Metabolism of the Raffinose Family Oligosaccharides in Leaves of Ajuga reptans L. (Cold Acclimation,Translocation, and Sink to Source Transition: Discovery of Chain Elongation Enzyme)

Ajuga reptans is a frost-hardy, perennial labiate that is known for its high content of raffinose family oligosaccharide(s) (RFO). Seasonal variations in soluble nonstructural carbohydrate levels in above-ground parts of Ajuga showed that the RFO were by far the most predominant components throughout the whole year. RFO were lowest in summer (75 mg/g fresh weight) and highest in fall/winter (200 mg/g fresh weight), whereas sucrose and starch were only minor components. Cold treatment (14 d at 10/3[deg]C, day/night) of plants that were precultivated under warm conditions (25[deg]C) lowered the temperature optimum of net photosynthesis from 16[deg] to 8[deg]C, decreased the maximum rate, and increased the total nonstructural carbohydrate content of leaves by a factor of about 10, mainly because of an increase of RFO. The degree of polymerization of the RFO increased sequentially up to at least 15. A novel, galactinol-independent galactosyltransferase enzyme was found, forming from two molecules of RFO, the next higher and lower degree of polymerization of RFO. The enzyme had a pH optimum of 4.5 to 5.0 and may be responsible for RFO chain elongation. RFO were the main carbohydrates translocated in the phloem, with stachyose being by far the most dominant form. Studies of carbon balance during leaf development revealed a transition point between import and export at approximately 25% maximal leaf area. RFO synthesis could be detected even before the commencement of export, suggesting the existence of a nonphloem-linked RFO pool even in very young leaves. Taken together, it seems that Ajuga leaves contain two pools of RFO metabolism, a pronounced long-term storage pool in the mesophyll, possibly also involved in frost resistance, and a transport pool in the phloem.     

Effects of Growth Temperature on the Responses of Ribulose-1,5-Biphosphate Carboxylase,Electron Transport Components,and Sucrose Synthesis Enzymes to Leaf Nitrogen in Rice,and Their Relationships to Photosynthesis

Effects of growth temperature on the photosynthetic gas-exchange rates and their underlying biochemical properties were examined in young, fully expanded leaves of rice (Oryza sativa L.). The plants were grown hydroponically under day/night temperature regimes of 18/15[deg]C, 23/18[deg]C, and 30/23[deg]C and all photosynthetic measurements were made at a leaf temperature of 25[deg]C and an irradiance of 1800 [mu]mol quanta m-2 s-1. Growth temperature affected the photosynthetic CO2 response curve. The relative ratio of the initial slope to the CO2-saturated photosynthesis increased with rising growth temperature. This was caused mainly by an increase in CO2-limited photosynthesis for a given leaf nitrogen content with rising growth temperature. However, there was no difference in ribulose-1,5-bisphosphate carboxylase (Rubisco) content at any given leaf nitrogen content among temperature treatments. In addition, the activation state and catalytic turnover rate of Rubisco were not affected by growth temperature. The increase in CO2-limited photosynthesis with rising growth temperature was the result of an increase in the CO2 transfer conductance between the intercellular airspaces and the carboxylation sites. The amounts of total chlorophyll and light-harvesting chlorophyll a/b protein II increased for the same leaf nitrogen content with rising growth temperature, but the amounts of cytochrome f and coupling factor 1 and the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were the same between plants grown at 23/18[deg]C and those grown at 30/23[deg]C. Similarly, CO2-saturated photosynthesis was not different for the same leaf nitrogen content between these treatments. For the 18/15[deg]C-grown plants, a slight decrease in the amounts of cytochrome f and coupling factor 1 and an increase in the activities of cytosolic fructose-1,6-bisphosphatase and sucrose-phosphate synthase were found, but these were not reflected in CO2-saturated photosynthesis.     

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.     

