RuBPCase Activity Research Articles

The role of calcium in regulating photosynthesis and related physiological indexes of cucumber seedlings under low light intensity and suboptimal temperature stress

The response of photosynthesis, antioxidant enzyme activity, and proline content to low light intensity and suboptimal temperature in Cucumis sativa L. seedlings pretreated with either distilled water, 10 mM CaCl 2, 1 mM LaCl 3, 3 mM ethyleneglycol-bis-(2-aminoethyl) tetraacetic acid (EGTA) or 0.05 mM chlorpromazine (CPZ) were investigated. The results showed that 10 mM CaCl 2 led to an increase in photosynthetic rate (Pn), carboxylation efficiency (CE), ribulose 1,5-biphosphate carboxylase (RuBPCase) activity, chlorophyll content, peroxidase (POD) and catalase (CAT) activity, and proline content of cucumber seedlings under low light intensity and suboptimal temperature, in comparison with the distilled water-pretreated seedlings. However, LaCl 3, EGTA and CPZ were in contrast to CaCl 2. These results suggest that CaCl 2 has beneficial effect on photosynthetic adaptation to low light and suboptimal temperature stress in cucumber seedlings. This might be related to the observed increase in RuBPCase activity, alleviation of lipid peroxidation and enhancement of osmoregulation, with these effects being inhibited by LaCl 3, EGTA and CPZ.

Effects of Brassinosteroid on Photosynthetic Characteristics in Soybean under Aluminum Stress

Al sensitive soybean genotype BD2 was pretreated by soaking seeds with two kinds of brassinosteroids,24-epibrassinosteroid(EBR) and long-effective brassinosteroid(coded as TS303),and then hydroponically cultured with 1/5 strength Hoagland solution with or without 1 mmol L-1 Al. Root elongation,root tip Al and callose concentrations were determined to find the optimal applicative concentration of BRs. Effects and mechanisms of EBR and TS303 on photosynthetic characteristics were subsequently investigated at optimal applicative concentration. The results showed that EBR and TS303 enhanced soybean tolerance to Al,as indicated by increased root elongation and in accordance with decreased Al and callose concentrations in root tips. The optimal effects occurred at concentration of 0.1 mg L-1 for both EBR and TS303,and TS303 showed a better effect than EBR. The results also showed that 0.1 mg L-1 of EBR and 0.1 mg L-1 of TS303 both ameliorated the decrease in leaf area and net photosynthetic rate caused by Al stress,indicating an enhancement in photosynthetic capacity. EBR and TS303 could counteract,at least in part,the decrease in chlorophyll content,maximal PS II efficiency(Fv/Fm),stomatal conductance,carbonic anhydrase activity,concentration and activity of RuBPCase caused by Al stress. The results suggested that,under Al stress,the increased net photosynthetic rate of soybean caused by EBR and TS303 was due to the increase in light energy harvest and transfer,and CO2 transportation and fixation as well.

Effect of Ar+ Implantation and Maize Genome DNA on Autotetraploid Rice

The effect of Ar+ beam implantation and maize genome DNA on autotetraploid rice is studied. Better mutation types and higher mutation rates were discovered in M2 of T3 with ion implantation and immersion in maize genome DNA. In the five agronomic categories investigated, the mutation rate of the seed setting rate was 9.1%, and the total mutation rate was 14.8% in the T3. However, the total mutation rate was 2.1% with the treatment of only ion implantation and 1.3% with the treatment of only immersion in maize genome DNA. Mutant FA36(4) was discovered in M1 with ion beam implantation and immersion in maize genome DNA. Its RuBPCase activity, PEPCase activity and seed setting rate were 32%, 153%, and 36.79%, respectively, higher than its parent IR36(4). Rapid analysis of polymorphicDNA (RAPD) analysis of three M2 plants of FA36(4) (FM1, FM2, FM3) and two controls (purple maize and IR36(4)) were also conducted with 40 random primers. S5-3 was RAPD fragment amplified with a template of purple maize, FM2 and FM3 genome DNA using primer S5. There was no S5-3 in the RAPD pattern of IR36(4) or FM1.

