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Chafik Hdider and Yves Desjardins

To identify the physiological and biochemical events leading to the negative effects of sucrose in culture medium on the photosynthetic capacity of plantlets cultivated in vitro, time-course changes in photosynthesis, metabolize pool sizes, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity were investigated in strawberry (Fragaria × ananassa Duch. cv. Kent) plantlets following their transfer to medium with or without sucrose. When the plantlets grown in medium without sucrose were transferred to a similar medium with 30 g sucrose/liter, their net photosynthesis decreased and their level of phosphorylated compounds increased with time. In addition, initial Kcat, total Kcat, and the activation state of Rubisco decreased in these plantlets. Conversely, when the plantlets grown in medium with 30 g sucrose/liter were transferred to a similar medium without sucrose, their net photosynthesis slowly increased with time and their level of phosphorylated compounds slowly decreased. A slow increase with time of initial Kcat, total Kcat, and the activation state of Rubisco was also observed in these plantlets. The results of the present research suggest that the reduced photosynthetic capacity of strawberry plantlets cultivated in vitro in the presence of sucrose was the consequence of reduced Rubisco efficiency due to its deactivation and the possible presence of a putative tight binding inhibitor.

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Tao Hu, Haiying Yi, Longxing Hu, and Jinmin Fu

, these studies in various plant species are mainly in annual crops. Ribulose-1, 5-bisphosphate carboxylase: oxygenase (Rubisco), a 4.8 to 5.9 GDa complex, is a chloroplast-specific photosynthetic protein, which consists of eight large subunits (≈50 to 55

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Li-Song Chen and Lailiang Cheng

To determine the cause of a characteristic zonal chlorosis of `Honeycrisp' apple (Malus ×domestica Borkh.) leaves, we compared CO2 assimilation, carbohydrate metabolism, the xanthophyll cycle and the antioxidant system between chlorotic leaves and normal leaves. Chlorotic leaves accumulated higher levels of nonstructural carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, and no difference was found in total nonstructural carbohydrates between predawn and dusk. This indicates that carbon export was inhibited in chlorotic leaves. CO2 assimilation and the key enzymes in the Calvin cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, and the key enzymes in starch and sorbitol synthesis, ADP-glucose pyrophosphorylase, cytosolic fructose-1,6-bisphosphatase, and aldose 6-phosphate reductase were significantly lower in chlorotic leaves than in normal leaves. However, sucrose phosphate synthase activity was higher in chlorotic leaves. In response to a reduced demand for photosynthetic electron transport, thermal dissipation of excitation energy (measured as nonphotochemical quenching of chlorophyll fluorescence) was enhanced in chlorotic leaves under full sun, lowering the efficiency of excitation energy transfer to PSII reaction centers. This was accompanied by a corresponding increase in both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to antheraxanthin and zeaxanthin. The antioxidant system, including superoxide dismutase and ascorbate peroxidase and the ascorbate pool and glutathione pool, was up-regulated in chlorotic leaves in response to the increased generation of reactive oxygen species via photoreduction of oxygen. These findings support the hypothesis that phloem loading and/or transport is partially or completely blocked in chlorotic leaves, and that excessive accumulation of nonstructural carbohydrates may cause feedback suppression of CO2 assimilation via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.

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Franco Famiani and Robert P. Walker

aminotransferase, aldolase, glutamine synthetase, and ribulose-1,5-bisphosphate carboxylase/oxygenase. It is difficult to measure the activity of enzymes in extracts of many soft fruit ( Manning, 1993 ). Therefore, in this study, the approach taken was to use

Open access

Wei Hu, Qing Di, Jingyi Wei, Jie Zhang, and Jia Liu

. Effects of grafting on the initial activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) in the flue-cured tobacco leaves. The tobacco graft combinations included the nongrafted tobacco ‘Wufeng No.2’ (W) and ‘Yunyan 87’ (Y) and the grafted

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Carole L. Bassett

Levels of the large subunit (LSU) of rubisco (ribulose 1,5 bisphosphate carboxylase/oxygenase) were measured in vegetative and floral organs of `Violet' Japanese morning glory (Ipomoea [Pharbitis] nil Roth). Identification of the LSU in polypeptides separated by two-dimensional gel electrophoresis allowed estimation of the relative abundance of this polypeptide in the organs examined. Further quantitation was achieved by immunoblotting protein extracts either alone or in combination with various amounts of extracts from other organs. The amount of LSU decreased in the order leaves > cotyledons > sepals > corollas > androecium, gynoecium. The relative abundance of LSU in sepals (74%) compared to photosynthetically competent organs [leaves (100%) and cotyledons (81%)] suggests that sepals may be photosynthetically competent in supporting development of the other floral organs.