The stoichiometry of photorespiration during C3-photosynthesis is not fixed: evidence from combined physical and stereochemical methods

The stoichiometry of photorespiration, S, is defined as the fraction of glycolate carbon photorespired. It is postulated that under steady-state conditions there are two determinants of the ratio of photorespiration to net photosynthesis: the partitioning of ribulose bisphosphate between oxidation and carboxylation, and the partitioning of glycolate between reactions leading to complete oxidation to CO2 (S = 100%) and those yielding CO2 plus serine (S = 25%). S may be calculated using two independent probes of the system. The physical probe, using an infrared gas analyzer, measured photorespiration and net photosynthesis, and hence their ratio PR/NPS = pn(phys). The metabolic probe employed tracer (3R)-D-[3-3H1,3-14C]glyceric acid to determine r, the fraction of 3H retained in the triose phosphates leaving the chloroplasts. It is deduced from the postulated model that S = pn(phys) . r/(1 - r). Experiments have been performed with illuminated tobacco leaf discs (inverted) under varying concentrations of O2 and CO2. Increasing O2 at constant CO2 increased pn(phys) and decreased r, whereas increasing CO2 at constant O2 had the opposite effect. S more than doubled at 32 degrees C on going from 16 to 40% O2 (340 microliters CO2/liter) and decreased 40% on going from 200 to 700 microliters CO2/liter (21% O2). For discs in normal air S was somewhat greater than 27%. It is suggested that net photosynthesis, and therefore crop yields, could be increased by selecting for crop plants with reduced photorespiration stoichiometry.     

Sugar synthesis and phloem loading in Coleus blumei leaves

Sugar-synthesis and -transport patterns were analyzed in Coleus blumei Benth. leaves to determine where galactinol, raffinose, and stachyose are made and whether phloem loading includes an apoplastic (extracellular) step or occurs entirely within the symplast (plasmodesmata-connected cytoplasm). To clarify the sequence of steps leading to stachyose synthesis, a pulse (15 s) of ¹⁴CO₂ was given to attached leaves followed by a 5-s to 20-min chase: sucrose was rapidly labeled while galactinol, raffinose and stachyose were labeled more slowly and, within the first few minutes, to approximately the same degree. Leaf tissue was exposed to either ¹⁴CO₂ or [¹⁴C]glucose to identify the sites of synthesis of the different sugars. A 2-min exposure of peeled leaf tissue to [¹⁴C]glucose resulted in preferential labeling of the minor veins, as opposed to the mesophyll; galactinol, raffinose and stachyose were more heavily labeled than sucrose in these preparations. In contrast, when leaf tissue was exposed to ¹⁴CO₂ for 2 min for preferential labeling of the mesophyll, sucrose was more heavily labeled than galactinol, raffinose or stachyose. We conclude that sucrose is synthesized in mesophyll cells while galactinol, raffinose and stachyose are made in the minorvein phloem. Competition experiments were performed to test the possibility that phloem loading involves monosaccharide uptake from the apoplast. Two saturable monosaccharide carriers were identified, one for glucose, galactose and 3-O-methyl glucose, and the other for fructose. Washing the apoplast of peeled leaf pieces with buffer or saturating levels of 3-O-methyl glucose, after providing a pulse of ¹⁴CO₂, did not inhibit vein loading or change the composition of labeled sugars, and less than 0.5% of the assimilated label was recovered in the incubation medium. These and previous results (Turgeon and Gowan, 1991, Plant Physiol. 94, 1244–1249) indicate that the phloem loading pathway in Coleus is probably symplastic.Abbreviations 3-OMG 3-O-methyl glucose - PCMBS p-chloromercuribenzenesulfonic acid - SE-CCC sieve-element-companion-cell complex This research was supported by National Science Foundation Grant DCB-9104159, U.S. Department of Agriculture Competetive Grant 90000854, and Hatch funds.     