Analysis of Reduced Photosynthesis in the Apple Leaf under Sink-limited Conditions Due to Girdling

On the 6th day after girdling, the photosynthesis rate of leaves on the 3-year-old stem, 2-year-old stem and new shoot of a branch of a field-grown 4-year-old apple tree decreased. The starch content in the leaves, bark and wood significantly increased, especially in shoot leaves. In the girdled branch, concomitant with the photosynthesis reduction, on the 6th day after girdling, the stomatal conductance of leaves on the 3-year-old stem, 2-year-old stem and new shoot declined. Under this condition, a significant correlation between the photosynthesis rate and stomatal conductance could be recognized. On the other hand, with girdling, the activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) per leaf area decreased on the basis of the decrease in RuBPcase activity per unit enzyme protein. The initial activity of RuBPcase per leaf area in shoot leaves decreased. Moreover, the activity of RuBPcase per leaf area significantly correlated with the photosynthesis rate. Based on these results, it is suggested that the decrease in the photosynthesis rate of the sink-limited branch of a young apple tree 6 days after girdling is mainly caused by closing of the stomatal aperture and reduction of RuBPcase activity per leaf area.

Characteristics of ribulose-1,5-bisphosphate carboxylase and C4 pathway key enzymes in flag leaves of a super-high-yield hybrid rice and its parents during the reproductive stage

A newly-developed super-high-yield two-line hybrid rice ( Oryza sativa L.) LiangYouPeiJiu (LYPJ) together with its paternal line Wumang 9311 (WM9311) and maternal line Peiai 64S (PA64S) were used as experimental materials. The activities of C4 pathway photosynthetic enzymes, i.e. phosphoenolpyruvatecarboxylase (PEPcase), NADP-malic enzyme (NADP-ME), NADP-malate dehydrogenase (NADP-MDH), pyruvate phosphate dikinase (PPDK) and the key C3 pathway enzyme–ribulose-1,5-bisphosphate carboxylase (RuBPcase), as well as soluble protein were assayed during the days after emergence (DAE) of flag leaves. The results indicated that C4 pathway enzymes were expressed differently during the flag leaf's span of life. The activity of PEPcase, NADP-MDH, and PPDK were increased with leaf age after full expansion, reached the peak at around 32 DAE, and then fell down significantly, and the activity of NADP-ME increased before 23 DAE and decreased earlier than other C4 enzymes, at 32 DAE, indicating that NADP-ME may be more sensitive to senescence than other C4 enzymes. The activity of RuBPcase was significantly higher in LYPJ than that in its parents during the whole life span of flag leaves. The decrease in RuBPcase activity was greater than that in soluble protein content. The onset of loss of RuBPcase activity (11 DAE) occurred earlier than that of soluble protein content (23 DAE). In comparison, there was no significant difference in C4 pathway enzymes between LYPJ and its parents.

Molecular Interactions between the Specialist Herbivore Manduca sexta (Lepidoptera, Sphingidae) and Its Natural Host Nicotiana attenuata. VII. Changes in the Plant's Proteome

When Manduca sexta attacks Nicotiana attenuata, fatty acid-amino acid conjugates (FACs) in the larvae's oral secretions (OS) are introduced into feeding wounds. These FACs trigger a transcriptional response that is similar to the response induced by insect damage. Using two-dimensional gel electrophoresis, matrix-assisted laser desorption ionization-time of flight, and liquid chromatography-tandem mass spectrometry, we characterized the proteins in phenolic extracts and in a nuclear fraction of leaves elicited by larval attack, and/or in leaves wounded and treated with OS, FAC-free OS, and synthetic FACs. Phenolic extracts yielded approximately 600 protein spots, many of which were altered by elicitation, whereas nuclear protein fractions yielded approximately 100 spots, most of which were unchanged by elicitation. Reproducible elicitor-induced changes in 90 spots were characterized. In general, proteins that increased were involved in primary metabolism, defense, and transcriptional and translational regulation; those that decreased were involved in photosynthesis. Like the transcriptional defense responses, proteomic changes were strongly elicited by the FACs in OS. A semiquantitative reverse transcription-PCR approach based on peptide sequences was used to compare transcript and protein accumulation patterns for 17 candidate proteins. In six cases the patterns of elicited transcript accumulation were consistent with those of elicited protein accumulation. Functional analysis of one of the identified proteins involved in photosynthesis, RuBPCase activase, was accomplished by virus-induced gene silencing. Plants with decreased levels of RuBPCase activase protein had reduced photosynthetic rates and RuBPCase activity, and less biomass, responses consistent with those of herbivore-attacked plants. We conclude that the response of the plant's proteome to herbivore elicitation is complex, and integrated transcriptome-proteome-metabolome analysis is required to fully understand this ubiquitous ecological interaction.

