Reversible phosphorylation and turnover of the D1 protein under various redox states of Photosystem II induced by low temperature photoinhibition

Reversible phosphorylation and turnover of the D1 protein in vivo were studied under low-temperature photoinhibition of pumpkin leaves and under subsequent recovery at low light at 4 °C or 23 °C. The inactivation of PS II and photodamage to D1 were not enhanced during low-temperature photoinhibition when compared to that at room temperature. The PS II repair cycle, however, was completely blocked at 4 °C at the level of D1 degradation. Both the recovery of the photochemical activity of PS II and the degradation of the damaged D1 protein at low light at 23 °C were delayed about 1 hour after low-temperature photoinhibition, suggesting that in addition to the decrease in catalytic turnover of the enzyme, the protease was specifically inactivated in vivo at low temperature. The effect of low temperature on the other regulatory enzymes of PS II repair, protein kinase and phosphatase [Rintamäki et al. (1996) J Biol Chem 271: 14870-14875] was variable. The D1 protein kinase was operational at low temperature while dephosphorylation of the D1 protein seemed to be completely inhibited during low temperature treatment. Under subsequent recovery conditions at low light and 23 °C, the high phosphorylation level of D1 was sustained in leaf discs photoinhibited at low temperature, despite the recovery of the phosphatase activity. This high phosphorylation level of D1 was due to the persistently active kinase. The D1 kinase, previously shown to get activated by reduction of plastoquinone, was, however, found to be maximally active already at relatively low redox state of the plastoquinone pool. We suggest that phosphorylation of PS II centers increases the stability of PS II complexes and concomitantly improves their survival under stress conditions.     

Effect of light on lipid composition of Ricinus cell cultures

The changes of lipid composition were determined in callus cultures from Ricinus communis endosperm upon transfer from darkness into light. Culture in light induced chlorophyll synthesis and formation of differentiated chloroplasts. In light-grown cultures the major lipid classes were phospho- and glycolipids, dark-grown cultures were rich in triacylglycerol. The major fatty acids were linolenic acid and palmitic acid in both cultures. In the green cultures linolenic acid was predominantly esterified in glycolipids whereas in the dark-grown cultures this fatty acid was the major component of phospholipids. Ricinoleic acid was not found.Abbreviations PC phosphatidylcholine - PE phosphatidylethanolamine - MGD monogalactosyldigylceride - DGD digalactosyldiglyceride - SE steryl esters - NL neutral lipids     

Stomatal Behaviours of Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) Plants in Response to Low Root Temperature in Hydroponics

Hydroponic-grown seedlings of aspen (Populus tremuloides Michx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the stomatal behaviour was found with the seedlings when their roots were subjected to the low temperature. Stomatal conductance rapidly and dramatically reduced in response to the low root temperature, while the xylem water potential did not significantly alter. Under the low root temperature, pH value of the xylem sap increased from 6.15 to 6.72 within the initial 4 h, while abscisic acid (ABA) concentration increased by the eighth hour of treatment. K⁺ concentration of the xylem sap significantly decreased within the 8th h and then reversed by the 24th h. The ion change was accompanied by a decrease and then an increase in the electrical conductivity, and an increase and then a decrease in the osmotic potential. The tempo of physiological responses to the low root temperature suggests that the rapid pH change of the xylem sap was the initial factor which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. Xylem sap from the seedlings subjected low root temperature affected stomatal aperture on leaf discs when they were floated on the sap solution. The stomatal aperture correlated (P = 0.006) with the changed pattern of [K⁺] in the sap while the range of pH or ABA found in the xylem sap did not influence stomatal aperture of leaf discs in solution. The effect of xylem sap on stomatal aperture on leaf discs was different from on stomatal conductance in the intact seedlings. Comparison was made with previous study with the soil-grown seedlings.     

Temperature induced changes of the ultrastructure and lipid composition in green and senescent leaves of Dicranum elongatum

Fatty acid content of the triglycerides in subarctic Dicranum elongatum Schleich was 4 times higher in low (1°C) than in high (23°C) temperature treated green tissue. The respective value was 3 times higher in low than in high temperature treated senescent portions of the shoot. Differences in the triglyceride concentration were corroborated with both light and electron microscopic observations. In addition, low temperature treatment resulted in thick cell walls and in an increase in dry weight of the tissue, whereas the opposite effects were caused by high temperature. Senescence of leaf tissue was accompanied by an over-all decrease in membraneous cell structures. In senescent leaf cells an inner wall-like layer, less electron dense than the thick secondary wall, was particularly well developed in low temperature treated material. Characteristic of the senescent leaf cells were cytoplasmic structures, which were described as dormant endospores.     