High glycolate oxidase activity is required for survival of maize in normal air

A mutant in the maize (Zea mays) Glycolate Oxidase1 (GO1) gene was characterized to investigate the role of photorespiration in C4 photosynthesis. An Activator-induced allele of GO1 conditioned a seedling lethal phenotype when homozygous and had 5% to 10% of wild-type GO activity. Growth of seedlings in high CO2 (1%-5%) was sufficient to rescue the mutant phenotype. Upon transfer to normal air, the go1 mutant became necrotic within 7 d and plants died within 15 d. Providing [1-14C]glycolate to leaf tissue of go1 mutants in darkness confirmed that the substrate is inefficiently converted to 14CO2, but both wild-type and GO-deficient mutant seedlings metabolized [1-14C]glycine similarly to produce [14C]serine and 14CO2 in a 1:1 ratio, suggesting that the photorespiratory pathway is otherwise normal in the mutant. The net CO2 assimilation rate in wild-type leaves was only slightly inhibited in 50% O2 in high light but decreased rapidly and linearly with time in leaves with low GO. When go1 mutants were shifted from high CO2 to air in light, they accumulated glycolate linearly for 6 h to levels 7-fold higher than wild type and 11-fold higher after 25 h. These studies show that C4 photosynthesis in maize is dependent on photorespiration throughout seedling development and support the view that the carbon oxidation pathway evolved to prevent accumulation of toxic glycolate.     

Carbohydrate Metabolism in Photosynthetic and Nonphotosynthetic Tissues of Variegated Leaves of Coleus blumei Benth

Mature, variegated leaves of Coleus blumei Benth. contained stachyose and other raffinose series sugars in both green, photosynthetic and white, nonphotosynthetic tissues. However, unlike the green tissues, white tissues had no detectable level of galactinol synthase activity and a low level of sucrose phosphate synthase indicating that stachyose and possibly sucrose present in white tissues may have originated in green tissues. Uptake of exogenously supplied [¹⁴C]stachyose or [¹⁴C]sucrose into either tissue type showed conventional kinetic profiles indicating combined operation of linear first-order and saturable systems. Autoradiographs of white discs showed no detectable minor vein labelling with [¹⁴C]stachyose, but some degree of vein labeling with [¹⁴C]sucrose. Autoradiographs of green discs showed substantial vein loading with either sugar. In both tissues, p-chloromercuribenzenesulfonic acid had no effect on the linear component of sucrose or stachyose uptake but inhibited the saturable component. Both tissues contained high levels of invertase, sucrose synthase and α-galactosidase and extensively metabolized exogenously supplied ¹⁴C-sugars. In green tissues, label from exogenous sugars was recovered as raffinose-series sugars. In white tissues, exogenous sugars were hydrolysed and converted to amino acids and organic acids. The results indicate that variegated Coleus leaves may be useful for studies on both phloem loading and phloem unloading processes in stachyose-transporting species.     

Low night temperatures inhibit galactinol synthase gene expression and phloem loading in melon leaves during fruit development

Low night temperatures seriously affect plant growth and fruit quality. To investigate the effect of low night temperatures on the expression of galactinol synthase genes (GOLS) and phloem loading of raffinose family oligosaccharides, particular stachyose and raffinose (RFO represents stachyose and raffinose in this paper) and to gain a better understanding of the relationship between the phloem loading of RFO and fruit development, melon (Cucumis melo L.) plants at the fruit development stage were treated with temperatures of 28/12°C or 28/9°C (day/night) with 28/15°C as the control. Both the CmGOLS1 and CmGOLS2 gene expression and the activity of galactinol synthase were clearly repressed after treatments with 9 and 12°C at night, and the effect of 9°C was more obvious. Furthermore, low night temperatures inhibited photosynthesis and caused the lower amounts of sucrose to supply the RFO synthesis. However, the total soluble sugar, RFO, and sucrose contents were increased in leaves subjected to low night temperatures. It is supposed that low night temperature blocked symplastic phloem loading, which led to the accumulation of RFO in the leaf cells. With increasing content of RFO in the leaves, the expression of GOLS genes was inhibited according to the principle of feedback, and therefore the decreased expression of GOLS limited RFO synthesis and was indirectly harmful to phloem loading, thereby affecting fruit development.     