Post-anthesis changes in photosynthetic traits of maize hybrids released in different years

Changes in post-anthesis physiological attributes related to genetic improvement for grain-yield were studied for six Chinese maize ( Zea mays L.) hybrids widely grown in North China during the past 50 years. The characteristics assessed included light-saturated photosynthetic rate ( P sat), chlorophyll content, soluble protein content, maximal efficiency of PS2 photochemistry ( F v/ F m), PS2 efficiency ( Φ PS2), ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity and phosphoenolpyruvate carboxylase (PEPCase) activity. We found that P sat of the newer hybrids was not always higher than the older ones. However, P sat of the 1950s hybrids was the highest among all at flowering stage, which was associated with their high PEPCase activity and soluble protein content. Post-anthesis changes in P sat of the older hybrids can be divided into two phases. During the first phase, which was the decisive phase of grain filling, P sat of the older hybrids declined gradually whereas the new hybrids remained relatively constant. P sat of 1950s hybrids was not the highest among all maize hybrids any longer although their PEPCase activities were also the highest. The reduction in P sat of the older hybrids during this phase was associated with a reduction in the chlorophyll content and the soluble protein content but was not influenced by the specific activity of RuBPCase and PEPCase. Adapting to the lowered CO 2 assimilation capacity, a down-regulation of Φ PS2 occurred in the older hybrids to protect the photosynthetic apparatus from photo-damage by strong light, while F v/ F m only decreased slightly during this phase, suggesting a functional PS2 apparatus in senescent flag leaves. During the second phase, the older hybrids underwent an irreversible leaf senescence that certain photosynthetic traits declined substantially with P sat, whereas the photosynthetic components of the newer hybrids remained functional. Newer maize hybrids produced higher grain yield compared to the old ones mainly because they could remain photosynthetically active when the older hybrids aged during the grain-filling period. The decline in photosynthetic rate of older hybrids during senescence is generally attributed to the degradation of both structural and functional components of chloroplasts.

Effect of Grapevine Leafroll on the Photosynthesis of Field Grown Grapevine Plants (Vitis vinifera L. cv. Lagrein)

AbstractWe have studied the effect of grapevine leafroll infection on some features of the thylakoids from field grown grapevine (Vitis vinifera L.) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose‐1,5‐bisphosphate carboxylase (RuBP), nitrate reductase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll (Chl) and carotenoids were reduced in virus‐infected leaves. Similar results were also observed for soluble proteins and RuBP case activity. The in vivo nitrate reductase activity was significantly reduced in infected leaves. Virus infection considerably decreased leaf net photosynthetic rate (Pn), stomatal conductance (gs) and transpiration rate (E) in grapevine leaves. When various photosynthetic activities were followed in isolated thylakoids, virus infection caused marked inhibition of whole chain and photosystem (PS) II activity while the inhibition of PSI activity was only marginal. The artificial exogenous electron donors, diphenyl carbazide and hydroxylamine (NH2OH) significantly restored the loss of PSII activity in infected leaves. The same results were obtained when Fv/Fm was evaluated by Chl fluorescence measurements. The marked loss of PSII activity in infected leaves could be due to the loss of 47, 43, 33, 28–25, 23 and 17 kDa polypeptides. It is concluded that virus infection inactivates the donor side of PSII. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water‐splitting complex was diminished significantly in infected leaves.

Analysis of the feed-forward effects of sink activity on the photosynthetic source-sink balance in single-rooted sweet potato leaves. I. Activation of RuBPcase through the development of sinks.

Single-rooted sweet potato leaves having a petiole with a fragment of stem allocated exceptionally large amounts of photosynthates to tuberous roots, the only major storage organ, throughout an experimental period of 50 d. The increase in photosynthetic activity for CO(2) fixation depended exclusively on the development of sink activity due to the growth of tuberous roots. Thus this model expressed a remarkable feed-forward effect on the photosynthetic source-sink balance. The level of ribulose-1,5-bisphosphate carboxylase (RuBPcase) protein in the leaves increased continuously during the period. The lowered initial as well as total activity of RuBPcase observed at the start of the experiment was raised with the cancellation of the sink-limited state due to the development of tuberous roots. The maximum activity determined after removing some inhibitor(s) from the enzyme by treating the leaf extract with SO(4)(2-) was much greater than the total activity and remained approximately constant throughout the experimental period. The clear decrease in the difference between maximum and total activities with the development of tuberous roots might reflect the reactivation of RuBPcase due to the removal of some inhibitor(s) from the enzyme through the cancellation of the sink-limited state.