Phenotypic Adaptation of Tonoplast Fluidity to Growth Temperature in the CAM Plant Kalanchoë daigremontiana Ham. et Per. is Accompanied by Changes in the Membrane Phospholipid and Protein Composition

The present study deals with the phenotypic adaptation of tonoplast fluidity in the CAM plant Kalancho? daigremontiana to changes in growth temperature. Tonoplast fluidity was characterized by measuring fluorescence depolarization in membranes labeled with fluorescent fatty acid analogues and by following formation of eximeres in membranes labeled by eximere-forming fluorophores. With both techniques it was found that exposure of the plants to higher growth temperature compared with the control decreased the fluidity of the tonoplast while exposure to lower growth temperature caused the opposite. Three hours of high temperature treatment (raised from 25°C to 35°C; ``heat shock') were sufficient to decrease the tonoplast fluidity to roughly the same extent as growth under high temperature for 30 days. The phenotypic response of tonoplast fluidity to changes in growth temperature was found only in the complete membrane, not however in the lipid matrix deprived of the membrane proteins. Heat treatments of the plants decreased the lipid/protein ratio while exposure to low temperature (for 30 days) increased it. Heat treatments led to a decrease in the percentage of linolenic acid (C18:3) and linoleic acid (C18:2), heat shock and low temperature treatments induced an increase in the percentage of linoleic acid (C18:3), with concomitant decrease in the percentage of linoleic acid (C18:2). However, in the case of heat shock, increase in linolenic acid concerned mainly monogalactosyldiacylglycerol, while with low temperature treatment linoleic acid increased in phosphatidylcholine. Both treatment of the plants with high and low temperature led to a slight decrease in the contribution of phosphatidylcholine and phosphoethanolamine to the total phospholipid content of the tonoplast. High-temperature treatment of the plants not only decreased the phospholipid/protein ratio in the tonoplast, but also led to the occurrence of a 35 kDa polypeptide in the tonoplast which cross-reacted with an antiserum against the tonoplast H⁺-ATPase holoenzyme. The important role of membrane proteins in bringing about the phenotypic rigidization of the tonoplast was mimicked by reconstitution experiments showing that incorporation of the proteins isolated from the tonoplast into phosphatidylcholine vesicles decreased the fluidity of this membrane system. As to be expected from the analyses in the natural membrane, the degree of this effect depended on the phospholipid/protein ratio. Received: 4 March 1998/Revised: 28 July 1998     

The Relation Between Activity of Phosphoenolpyruvate Carboxykinase and Photosynthesis in Aloe vera Leaves

The chlorophyllous layer of leaf of a PEP-CK type CAM plant Aloe vera was stripped tiff from the colorIess water storage tissue and used to stuly the interrelation between the activity of decarboxylating enzyme phosphoenolpyruvate carboxykinase (PEPCK) and photosynthesis. Oxaloacetate, malate+ADP, and NaHCO3 were found to stimulate photosynthetic oxygen evolution. During the period from 6:00 to 18:00 of the day time, a diurnal fluctuation was observed in both PEPCK activity and the rate of oxygen evolution. The maximum of photosynthesis appeared at 10-12:00, but the maximum PEPCK activity appeared at 14:00. The PEPCK activity and photosynthetic rate in leaf discs increased with temperature from 10 to 35℃, then decreased at 45℃. Similar decline of both parameters was found in the leaf discs stressed by different concentration of PEG-6000 solution for 4.5 h. At light intensity of 900 mol m-2 s-1 and 25℃, the PEPCK activity and photosynthetic rate of leaf discs rised with the illumination time, then a slight inhibition followed at the time of 30 min (Pn) or 40 min (PEPCK). The strong response of PEPCK activity to high light intensity in leaf discs, and a progressive increase of PEPCK activity in direct illumination of crude enzyme extractm the range of 0-55 min, indicated that light s likely to be an activator for PEPCK. Leaf discs were infiltrated with 3-(3,4-dichlorophenyl)-l, 1-dimethylurea, DL-glyceraldehyde and 2,4-dimitrophenol resulted in the partial inhibition of light-ependent photosynthesis and decarboxylation of C4 acid. The activity of PEPCK was also stimulated by Mg2+ or Mg2++ATP infiltrated into the leaf discs in the dark. The evidence presented here suggested that PEPCK activity of CAM plants showed a close interrelation with photosynthesis. Both of them were regulated by the environmental changes. The activity of PEPCK might be coupled to electron trsnsport and photophosphorylatiou.     

