂ (about 370 μl l^?1). A similar increase in photosynthesis rate was detected throughout the growth period when measured under growth CO₂ conditions. Concomitantly, there was a decline in leaf Rubisco activity in plants in elevated CO₂ after 10 days of growth. Over the growth period, leaf SPS and SS activities were up‐regulated by an average of 20% and 40% for plants grown at 750 and 1100 μl l^?1 CO₂, respectively. Leaf sucrose content and starch content were significantly higher throughout the growth period in plants grown at elevated CO₂ than those at ambient CO₂. The partitioning of photosynthetically fixed carbon between sucrose and starch appeared to be unaffected by the 750 μl l^?1 CO₂ treatment, but it was favored into starch under the 1 100 μl l^?1 CO₂ condition. The activities of SPS and SS in leaf extracts were closely associated with photosynthetic rates and with partitioning of carbon between starch and sucrose in leaves. The data are consistent with the hypothesis that the up‐regulation of leaf SPS and SS might be an acclimation response to optimize the utilization and export of organic carbon with the increased rate of inorganic‐carbon fixation in elevated CO₂ conditions.     

Source-sink relations in Lolium perenne L. as reflected by carbohydrate concentrations in leaves and pseudo-stems during regrowth in a free air carbon dioxide enrichment (FACE) experiment*

The effect of an elevated partial pressure of CO₂ (p_CO2) on carbohydrate concentrations in source leaves and pseudo-stems (stubble) of Lolium perenne L. (perennial ryegrass) during regrowth was studied in a regularly defoliated grass sward in the field. The free air carbon dioxide enrichment (FACE) technology enabled natural environmental conditions to be provided. Two levels of nitrogen (N) supply were used to modulate potential plant growth. Carbohydrate concentrations in source leaves were increased at elevated p_CO2, particularly at low N supply. Elevated leaf carbohydrate concentrations were related to an increased structural carbon (C) to N ratio and thus reflected an increased C availability together with a N-dependent sink limitation. Immediately after defoliation, apparent assimilate export rates (differences in the carbohydrate concentrations of young source leaves measured in the evening and on the following morning) showed a greater increase at elevated p_CO2 than at ambient p_CO2; however, replenishment of carbohydrate reserves was not accelerated. Distinct, treatment-dependent carbohydrate concentrations in pseudo-stems suggested an increasing degree of C-sink limitation from the treatment at ambient p_CO2 with high N supply to that at elevated p_CO2 with low N supply. During two growing seasons, no evidence of a substantial change in the response of the carbohydrate source in L. perenne to elevated p_CO2 was found. Our results support the view that the response of L. perenne to elevated p_CO2 is restricted by a C-sink limitation, which is particularly severe at low N supply.     

Induced systemic resistance in groundnut by foliar application of Headline (Pyraclostrobin 20% WG)

Plant’s own defence mechanisms can be stimulated by the application of elicitors for its protection. We evaluated the ability of Headline (Pyraclostrobin 20% WG) to induce systemic resistance in 40-day-old field-grown groundnut plants by foliar application. Elevated levels of defence-related enzymes such as phenylalanine ammonia lyase, peroxidase, β 1,3 glucanase, nitrate and nitrite reductase and other biochemical parameters such as total phenol, total protein, total carbohydrate and total chlorophyll content were found highest in Headline treated plants on fourth day after treatment in comparison with untreated plants. The treated plants showed enhanced growth and development as well as increased yield up to 16%. About 10% mortality rate was found in groundnut seeds treated with Headline when grown on Sclerotium rolfsii-infested soil than control. These results indicate that foliar application of Headline can be successfully developed the broad-spectrum resistance against pathogens in groundnut.     