Effects of Certain Inhibitors on Photorespiration by Wheat Leaf Segments

The effect on the carbon metabolism of wheat leaf segments ofcertain inhibitors of photorespiration was studied. Sodium 2-hydroxy-3-butynoatesupplied for 40 min resulted in accumulation of ¹⁴C in glycolicacid with only a 7% inhibition of photosynthesis; when suppliedfor 90 min, photosynthesis was inhibited by 47%. When ¹⁴CO₂was replaced by 1000 vpm ¹²CO₂, radioactivity in glycine decreasedbut increased more rapidly in sucrose with less release of ¹⁴CO₂.Isonicotinyl hydrazide (INH) inhibited photosynthesis from ¹⁴CO₂by 50% and glycine replaced sucrose as the main product. When,after 15 min, ¹⁴CO₂ was replaced by 150 vpm ¹²CO₂, in the presenceof INH less ¹⁴CO₂ was released, ¹⁴CO in glycine decreased moreslowly, and less [¹⁴CO]sucrose accumulated. Glycidate (potassium2,3-epoxypropionate) at 2 mM had no effect on photosyntheticrate and little effect on carbon metabolism; 20 mM glycidateinhibited photosynthesis by 64% and resulted in less radioactivityin glycine, more in phosphate esters, and less ¹⁴CO₂ released.When photosynthesis was measured in 1000 vpm CO₂ the inhibitorsgave smaller effects on metabolism than during photosynthesisfrom 150 vpm ¹⁴CO₂ but 20 mM glycidate still resulted in a 42%inhibition of photosynthesis. When U- [¹⁴CO]glycerate was appliedto leaf segments in air with 320 vpm ¹⁴CO₂ the total uptakeof glycerate was not changed by the inhibitors. INH and glycidateboth decreased the amount of glycerate metabolised. More ¹⁴COaccumulated in glycine in the presence of INH and in phosphateesters and serine in the presence of glycidate. Hydroxybutynoateincreased the production of glycolate from glycerate but didnot affect the total amount of glycerate metabolised. Although all three inhibitors affected photorespiratory metabolismnone stimulated photosynthesis. The results are consistent withthe main release of CO₂ in photorespiration arising from theconversion of glycine to serine.     

Phloem loading in cucumber: combined symplastic and apoplastic strategies

Phloem loading, as the first step of transporting photoassimilates from mesophyll cells to sieve element‐companion cell complex, creates a driving force for long‐distance nutrient transport. Three loading strategies have been proposed: passive symplastic loading, apoplastic loading and symplastic transfer followed by polymer‐trapping of stachyose and raffinose. Although individual species are generally referred to as using a single phloem loading mechanism, it has been suggested that some plants may use more than one, i.e. ‘mixed loading’. Here, by using a combination of electron microscopy, reverse genetics and ¹⁴C labeling, loading strategies were studied in cucumber, a polymer‐trapping loading species. The results indicate that intermediary cells (ICs), which mediate polymer‐trapping, and ordinary companion cells, which mediate apoplastic loading, were mainly found in the fifth and third order veins, respectively. Accordingly, a cucumber galactinol synthase gene (CsGolS1) and a sucrose transporter gene (CsSUT2) were expressed mainly in the fifth/third and the third order veins, respectively. Immunolocalization analysis indicated that CsGolS1 was localized in companion cells (CCs) while CsSUT2 was in CCs and sieve elements (SEs). Suppressing CsGolS1 significantly decreased the stachyose level and increased sucrose content, while suppressing CsSUT2 decreased the sucrose level and increased the stachyose content in leaves. After ¹⁴CO₂ labeling, [¹⁴C]sucrose export increased and [¹⁴C]stachyose export reduced from petioles in CsGolS1i plants, but [¹⁴C]sucrose export decreased and [¹⁴C]stachyose export increased into petioles in CsSUT2i plants. Similar results were also observed after pre‐treating the CsGolS1i leaves with PCMBS (transporter inhibitor). These results demonstrate that cucumber phloem loading depends on both polymer‐trapping and apoplastic loading strategies.     