Increased nitrate reductase activity in leaf tissue after application of the fungicide Kresoxim-methyl

Spinach (Spinacia oleracea L.) leaf discs floating on buffer solution were treated with Kresoxim-methyl (KROM), an inhibitor of respiratory electron transport. In the leaf tissue, actual and maximal nitrate reductase (NR) activities, nitrite content and ATP levels were determined. In darkened leaf discs incubated without KROM (control) actual NR activity decreased to 20% after 6 h in the dark. Treatment with 10 μg ml⁻¹ (corresponding to 32 μM) KROM totally prevented inactivation of NR in the dark and also diminished NR-protein degradation during prolonged darkness. Due to restricted nitrite reduction in darkened leaf tissues, nitrite accumulated in KROM-treated discs. Inhibition of respiration decreased ATP and increased AMP levels in KROM-treated discs. In illuminated leaf discs, NR was highly activated to 65%. Nevertheless, KROM-treatment caused an additional activation of NR (activation state 76%) in the light. Possible side-effects of KROM on nitrite reduction and photosynthesis were also checked in the leaf-disc system. Neither nitrite reduction nor photosynthesis were altered in KROM-treated discs. The extent of KROM-induced activation of NR was dependent on the applied concentration and on the pH of the external medium. The highest activation of NR was achieved at an external pH of 4.8, confirming previous results (Kaiser and Brendle-Behnisch, 1995, Planta 196: 1–6) that cytosolic acidification might play an important role in the modulation of NR activity. Received: 17 June 1998 / Accepted: 2 September 1998     

Growth temperature effects on thylakoid membrane lipid and protein content of pea chloroplasts

The lipid composition and level of unsaturation of fatty acids has been determined for chloroplast thylakoid membranes isolated from Pisum sativum grown under cold (4°/7°C) or warm (14°/17°C) conditions. Both the relative amounts of lipid classes and degree of saturation were not greatly changed for the two growth conditions. In cold-grown plants, there was a slightly higher linolenic and lower linoleic acid content for the glycolipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyldiacylglycerol. In contrast to thylakoid membranes, a non-thylakoid leaf membrane fraction including the chloroplast envelope, had a higher overall level of fatty acid unsaturation in cold-grown plants due mainly to an increase in the linolenic acid content of MGDG, DGDG, phosphatidylglycerol, and phosphatidylcholine. The most clear cut change in the thylakoid membrane composition was the lipid to protein ratio which was higher in the cold-grown plants.     

Absence of red-light enhancement of phototropism in pea seedlings at limiting irradiances of blue light

The effect of different light qualities (blue, green, white, red and far-red) on ethylene production in leaf discs and flower petal discs of Begonia × hiemalis cv. Schwabenland Red was studied. All the light qualities, except far-red, reduced the ACC-conversion to ethylene in leaf discs by about 70% at a photosynthetic photon flux density (PPFD) of 20 mol m^–2s^–1.Blue and green light were less inhibitory than white and red light at lower PPFD. In all treatments far-red light at 0.5 mol m^–2s^–1 of photon flux density (PFD) stimulated the ACC-conversion to ethylene in leaf discs by about 60–90% compared to the dark-incubated control. White and red light strongly inhibited the -naphthalene-acetic acid (NAA) stimulated ethylene synthesis in leaf discs. The results may suggest that the ethylene production is controlled by phytochrome in the leaves but not in the petals. Lack of coaction of any light quality with silver ions on ethylene production in leaf and petal discs was also observed.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - EFE ethylene forming enzyme - NAA -naphthalene-acetic acid - PFD photon flux density - PPFD photosynthetic photon flux density - RH relative air humidity - SAM S-adenosylmethionine - STS silver thiosulphate     

Phytoalexin production by leaves of Pisum sativum in relation to senescence

A sterile culture nitrate of Penicillium expansum was shown to induce pisatin synthesis in pea leaf discs. The amount of pisatin produced by pea leaves was shown to decrease as they underwent senescence. N⁶-benzyladenine delayed senescence, and at the same time maintained the production of pisatin at a high level. In darkness, leaf discs maintained on either benzyl-adenine solution or distilled water produced greater amounts of pisatin than leaf discs which were not treated in this way. Benzyladenine also increased pisatin production by leaf discs kept in the light. The relevance of these results to disease resistance and possible mechanisms involved are discussed.     