Photosynthesis with single-rooted Amaranthus leaves. II. Regulation of ribuelose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, NAD-malic enzyme and NAD-malate dehydrogenase and coordination between PCR and C4 photosynthetic metabolism in response to changes in the source-sink balance.

We have studied source-sink relationships with a model consisting of single-rooted leaves without petioles. We previously reported that the rate of photosynthesis decreased when C4 model plants prepared from Amaranthus cruentus leaves were subjected to sink-limited conditions by exposure to continuous light for a few days. It was suggested that the inhibition is due to a coordinated decrease in the activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) and phosphoenol-pyruvate carboxylase (PEPcase), both essential enzymes for photosynthesis in C4 plants. We further investigated the mechanisms behind the decreased activity of RuBPcase, PEPcase, NAD-malic enzyme and NAD-malate dehydrogenase. The results suggested that (1) the initial activity of RuBPcase is suppressed by a lowering of the P(i) level in chloroplasts, (2) the inhibition of PEPcase is due to dephosphorylation of the enzyme via the inhibition of PEPcase kinase and PEPcase phosphatase, (3) the inhibition of NAD-malic enzyme and NAD-malate dehydrogenase is derived from the oxidation of these enzymes, and (4) some proteinous factor(s) may be involved in the inhibition of the activity of these latter three enzymes. The significance of a coordinated decrease in these enzymes in response to a change in the source-sink balance is discussed.

Effects of elevated CO2 concentrations on photosynthesis, dark respiration and RuBPcase activity of three species seedlings in Changbai Mountain

Two-year-old seedlings ofPinus koraiensis, Pinus sylvestriformis andFraxinus mandshurica were treated in open-top chambers with elevated CO2 concentrations (700 μL·L−1, 500 μL·L−1) and ambient CO2 concentrations (350 μL·L−1) in Changbai Mountain from June to Sept. in 1999 and 2001. The net photosynthetic rate, dark respiration rate, ribulose-1,5-bisphosphate carboxlase (RuBPcase) activity, and chlorophyll content were analyzed. The results indicated the RuBPcase activity of the three species seedlings increased at elevated CO2 concentrations. The elevated CO2 concentrations stimulated the net photosynthetic rates of three tree species exceptP. sylvestriformis grown under 500 μL·L−1 CO2 concentration. The dark respiration rates ofP. koraiensis andP. sylvestriformis increased under concentration of 700 μL·L−1 CO2, out that ofF. mandshurica decreased under both concentrations 700 μL·L−1 and 500 μL·L−1 CO2. The seedlings ofF. mandshurica decreased in chlorophyll contents at elevated CO2 concentrations.

The characteristics of CO2 assimilation of photosynthesis and chlorophyll fluorescence in transgenic PEPC rice

With PEPC, PPDK, NADP-ME and PEPC+ PPDK transgenic and untransformed rice (Orysa sativa L.), the activities of related C4 photosynthesis enzymes, the chlorophyll fluorescence parameters, CO2 exchange and other physiological indexes were compared, in which the physiological characteristics of PEPC transgenic rice were mainly studied. The results were as follows: ( i ) The activities of PEPC in PEPC transgenic rice were 20-fold higher than those in untransformed rice; the light-saturation photosynthetic rates and the carboxylation efficiency of PEPC transgenic rice were increased by 55% and 50% more than those of untransformed rice, respectively, while the CO2 compensation point decreased by 27%. (ii) The PS II photochemical efficiency (F v/F m) and photochemical quenching (q p) of transgenic PEPC rice decreased less in comparison with those of untransformed rice after the treatment with high light intensity (3 h) or methyl viologen (MV), a photooxidative reagent, which demonstrated that the tolerance of PEPC transgenic rice to photoinhibition and photooxidation was enhanced. (iii) Under the condition of high light intensity, the activity of RuBPCase in PEPC transgenic rice did not obviously vary while the activity induced of carbonic anhydrase (CA) in PEPC transgenic rice increased by 1.8 fold. These results would provide some beneficial enlightment for revealing the mechanism of high photosynthetic efficiency and breeding with high photosynthetic efficiency in rice.