Temperature Dependence of the Linkage of Quantum Yield of Photosystem II to CO2 Fixation in C4 and C3 Plants

The temperature dependence of quantum yields of electron transport from photosystem II (PSII) ([phi]II, determined from chlorophyll a fluorescence) and CO2 assimilation ([phi]CO2, apparent quantum yield for CO2 assimilation) were determined simultaneously in vivo. With C4 species representing NADP-malic enzyme, NAD-malic enzyme, and phosphoenolpyruvate carboxykinase subgroups, the ratio of [phi]II/[phi]CO2 was constant over the temperature range from 15 to 40[deg]C at high light intensity (1100 [mu]mol quanta m-2 s-1). A similar response was obtained at low light intensity (300 [mu]mol quanta m-2 s-1), except the ratio of [phi]II/[phi]CO2 increased at high temperature. When the true quantum yield for CO2 fixation ([phi]CO2*) was calculated by correcting for respiration in the light (estimated from temperature dependence of dark respiration), the ratio of [phi]II/[phi]C02* remained constant with varying temperature and under both light intensities in all C4 species examined. Because the [phi]II/[phi]CO2* ratio was the same in C4 monocots representing the three subgroups, the ratio was not affected by differences in the bio-chemical mechanism of concentrating CO2 in the bundle sheath cells. The results suggest that PSII activity is closely linked to the true rate of CO2 fixation in C4 plants. The close relationship between [phi]II and [phi]CO2* in C4 species under varying temperature and light intensity conditions is apparently due to a common low level of photorespiration and a primary requirement for reductive power in the C3 pathway. In contrast, in a C3 plant the [phi] II/[phi]CO2* ratio is higher under normal atmospheric conditions than under nonphotorespiratory conditions and it increases with rising temperature. This decrease in efficiency in utilizing energy derived from PSII for CO2 fixation is due to an increase in photorespiration. In both the C3 and C4 species, photochemistry is limited under low temperature, and thus excess energy must be dissipated by nonphotochemical means.     

Carbon Partitioning in Eelgrass (Regulation by Photosynthesis and the Response to Daily Light-Dark Cycles)

Diel variations in rates of C export, sucrose-phosphate synthase (SPS) and sucrose synthase (SS) activity, and C reserves were investigated in Zostera marina L. (eelgrass) to elucidate the environmental regulation of sucrose formation and partitioning in this ecologically important species. Rates of C flux and SPS activity increased with leaf age, consistent with the ontogenic transition from sink to source status. Rates of C export and photosynthesis were low but quantitatively consistent with those of many terrestrial plant species. The Vmax activity of SPS approached that of maize, but substrate-limited rates were 20 to 25% of Vmax, indicating a large pool of inactive SPS. SPS was unresponsive to the day/night transition or to a 3-fold increase in photosynthesis generated by high [CO2] and showed little sensitivity to inorganic phosphate. Consequently, regulation of eelgrass SPS appeared similar to starch- rather than to sugar-accumulating species even though eelgrass accumulates sucrose. Leaf [sucrose] was constant and high throughout the diel cycle, which may contribute to the down-regulation of SPS. Root sucrose synthase activity was high but showed no response to nocturnal anoxia. Root [sucrose] also showed no diel cycle. The temporal stability of [sucrose] confers an ability for eelgrass to buffer the effects of prolonged light limitation that may be key to its survival and ecological success in environments subject to periods of extreme light limitation and chaotic daily variation in light availability.     