Biosynthesis of Leaf Alcohol Formation of 3Z-Hexenal from Linolenic Acid in Chloroplasts of Thea sinensis Leaves

The synthetic activity for 3Z-hexenal, an important precursor of 3Z-hexenol (leaf alcohol), was localized in chloroplasts of Thea sinensis leaves. 3Z-Hexenal, which is easily isomerized to 2E-hexenal (leaf aldehyde), was formed from linolenic acid in the presence of oxygen. 13-l-Hydroperoxy-linolenic acid also served as a precursor, but the triglyceride and methyl ester of linolenic acid did not. This enzyme system appeared to be tightly bound to the lamellae membranes of chloroplasts.     

Pigment Degradation in Discs of the Thermophilic Cucumis sativus as Affected by Light, Temperature, Sugar Application and Inhibitors

Chlorophyll degradation in Cucumis leaf discs was measured at different temperatures between 1 and 25°C in the light and in darkness, and in the presence or absence of sucrose. Two different processes of chlorophyll degradation could be distinguished, a light-requiring process operating at 1 and 5°C and another, light and sucrose enhanced degradation process which was evident at 25°C. Degradation of leaf pigments at low temperatures was of a photo-oxidative nature since there was no degradation in the dark. The chlorophyll a/b ratio was decreased, carotene was degraded at a faster rate than chlorophyll, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and triphenyltetrazolium chloride (TTC) which prevent photo-oxidation, protected against chlorophyll degradation. The light and sucrose enhanced chlorophyll degradation at 25°C was of an enzymatic nature since it occurred in the dark as well as in the light. The chlorophyll a/b ratio was not affected, and carotene and chlorophyll degradation occurred at the same rate. Since DCMU completely inhibited the light enhancement at 25°C and experimentation in a low oxygen atmosphere also protected chlorophyll against the effect of light and sugar application, it is suggested that the enhancement of chlorophyll degradation by light and sucrose at 25°C may be due to increased sugar uptake of the chloroplasts and consequently excessive starch formation in the organelles.     

Mesure d'une activité métabolique in vivo. Cas du catabolisme purique chez le Pharbitis nil

In vivo measurement of a metabolic activity. — Purine catabolism in Pharbitis nil cotyledons. To study the influence of light and darkness on the level of purine degradation, an in vivo method of measuring this catabolic pathway has been developed. Cotyledon discs of Pharbitis nil were incubated in an aqueous solution of hypoxanthine-8-¹⁴C. This purine base was first oxidized by a NAD⁺ dependent xanthine dehydrogenase. The metabolic flow was expressed either by the quantity of products (xanthine + uric acid + allantoin + allantoic acid) or by the rate of hypoxanthine degradation in the discs. This easy, reproducible and very sensitive technique of measurement has been investigated with respect to several variables including tissue quantity, length of incubation time, hypoxanthine and NAD⁺ concentrations, and pH. The quantity of products varied proportionally with the tissue quantity but the catabolic rate was not linked to this. The best conditions of measurement were to incubate discs for at least 60 min in distilled water containing only hypoxanthine-8-¹⁴C at low concentrations (below 0.25 mM). The values obtained represent the actual in vivo level of the studied metabolic pathway. The NAD⁺ concentration within the tissue does not seem to be a limiting factor for hypoxanthine degradation, the hypoxanthine concentration itself appears to be the only limiting factor in endogenous purine catabolism.     

LIGHT,TEMPERATURE AND SALINITY EFFECTS ON GROWTH,LEAF ANATOMY AND PHOTOSYNTHESIS OF DISTICHLIS SPICATA (L.) GREENE

The effects of light, temperature, and salinity on growth, net CO₂ exchange and leaf anatomy of Distichlis spicata were investigated in controlled environment chambers. When plants were grown at low light, growth rates were significantly reduced by high substrate salinity or low temperature. However, when plants were grown at high light, growth rates were not significantly affected by temperature or salinity. The capacity for high light to overcome depressed growth at high salinity cannot be explained completely by rates of net photosynthesis, since high salinity caused decreases in net photosynthesis at all environmental conditions. This salinity-induced decrease in net photosynthesis was caused largely by stomatal closure, although plants grown at low temperature and low light showed significant increases in internal leaf resistance to CO₂ exchange. Increased salinity resulted in generally thicker leaves with lower stomatal density but no significant differences in the ratio of mesophyll cell surface area to leaf area. Salinity and light during growth did not significantly affect rates of dark respiration. The mechanisms by which Distichlis spicata tolerates salt appear to be closely coulpled to the utilization of light energy. Salt-induced leaf succulence is of questionable importance to gas exchange at high salinity in this C₄ species.     