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Dean E. Knavel and Robert L. Houtz

Plants of Main Dwarf, a short-internode mutant of the normal-internode `Mainstream' muskmelon (Cucumis melo L.), have shorter internodes, fewer nodes, less total vine length, less total dry weight, smaller leaves, increased chlorophyll concentrations, increased specific leaf dry weight, and increased ribulose-1, 5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39, rubisco) activity per unit leaf area than `Mainstream' plants. Main Dwarf plants produce an equal number of fruit as `Mainstream' plants but are only half their size. Many of the plant and fruit characteristics for F1(Main Dwarf × `Mainstream') are similar to those of `Mainstream', except for greater leaf chlorophyll and rubisco activity per unit leaf area. The F1 (`Mainstream' × Main Dwarf) produced fewer and lower weight fruit than its reciprocal, F1 (Main Dwarf × `Mainstream').

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John M. Ruter and Dewayne L. Ingram

Plants of `Rotundifolia' holly (Ilex crenata Thunb.) were grown for 3 weeks with root zones at 30,34,38, or 42C for 6 hours daily to evaluate the effects of supraoptimal root-zone temperatures on various photosynthetic processes. After 3 weeks, photosynthesis of plants grown with root zones at 38 or 42C was below that of plants grown at 30 or 34C. Chlorophyll and carotenoid levels decreased while leaf soluble protein levels increased as root-zone temperature increased. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity per unit protein and per unit chlorophyll responded quadratically, while RuBisCO activity per unit fresh weight increased linearly in response to increasing root-zone temperature. Results of this study suggest that `Rotundifolia' holly was capable of altering metabolism or redistributing available assimilates to maintain CO2 assimilation rates in response to increasing root-zone temperatures.

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Tridate Khaithong, Brent S. Sipes, and Adelheid R. Kuehnle

Lack of a conclusive evidence of ingestion of plastid components by plant-parasitic nematodes cautions the use of plastid transformation technology for nematode resistance. Nematode-resistant effector proteins generally require ingestion by the nematode to be effective. Transgene-encoded proteins produced in plastids are not known to be exported into the cytoplasm. Disintegration of plant cell organelles after nematode feeding suggests that nematodes possibly ingest plastid components. Proof of ingestion will validate the development of plastid transformation for nematode resistance. Small subunit ribulose-1,5-bisphosphate carboxylase/oxygenase (SSRubisco) protein is prevalent in chloroplasts and thus chosen as a study molecule. The migratory endoparasitic nematodes Pratylenchus penetrans and Radopholus similis were cultured on green carrot callus containing chloroplasts. Total nematode proteins were extracted and subjected to western blot analysis using cross-reactive polyclonal antibody raised against spinach SSRubisco. Positive detection of SSRubisco occurred in protein extracts from nematodes fed on green carrot callus, but not in extracts from nematodes cultured on non-green alfalfa root callus as negative control. These results confirm the ingestion of plastid components of MW 14 kDA by migratory endoparasitic nematodes.

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Li-Song Chen and Lailiang Cheng*

To determine the cause of zonal chlorosis of `Honeycrisp' apple leaves, we compared CO2 assimilation, carbohydrate metabolism, xanthophyll cycle and the antioxidant system between chlorotic leaves and normal leaves. Chlorotic leaves accumulated higher levels of non-structural carbohydrates, particularly starch, sorbitol, sucrose, and fructose at both dusk and predawn, and no difference was found in total non-structural carbohydrates between predawn and dusk. CO2 assimilation and the key enzymes in the Calvin cycle, ribulose 1,5-bisphosphate carboxylase/oxygenase, NADP-glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, stromal fructose-1,6-bisphosphatase, and enzymes in starch and sorbitol synthesis, ADP-glucose pyrophosphorylase, cytosolic fructose-1,6-bisphosphatase, and aldose 6-phosphate reductase were significantly lower in chlorotic leaves than in normal leaves. However, sucrose phosphate synthase activity was higher in chlorotic leaves. Thermal dissipation of excitation energy was enhanced in chlorotic leaves under full sun, lowering the efficiency of excitation energy transfer to PSII reaction centers. This was accompanied by a corresponding increase in both xanthophyll cycle pool size (on a chlorophyll basis) and conversion of violaxanthin to antheraxanthin and zeaxanthin. The antioxidant system was up-regulated in chlorotic leaves in response to the increased generation of reactive oxygen species. These findings support the hypothesis that phloem loading and/or transport is partially or completely blocked in chlorotic leaves, and that excessive accumulation of non-structural carbohydrates may cause feedback suppression of CO2 assimilation via direct interference with chloroplast function and/or indirect repression of photosynthetic enzymes.