Grafting helps improve photosynthesis and carbohydrate metabolism in leaves of muskmelon

The most important quality for muskmelon (Cucumis melo L.) is their sweetness which is closely related to the soluble sugars content. Leaves are the main photosynthetic organs in plants and thus the source of sugar accumulation in fruits since sugars are translocated from leaves to fruits. The effects of grafting muskmelon on two different inter-specific (Cucurbita maxima×C. moschata) rootstocks was investigated with respect to photosynthesis and carbohydrate metabolism. Grafting Zhongmi1 muskmelon on RibenStrong (GR) or Shengzhen1 (GS) rootstocks increased chlorophyll a, chlorophyll b and chlorophyll a+b content and the leaf area in middle and late developmental stages of the plant compared to the ungrafted Zhongmi1 check (CK). Grafting enhanced the net photosynthesis rate, the stomatal conductance, concentration of intercellular CO(2) and transpiration rate. Grafting influenced carbohydrates contents by changing carbohydrate metabolic enzymes activities which was observed as an increase in acid invertase and neutral invertase activity in the functional leaves during the early and middle developmental stages compared to CK. Grafting improved sucrose phosphate synthase and stachyose synthase activities in middle and late developmental stages, thus translocation of sugars (such as sucrose, raffinose and stachyose) in GR and GS leaves were significantly enhanced. However, compared with CK, translocation of more sugars in grafted plants did not exert feedback inhibition on photosynthesis. Our results indicate that grafting muskmelon on inter-specific rootstocks enhances photosynthesis and translocation of sugars in muskmelon leaves.     

Molecular Genetics of Crassulacean Acid Metabolism

Most higher plants assimilate atmospheric CO2 through the C3 pathway of photosynthesis using ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). However, when CO2 availability is reduced by environmental stress conditions, the incomplete discrimination of CO2 over O2 by Rubisco leads to increased photorespiration, a process that reduces the efficiency of C3 photosynthesis. To overcome the wasteful process of photorespiration, approximately 10% of higher plant species have evolved two alternate strategies for photosynthetic CO2 assimilation, C4 photosynthesis and Crassulacean acid metabolism. Both of these biochemical pathways employ a "CO2 pump" to elevate intracellular CO2 concentrations in the vicinity of Rubisco, suppressing photorespiration and therefore improving the competitiveness of these plants under conditions of high light intensity, high temperature, or low water availability. This CO2 pump consists of a primary carboxylating enzyme, phosphoenolpyruvate carboxylase. In C4 plants, this CO2-concentrating mechanism is achieved by the coordination of two carboxylating reactions that are spatially separated into mesophyll and bundle-sheath cell types (for review, see R.T. Furbank, W.C. Taylor [1995] Plant Cell 7: 797-807;M.S.B. Ku, Y. Kano-Murakami, M. Matsuoka [1996] Plant Physiol 111: 949-957). In contrast, Crassulacean acid metabolism plants perform both carboxylation reactions within one cell type, but the two reactions are separated in time. Both pathways involve cell-specific changes in the expression of many genes that are not present in C3 plants.     

Influence of Environmental Factors and Air Composition on the Emission of [alpha]-Pinene from Quercus ilex Leaves

We studied the emission of [alpha]-pinene from Quercus ilex leaves. Only the abaxial side of the hypostomatous Q. ilex leaf emits [alpha]-pinene. Light induced photosynthesis and [alpha]-pinene emission. However, the response of photosynthesis to dark-to-light transitions was faster than that of [alpha]-pinene, suggesting that ATP controls the emission. The emission was higher at 30 than at 20[deg]C, whereas photosynthesis did not change. Therefore, the relationship between photosynthesis and [alpha]-pinene emission does not always hold. When CO2 was removed from the air, transpiration was stimulated but photosynthesis and [alpha]-pinene emission were inhibited. [alpha]-Pinene inhibition was more rapid under low O2. When CO2 in the air was increased, photosynthesis was stimulated and transpiration was reduced, but [alpha]-pinene emission was unaffected. Therefore, the emission depends on the availability of photosynthetic carbon, is not saturated at ambient CO2, and is not dependent on stomatal opening. The pattern of [alpha]-pinene emission from Q. ilex is different from that of plants having specialized structures for storage and emission of terpenes. We suggest that [alpha]-pinene emitted by Q. ilex leaves is synthesized in the chloroplasts and shares the same biochemical pathway with isoprene emitted by isoprene-emitting oak species.     