Stimulation effect of gibberellic acid short-term treatment on leaf photosynthesis related to the increase in Rubisco content in broad bean and soybean.

Short-term (one hour) application (painting on surfaces of leaves) of 9 muM GA(3) increased net photosynthetic rate (Pn) in broad bean leaves at 31 Pa CO(2) and saturating light by more than 20% compared with that of control. The increased Pn was accompanied by an increase in stomatal conductance and a decrease in intercellular CO(2) partial pressure. Moreover, the GA(3)treatment increased the rate of photosynthetic oxygen evolution in isolated broad bean protoplasts to an extent similar to that of leaves. It had little effect on apparent photosynthetic quantum yield and photosynthetic electron transport rate, but could significantly increase carboxylation efficiency in leaves. In consonance with the increase in the carboxylation efficiency, RuBPCase activity and relative content of Rubisco large subunits were also increased by GA(3) treatment. Chloramphenicol, an inhibitor of chloroplast protein synthesis, could eliminate the enhancing effect of GA(3) on photosynthetic oxygen evolution and relative content of Rubisco large subunits in broad bean protoplasts. Nevertheless, actinomycin D and rifampicin, DNA transcription inhibitors, could not eliminate the enhancement effect of GA(3). Similar results were obtained with soybean leaves treated by 90 muM GA(3). These results suggest that the increase in leaf net photosynthetic rate caused by GA(3) short-term treatment is mainly due to the increases in content and activity of RuBPCase, and that GA(3) stimulates the synthesis of Rubisco subunits at translation rather than transcription level.

Temperature acclimation of the photosynthetic apparatus in an evergreen shrub, Nerium oleander

It is known that temperature dependence of the photosynthetic rate changes with growth temperatures. As one of the causes of this change, changes in thermal tolerance of photosynthetic enzymes have been suggested (Badger et al. 1982, PCE Hikosaka et al. 1999, PC&E). In the present study, Nerium oleander plants were grown at 20 and 35oC and their gas exchange characteristics and photosynthetic apparatus were analysed. Temperature dependence of the light-saturated rate of photosynthesis at ambient CO2 (P360) differed between plants grown at different temperatures: P360 at 35oC was higher than that at 20oC in 35oC-grown plants, while P360 was similar between 20 and 35oC in 20oC-grown plants. However, initial slope of the CO2 dependence of the photosynthetic rate exhibited similar temperature dependence irrespective of growth temperatures. The ratio of the initial slope to RuBPCase (ribulose-1,5-bisphosphate carboxylase) content was also similar between plants grown at different temperatures. Analyses with gas exchange and chlorophyll fluorescence suggested that the CO2 compensation point and CO2 concentration at chloroplasts were similar between plants grown at different temperatures. These results suggest that P360 was limited by RuBPCase activity except for that at 35oC in 20oC-grown plants. We concluded that changes in the limiting step resulted in the different temperature responses of the photosynthesis between plants grown at different temperatures.

Ecophysiological responses and carbon distribution ofPinus koraiensis seedlings to elevated carbon dioxide

The net CO2 assimilation rate, stomatal conductance, RuBPcase (ribulose 1,5-biphosphate carboxylose) activity, dry weight of aboveground and belowgroud part, plant height, the length and diameter of taproot ofPinus koraiensis seedlings were measured and analyzed after six-week exposure to elevated CO2 in an open-top chamber in Changbai Mountain of China from May to Oct. 1999. Seedlings were planted in four different conditions: on an open site, control chamber, 500 μL·L−1 and 700 μL·L−1 CO2 chambers. The results showed that the total biomass of the seedlings increased whereas stomatal conductance decreased. The physiological responses and growth to 500 μL·L−1 and 700 μL·L−1 CO2 varied greatly. The acclimation of photosynthesis was downward to 700 μL·L−1 CO2 but upward to 500 μL·L−1 CO2. The RuBPcase activity, chlorophyll and soluble sugar contents of the seedlings grown at 500 μL·L−1 CO2 were higher than that at 700 μL·L−1 CO2. The concentration 500 μL·L−1 CO2 enhanced the growth of aboveground part whereas 700 μL·L−1 CO2 allocated more carbon to belowground part. Elevated CO2 changed the carbon distribution pattern. The ecophysiological responses were significantly different between plants grown under 500 μL·L−1 CO2 and 700 μL·L−1 CO2.