Effects of Foliar and Root Applications of Methanol on the Growth of Arabidopsis, Tobacco, and Tomato Plants

The effects of aqueous methanol solutions applied as a foliar spray or via irrigation were investigated in Arabidopsis, tobacco, and tomato plants. Methanol applied to roots leads to phytotoxic damage in all three species tested. Foliar application causes an increase of fresh and dry weight in Arabidopsis and tobacco plants, but not in tomato plants. The increase in fresh and dry weight of Arabidopsis plants does not correlate with increased levels of soluble sugars, suggesting that increased accumulation of other products is responsible for the differences in the methanol-treated leaves. Foliar application of methanol can induce pectin methylesterase (PME) gene expression in Arabidopsis and tomato plants, activating specific PME genes.     

New insights on sucrose metabolism: evidence for an active A/N-Inv in chloroplasts uncovers a novel component of the intracellular carbon trafficking

The presence of sucrose (Suc) in plastids was questioned for several decades. Although it was reported some decades ago, neither Suc transporters nor Suc metabolizing enzymes were demonstrated to be active in those organelles. By biochemical, immunological, molecular and genetic approaches we show that alkaline/neutral invertases (A/N-Invs) are also localized in chloroplasts of spinach and Arabidopsis. A/N-Inv activity and polypeptide content were shown in protein extracts from intact chloroplasts. Moreover, we functionally characterized the Arabidopsis At-A/N-InvE gene coding for a chloroplast-targeted A/N-Inv. The At-A/N-InvE knockout plants displayed a lower total A/N-Inv activity in comparison with wild-type plants. Furthermore, neither A/N-Inv activity nor A/N-Inv polypeptides were detected in protein extracts prepared from chloroplasts of mutant plants. Also, the measurement of carbohydrate content, in leaves harvested either at the end of the day or at the end of the night period, revealed that the knockout plants showed a decrease in starch accumulation but no alteration in Suc levels. These are the first results demonstrating the presence of a functional A/N-Inv inside chloroplasts and its relation with carbon storage in Arabidopsis leaves. Taken together our data and recent reports, we conclude that the participation of A/N-Invs in the carbon flux between the cytosol and the plastids may be a general phenomenon in plants. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Drought induces fructan synthesis and 1-SST (sucrose: sucrose fructosyltransferase) in roots and leaves of chicory seedlings (Cichorium intybus L.)

 Seeds of Cichorium intybus L. var. foliosum cv. Flash were sown in acid-washed vermiculite and grown in a controlled-environment growth chamber. After 1 month of growth, plantlets did not contain sucrose:sucrose 1-fructosyltransferase (1-SST), the key enzyme in fructan biosynthesis. No fructan could be observed. Some of the plants were submitted to drought for 2 weeks. Glucose, fructose and sucrose concentrations increased in roots and leaves of stressed plants and the fructan concentration in roots and leaves was ten times higher than in control plants. The onset of fructan synthesis coincided with the increase in 1-SST activity in roots. Expression of the 1-SST gene could be observed in roots and leaves of stressed plants. Received: 12 July 1999 / Accepted: 16 October 1999     

Elevated sucrose-phosphate synthase activity in transgenic tobacco sustains photosynthesis in older leaves and alters development

Constitutive over-expression of a maize sucrose-phosphate synthase (SPS) gene in tobacco (Nicotiana tabacum) had major effects on leaf carbohydrate budgets with consequences for whole plant development. Transgenic tobacco plants flowered earlier and had greater flower numbers than wild-type plants. These changes were not linked to modified source leaf carbon assimilation or carbon export, although sucrose to starch ratios were significantly higher in leaves expressing the transgene. The youngest and oldest leaves of plants over-expressing SPS had up to 10-fold wild-type maximal extractable SPS activity, but source leaf SPS activities were only 2-3 times greater in these lines than in the wild type. In the oldest leaves, where the expression of the transgene led to the most marked enhancement in SPS activity, photosynthesis was also increased. It was concluded that these increases in the capacity for sucrose synthesis and carbon assimilation, particularly in older leaves, accelerate the whole plant development and increase the abundance of flowers without substantial changes in the overall shoot biomass.     

Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment

Four South African Leucadendron congenerics with divergent soil N and P preferences were grown as juveniles at contrasting nutrient concentrations at ambient (350 mol mol^-1) and elevated (700 mol mol^-1) atmospheric CO2 levels. Photosynthetic parameters were related to leaf nutrient and carbohydrate status to reveal controls of carbon uptake rate. In all species, elevated CO2 depressed both the maximum Rubisco catalytic activity (Vc,max, by 19-44%) and maximum electron transport rate (Jmax, by 13-39%), indicating significant photosynthetic acclimation of both measures. Even so, all species had increased maximum light-saturated rate of net CO2 uptake (Amax)) at the elevated growth CO2 level, due to higher intercellular CO2 concentration (ci). Leaf nitrogen concentration was central to photosynthetic performance, correlating with Amax, Vc,max and Jmax, Vc,max and Jmax were linearly co-correlated, revealing a relatively invariable Jmax:Vc,max ratio, probably due to N resource optimization between light harvesting (RuBP regeneration) and carboxylation. Leaf total non-structural carbohydrate concentration (primarily starch) increased in high CO2, and was correlated with the reduction in Vc,max and Jmax. Apparent feedback control of Vc,max and Jmax was thus surprisingly consistent across all species, and may regulate carbon exchange in response to end-product fluctuation. If so, elevated CO2 may have emulated an excess end-product condition, triggering both Vc,max and Jmax down-regulation. In Leucadendron, a general physiological mechanism seems to control excess carbohydrate formation, and photosynthetic responsiveness to elevated CO2, independently of genotype and nutrient concentration. This mechanism may underlie photosynthetic acclimation to source:sink imbalances resulting from such diverse conditions as elevated CO2, low sink strength, low carbohydrate export, and nutrient limitation.     

Export of carbon from leaf blades of Poa alpina L. at elevated CO2 and two nutrient regimes

The hypothesis was tested that, in plants of the alpine meadow grass (Poa alpina L.) exposed to elevated CO2, net photosynthesis and export from source leaves is reduced as a result of feedback from sinks. Nutrient supply was used as one way of reducing photosynthesis and export. Single plants were grown in sand culture under specified controlled environmental conditions for a period of 50 d at two levels of nitrogen and phosphorus ('low': 0.2 mol m^-3 N, 0.04 mol m^-3 P; 'high': 2.5 mol m^-3 N, 0.5 mol m^-3 P). Compartmentation within, and export of carbon from, individual youngest fully expanded leaves of acclimated plants was determined using ¹⁴C feeding and efflux plus mass balance calculations of carbohydrate export. Independent of treatment, the bulk of soluble carbohydrate (65-75%) was present as fructan, with most of the remainder being sucrose. Depending on nutrient supply, CO2 could alter export from source leaves either by a reduction in the amount of sucrose present in a readily available pool for transport, or by altering the rate constant describing phloem loading.Key words: Poa alpina L., phloem transport, carbohydrate, compartmentation, export, elevated CO2, nutrients.      

The Arabidopsis phytochrome B gene influences growth of the apple rootstock M26

 The apple rootstock M26 (Malus domestica) was infected with a binary vector system of Agrobacterium tumefaciens carrying the neomycin phosphotransferase II and Arabidopsis phyB genes. Thirteen transformed clones were obtained from 329 infected leaves. Five of the clones had a single copy integration, six clones had two copies, one clone had five copies and one of the clones had eight copies of the phyB gene integrated. No differences in rooting were found between transformed and untransformed plants. The stem length was reduced in nine of the 13 transgenic clones, and shoot, root and plant dry weights were reduced in all transformed clones compared with untransformed control plants. Northern analysis showed that the Arabidopsis phyB gene was expressed in the transformed clones. Received: 28 April 1999 / Revision received: 28 February 2000 / Accepted:29 February 2000