Effect of High Temperature on Photosynthesis in Beans (II. CO2 Assimilation and Metabolite Contents)

The effect of high temperatures on CO2 assimilation, metabolite content, and capacity for reducing power production in non-photorespiratory conditions has been assessed in two different bean (Phaseolus vulgarus L.) varieties, Blue Lake (commercially available in the United Kingdom) and Barbucho (a noncommercially bred Chilean variety), which are known to differ in their resistance to extreme high temperatures. Barbucho maintains its photosynthetic functions for a longer period of time under extreme heat compared with Blue Lake. The CO2 assimilation rate was increased by increases in temperature, with a decrease in ratio of rates of temperatures differing by 10[deg]C. It is suggested that limitations to CO2 assimilation are caused by metabolic restrictions that can be differentiated between those occurring in the range of 20 to 30[deg]C and 30 to 35[deg]C. It is likely that changes in the capacity for Calvin cycle regeneration and starch synthesis affect photosynthesis in the range of 20 to 30[deg]C. But following an increase in temperature from 30 to 35[deg]C, the supply of reducing power becomes limiting. From analysis of adenylate concentration, transthylakoid energization, and, indirectly, NADPH/NADP+ ratio, it was concluded that the limitation in the assimilatory power was due to an oxidation of the NADPH/NADP+ pool. In the range of 30 to 35[deg]C, the photosystem I quantum yield increased and photosystem II maintained its value. We conclude that the reorganization of thylakoids observed at 30 to 35[deg]C increased the excitation of photosystem I, inducing an increase in cyclic electron transport and a decrease in the supply of NADPH, limiting carbon assimilation.     

Sources of sucrose translocated from illuminated sugar beet source leaves

A search for source leaf sucrose pools that differed in their relation to export was carried out in photosynthesizing leaves of Beta vulgaris L. The time course of depletion of [¹⁴C]sucrose in a leaf in unlabeled CO₂ following steady state labeling provided evidence for two distinct sucrose pools. After the start of the light period, leaf blade sucrose remained constant although it exchanged between the two pools. Newly synthesized sucrose destined for export passed through one pool more rapidly than through the other. All of the leaf blade sucrose appeared to exchange with export sucrose. Modeling and regression analysis of [¹⁴C]sucrose data provided a means for estimating the size of the two pools. From 20 to 40% of the sucrose was calculated to be present in the pool that provided the less direct path to export; this was likely vacuolar sucrose. The remainder of the sucrose in the blade was probably in the cytoplasm and veins. Added amounts of leaf blade sucrose, produced in response to elevated CO₂, appeared to be stored mainly in the vacuolar compartment.     

Phloem Loading in Coleus blumei in the Absence of Carrier-Mediated Uptake of Export Sugar from the Apoplast