Stimulation of ACC-dependent ethylene production in citrus leaf discs by light

The influence of light and darkness incubation on in vivo ethylene forming enzyme (EFE) activity in citrus ( Citrus sinensis L. Osbeck cv. Salustiana) mature leaf discs was studied. Leaf discs incubated in light produced higher amounts of ethylene than in darkness. Transfer of discs from light to the dark resulted in a marked inhibition of EFE activity, whereas transfer of discs from the dark to light enhanced ethylene forming activity considerably. Light did not affect 1-aminocyclopropane-l-carboxylie acid (ACC) uptake. Incubation in a CO₂-eniiched atmosphere enhanced EFE activity both in light and in darkness, but light stimulation of EFE activity was apparently not affected by CO₂. Effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU, inhibitor of photosynthetic electron flow) and KCN (inhibitor of cytochrome oxidase) were studied. DCMU at 0.2 m M inhibited EFE activity in light, whereas no effect was detected in the dark. On the other hand 1 m M KCN stimulated EFE activity in the light, and no significant effect was observed in the dark. CoCl₂ at 1 m M inhibited ACC-dependent ethylene production, suggesting that ethylene production from ACC is mediated by EFE in citrus leaf discs both in light and in the dark. Cycloheximide also inhibited EFE activity in the light and no effects were detected in the dark. Therefore protein synthesis in light (perhaps EFE synthesis) could be required for the light stimulation of the in vivo EFE activity.     

Photooxidative inhibition of nitrate reductase during chilling

The effect of a temperature close to the freezing point (chilling) on the nitrate reductase system of leaf discs of Cucumis sativus L. cv. Kleine Groene Scherpe was determined in the absence and presence of light. The capacity of leaf discs in the light (250 μE m⁻²s⁻¹) at 20°C to increase in vivo and in vitro nitrate reductase activity, was unaffected by chilling pretreatment in the dark, but 4 h of chilling pretreatment in the light (250 μE m⁻²s⁻¹) decreased the capacity to less than 50% of the unchilled control. The chilling inhibition of the capacity to increase nitrate reductase activity was of a photooxidative nature since it only occurred in the presence of light and oxygen. Plants grown at a low light intensity (65 μE m⁻²s⁻¹) lost 95% of their capacity to increase nitrate reductase activity, while plants grown at 195 μE m⁻²s⁻¹ retained 80% of their nitrate reducing capacity after 6 h chilling pretreatment in the 250 μE m⁻²s⁻¹ light. Previously induced nitrate reductase activity was also affected by light during chilling. A lag phase of 7 h preceded a fast phase of decrease in activity. Both in vivo and in vitro activity decreased to 15% of the control value after 18 h of chilling in the light. It is concluded that the induction mechanism of nitrate reductase is primarily affected by photooxidation during chilling. The decrease in nitrate reductase activity is attributed to a decrease in the amount of activity enzyme.     

Influence of S-ethyl Dipropylthiocarbamate (EPTC) on the Fatty Acid Composition of Wheat Leaf Galactolipids

Winter wheat (Triticum sativum L. cv. Nisu) was grown in sand which contained 0, 0.25, 0.5 and 1.0 mg S-ethyl dipropylthiocarbamate (EPTC) per kg air dry sand. The galactolipids of leaves of 21-day-old seedlings were isolated by preparative thin layer chromatography. The fatty acids of the mono- and digalactosyl diglycerides were analysed by gas liquid capillary chromatography. The major fatty acids of the wheat leaf galactolipids were palmitic, palmitoleic, stearic, oleic, vaccenic, linoleic and linolenic acids (in the monogalactosyl diglyceride fraction of untreated plants 22.5, 2.4, 3.1, 5.2, 2.5, 51.1 and 1526.6 μg and in the digalactosyl diglyceride fraction 108.8, 2.3, 10.4, 9.9, 8.2, 42.3 and 1120.7 μg/g leaf fresh weight, respectively). Total fatty acid content of the mono- and digalactosyl diglyceride fractions was decreased by 85 and 87%, respectively, at 1 mg EPTC/kg sand, while decrease in the fresh weight of the leaves was 79%. The content of linoleic and linolenic acids/g fresh weight of the leaves was decreased in the monogalactosyl diglyceride fraction by 27 and 43%, respectively, while the content of all other fatty acids was increased. In the digalactosyl diglyceride fraction the content of both linoleic and linolenic acids/g leaf fresh weight was decreased by 55%. The content of palmitic and vaccenic acids was also decreased, whereas the content of other fatty acids remained at the level of the untreated samples. The general quality of the fatty acids in the mono- and digalactosyl diglyceride fractions was altered slightly by EPTC.     