Effects of rare earth ions on activity of RuBPcase in tobacco

To investigate the physiological effects of rare earth ions, we have studied the effect of LaCl 3 on the activity of ribulose 1,5-bisphosphate carboxylase (RuBPcase) in tobacco ( Nicotiana tobacum). When treated with 20–80 μmol/l LaCl 3 in Hoagland solution by water culture or by spraying with 400–1600 μmol/l LaCl 3 in pot experiment, the total and initial activities of RuBPcase were raised gradually, but decreased when the concentrations of LaCl 3 were ≥200 μmol/l in water culture and >1600 μmol/l by spraying. It was indicated that LaCl 3 promoted the activation of RuBPcase at low concentrations in vivo. It was also proved that lanthanides (Ln 3+) increased the activity of RuBPcase in vitro. Upon addition of different concentrations of Ln 3+ to replace Mg 2+, the RuBPcase activity exhibited an apparently biphasic response curve. Among the three Ln 3+ tested, the activation effects of La 3+ and Ce 3+ on RuBPcase were greater than that of Gd 3+, but the activation effects of all the three Ln 3+ were lower at high concentrations than that of Mg 2+, showing strongly inhibitory function. The extended X-ray absorption fine structure study of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco)–La complex indicated that La is bound with the oxygen atoms of amino acid residues, with the distance of the LaO bond being 2.51 Å. The mechanism of molecular interaction between Ln 3+ and RuBPcase is discussed.

大気‐植生‐土壌系モデルによる高温・高ＣＯ２濃度条件下のダイズ群落におけるＣＯ２収支の解析

Increase in atmospheric carbon dioxide (CO2) concentration and associated global warming had been predicted in many papers. In order to reveal the effects of global change on CO2 and energy budgets between the atmosphere and agricultural plant canopy, a mechanistic simulation model in soybean canopy was applied.The model was based on the NEO Soil-Plant-Atmosphere Model (Kanda and Hino, 1990) and had been specified for soybean canopy using measured data (Harazono et al., 1996). The present model was improved by incorporating the biochemical sub-model on photosynthesis presented by Farquhar et al. (1980), in which leaves responded physiologically with daily changes of CO2 concentration and micrometeorological conditions.Numerical experiments were conducted for three different conditions, higher temperature (present+2°C), higher CO2 concentration (present+200ppm) and both higher temperature and CO2 (+2°C and +200ppm) conditions. Under the higher temperature condition, canopy photosynthesis decreased because of water stress and an increase in plant respiration, while under the higher CO2 concentration condition, canopy photosynthesis increased by more than 50% compared to present. Stomatal closure by higher CO2 reduced the effect of water stress, and an increase of both temperature and CO2 concentration enhanced canopy photosynthesis by about 30% compared to present. The decline of RuBPCase activity by higher CO2 did not much affect canopy photosynthesis, since photosynthesis in most leaves except for the top leaves were limited by RuBP regeneration rate.Evapotranspiration increased under higher temperature condition resulting in lower water use efficiency (WUE), while WUE under higher CO2 increased by the stomatal closure. WUE remained almost unchanged under 2°C higher temperature and 200ppm higher CO2 concentration. However, it was suggested that WUE should change extensively, depending on the degree of the CO2 increase and global warming.

Developmental Changes in Chloroplast Ultrastructure and Carbon‐Fixation Metabolism of Dendrobium Flowers (Orchidaceae)

The developmental changes in chloroplast ultrastructure and carbon‐fixation metabolism were studied in flowers of Dendrobium Sonia, an orchid with Crassulacean acid metabolism (CAM). The chloroplasts in the flowers were smaller and had fewer thylakoid membranes and grana than in the leaf. During flower development, the chloroplasts degenerate and become less distinct, with dilated thylakoid membranes. The chlorophyll a/b ratio of the flowers and leaf were similar, while total chlorophyll contents were lower than in the leaf. The total chlorophyll content and the RuBPcase activity in the flowers declined as they matured. In contrast, PEPcase activity increased with flower development. At all stages of flower development, PEPcase/RuBPcase ratios were ca. 10–20‐fold higher than in the leaf. As the flower aged, the PEPcase/RuBPcase ratio increased with decreased rate of 14CO2 fixation. The δ13C values of the flowers were less negative than those of the leaves. Following a 14CO2 feed in the light, a considerable amount of 14C‐metabolites appeared in the organic acids (35%–51%) and sugar (30%–50%) fractions in the flowers. The nocturnal malate increase (NMI) in the flower bud was only 2% of the leaf, and this increased to 14% at the fruit capsule stage. Our results indicate that flowers of Dendrobium Sonia had weak CAM activity in the dark and some degree of β‐carboxylation in the presence of C3 metabolism in the light.