Phloem loading in Coleus blumei Benth. leaves cannot be explained by carrier-mediated transport of export sugar from the apoplast into the sieve element-companion cell complex, the mechanism by which sucrose is thought to load in other species that have been studied in detail. Uptake profiles of the export sugars sucrose, raffinose, and stachyose into leaf discs were composed of two components, one saturable and the other not. Saturable (carrier-mediated) uptake of all three sugars was almost completely eliminated by the inhibitor p-chloromercuribenzenesulfonic acid (PCMBS). However, when PCMBS was introduced by transpiration into mature leaves it did not prevent accumulation of ¹⁴C-photosynthate in minor veins or translocation of labeled photosynthate from green to nonchlorophyllous regions of the leaf following exposure to ¹⁴CO₂. The efficacy of introducing inhibitor solutions in the transpiration stream was proven by observing saffranin O and calcofluor white movement in the minor veins and leaf apoplast. PCMBS introduced by transpiration completely inhibited phloem loading in tobacco leaves. Phloem loading in C. blumei was also studied in plasmolysis experiments. The carbohydrate content of leaves was lowered by keeping plants in the dark and then increased by exposing them to light. The solute level of intermediary cells increased in the light (phloem loading) in both PCMBS-treated and control tissues. A mechanism of symplastic phloem loading is proposed for species that translocate the raffinose series of oligosaccharides.     

Effects of a Short-Term Shift to Low Temperature and of Long-Term Cold Hardening on Photosynthesis and Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase and Sucrose Phosphate Synthase Activity in Leaves of Winter Rye (Secale cereale L.)

The effect of a short-term (hours) shift to low temperature (5[deg]C) and long-term (months) cold hardening on photosynthesis and carbon metabolism was studied in winter rye (Secale cereale L. cv Musketeer). Cold-hardened plants grown at 5[deg]C exhibited 25% higher in situ CO2 exchange rates than nonhardened plants grown at 24[deg]C. Cold-hardened plants maintained these high rates throughout the day, in contrast to nonhardened plants, which showed a gradual decline in photosynthesis after 3 h. Associated with the increase in photosynthetic capacity following cold hardening was an increase in ribulose-1,5-bisphosphate carboxylase/oxygenase and sucrose phosphate synthase activity and 3- to 4-fold increases in the pools of associated metabolites. Leaves of nonhardened plants shifted overnight to 5[deg]C required 9 h in the light at 5[deg]C before maximum rates of photosynthesis were reached. The gradual increase in photosynthesis in leaves shifted to 5[deg]C was correlated with a sharp decline in the 3-phosphoglycerate/triose phosphate ratio and by an increase in the ribulose bisphosphate/3-phosphoglycerate ratio, indicating the gradual easing of aninorganic phosphate-mediated feedback inhibition on photo-synthesis. We suggest that the strong recovery of photosynthesis in winter rye following cold hardening indicates that the buildup of photosynthetic enzymes, as well as those involved in sucrose synthesis, is an adaptive response that enables these plants to maximize the production of sugars that have both cryoprotective and storage functions that are critical to the performance of these cultivars during over-wintering.     

Photorespiratory and respiratory decarboxylations in leaves of C3 plants under different CO2 concentrations and irradiances

We used an advanced radiogasometric method to study the effects of short-term changes in CO2 concentration ([CO2]) on the rates and substrates of photorespiratory and respiratory decarboxylations under steady-state photosynthesis and in the dark. Experiments were carried out on Plantago lanceolata, Poa trivialis, Secale cereale, Triticum aestivum, Helianthus annuus and Arabidopsis thaliana plants. Rates of photorespiration and respiration measured at a low [CO2] (40 micromol mol(-1)) were equal to those at normal [CO2] (360 micromol mol(-1)). Under low [CO2], the substrates of decarboxylation reactions were derived mainly from stored photosynthates, while under normal [CO2] primary photosynthates were preferentially consumed. An increase in [CO2] from 320 to 2300 micromol mol(-1) brought about a fourfold decrease in the rate of photorespiration with a concomitant 50% increase in the rate of respiration in the light. Respiration in the dark did not depend on [CO2] up to 30 mmol mol(-1). A positive correlation was found between the rate of respiration in the dark and the rate of photosynthesis during the preceding light period. The respiratory decarboxylation of stored photosynthates was suppressed by light. The extent of light inhibition decreased with increasing [CO2]; no inhibition was detected at 30 mmol mol(-1) CO2.