Effect of Photoperiod and Chlorogenic Acid on Morphogenesis in Leaf Discs of Streptocarpus Nobilis

Leaf discs from vegetative plants greatly increase their phenolic content when cultivated in vitro. Under long days the values remained constant, and were higher when compared with short days cultures. Under short days total phenolics decreased after 10 d, corresponding to the induction and expression of in vitro flowering. The effect of photoperiod and chlorogenic acid (0.01 mM) on leaf discs cultured from induced and non-induced plants, were analyzed regarding the neo-formation of roots, as well as vegetative and flower buds. Chlorogenic acid enhances the regeneration of roots in all treatments tested, with the highest stimulation on induced leaf discs cultivated in short days. The flowering was not affected by chlorogenic acid, but an inhibitory effect was observed on the neo-formation of vegetative buds in non-induced explants maintained in short days. Vegetative buds were reduced by 50% in flower-induced leaf discs cultivated under short days.     

Identification of molecular markers associated with linoleic acid desaturation in Brassica napus

 Linolenic acid is a component of canola oil that is readily oxidized, which results in a reduced frying stability and shelf life of the oil. The reduction of linolenic acid in canola seed has therefore been an important breeding objective for many years. The inheritance of linolenic acid concentrations in seed oil is polygenic and is also strongly influenced by the environment. For these reasons, molecular markers are sought to assist in early and reliable selection of desired low linolenic acid genotypes in breeding programmes. Molecular markers associated with low linolenic acid loci were identified in a doubled-haploid population derived from a cross between the Brassica napus lines, ‘Apollo’ (low linolenic)×YN90-1016 (high linolenic) using RAPDs and bulked segregant analysis. A total of 16 markers were distributed over three linkage groups, which individually accounted for 32%, 14% and 5% of the phenotypic variation in linolenic acid content. The rapeseed fad3 gene was mapped near the locus controlling 14% of the variation. The mode of inheritance appeared to be additive, and a QTL analysis showed that collectively the three loci explained 51% of the phenotypic variation within this population. PCR fragments for low linolenic acid ‘Apollo’ alleles (3% linolenic acid) were identified at all three loci. Simultaneous selection for low linolenic acid ‘Apollo’ alleles at each locus resulted in a group of DH lines with 4.0% linolenic acid. The use of these makers in the breeding programme will enhance the breeding of low linolenic acid B. napus cultivars for production in Canada. Received: 23 September 1997 / Accepted: 21 October 1997     

Overexpression of the rubisco activase gene improves growth and low temperature and weak light tolerance in Cucumis sativus

Rubisco activase (RCA) is an important enzyme that can catalyze the carboxylation and oxygenation activities of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), which is involved in the photosynthetic carbon reduction cycle. Here, we studied the effects of changes in RCA activity on photosynthesis, growth and development, as well as the low temperature and weak light tolerance of RCA overexpressing transgenic cucumber (Cucumis sativus) plants. CsRCA overexpression increased the plant height, leaf area and dry matter, and decreased the root/top ratio in transgenic cucumber plants compared with the wild‐type (WT) plants. Low temperature and low light stress led to decreases in the CsRCA expression and protein levels, the photosynthetic rate (Pn) and the stomatal conductance (Gs), but an increase in the intercellular CO₂ (Ci) concentration in cucumber leaves. The actual photochemical efficiency and maximal photochemical efficiency of photosystem II in cucumber seedlings also declined, but the initial fluorescence increased during low temperature and weak light stress. Transgenic plants showed a lower decrease in the CsRCA expression level and actual and maximal photochemical efficiencies, as well as increases in the Ci and initial fluorescence relative to the WT plants. Low temperature and low light stress resulted in a significant increase in the malondialdehyde (MDA) content; however, this increase was reduced in transgenic plants compared with that in WT plants. Thus, the overexpression of CsRCA may promote the growth and low temperature and low light tolerance of cucumber plants in solar greenhouses.