Effects of micro‐environmental factors on photosynthetic CO2 uptake and carbon fixation metabolism in a spring ephemeral, Erythronium japonicum, growing in native and open habitats

Microenvironmental factors and physiological parameters (such as water potential, activity of ribulose 1,5‐bisphosphate carboxylase (RuBPcase), levels of ribulose 1,5‐bisphosphate (RuBP), 3‐phosphoglyceric acid (PGA) and sucrose in leaves) affecting photosynthetic processes of the typical vernal species Erythronium japonicum Decne. were examined on the floor of a deciduous broad‐leaved Quercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 7 years after forest clearing (bare stand). Daytime solar radiation and the air and leaf temperatures at the bare stand were significantly higher than those at the Q.m. stand. The relative air humidity was very low and did not differ much between the stands, whereas the leaf–air vapor pressure difference (VPD) at the bare stand was twice as high as that at the Q.m. stand. The water potential in leaves at the bare stand was lower than two times that at the Q.m. stand. Therefore, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress than those at the Q.m. stand. When these environmental factors observed at the bare stand were reproduced in an assimilation chamber, the rate of photosynthetic CO2 uptake, stomatal conductance and water potential in leaves were significantly low in comparison with those when the factors at the Q.m. stand were simulated. The internal CO2 partial pressure in leaves at the bare stand was considerably lower than that at the Q.m. stand. Consequently, the decrease in the photosynthetic rate of the plants at the bare stand was caused mainly by a decrease in stomatal conductance through a lowering of water potential due to subjection of the aboveground parts to much more severe heat stress than that at the Q.m. stand. The possibility that an inhibition of the photosynthetic carbon fixation metabolism induced by the decrease in water potential contributes to the reduction in photosynthetic CO2 uptake in the plants at the bare stand is also discussed in light of physiological characteristics such as the activity of RuBPcase and levels of PGA, RuBP and sucrose in the leaves.

Effect of Potassium, Magnesium, and Calcium Deficiencies on Nitrogen Constituents and Chloroplast Components in Citrus Leaves

The chlorotic appearance of mineral-deficient citrus leaves presumably reflects degradative changes in chloroplast components, most of which have nitrogen as a principal constituent. To examine this assumption the size of some major nitrogen pools, the SDS-PAGE pattern of soluble and chloroplast membranal proteins, and the activities of nitrate reductase and ribulose bisphosphate carboxylase (RuBPcase) were determined in leaves of rough-lemon (Citrus volkameriana Ten. & Pasq) plants grown hydroponically for 3 to 10 months under K, Mg, and Ca deficiencies. Plants grown under minerally deficient conditions produced less biomass. Leaves developing under K, Mg, and Ca-deficient conditions had significantly reduced concentrations of the respective elements. Chlorophyll levels of the chlorotic Mg and Ca-deficient leaves were lower than those of control leaves but chlorophyll a/b ratios were not markedly different. Calcium deficiency caused significant decreases in total nitrogen, nitrates, and the free amino acid pool. Proline which is the major component of the free amino acid pool decreased by 82.5%. Calcium-deficient leaves had significantly lower nitrate reductase and RuBPcase activities. The level of RuBPcase holoenzyme and its subunits were also reduced. Protein levels of K, Mg, and Ca-deficient leaves were not significantly altered. The SDS-PAGE patterns of soluble and chloroplast membranal proteins did not reveal major qualitative changes. In conclusion, the data do not demonstrate a general close link between chlorosis of minerally deficient citrus leaves and nitrogen metabolism. Calcium deficiency seems to specifically interfere with early stages of nitrogen assimilation and free amino acid accumulation but the metabolic integrity of the leaf is apparently maintained even under severe nutritional stress conditions.