Regulation of frost resistance during cold de-acclimation and re-acclimation in oilseed rape. A possible role of PSII redox state

A possible role of photosynthetic apparatus during cold de-acclimation was studied in oilseed rape ( Brassica napus var. oleifera ). Plants of spring (Star) and winter (Górczañski) cultivars were cold acclimated at + 5°C, and de-acclimated during 4 weeks at combinations of + 12 and + 20°C operating in the light or/and dark, with a 12-h photoperiod. Evidence is presented that the photosynthetic apparatus may be involved in temperature perception during de-acclimation. De-acclimation was faster under a 20/12°C (day/night) treatment than under the reverse 12/20°C (day/night). De-acclimation rate was constant when the day temperature was constant, irrespective of the night temperature both under cold day temperature regimes (12/20, 12/12°C (day/night) and warm-day treatments (20/12, 20/20°C (day/night). The fast decrease in frost resistance observed under warm-day de-acclimation was always accompanied by an acceleration of elongation growth. In the spring cultivar, elongation growth increased starting from the second week of de-acclimation, regardless of temperature conditions. Once elongation growth had commenced during de-acclimation, it continued throughout the period necessary for re-acclimation to low temperature. Re-acclimation to the initial freezing tolerance level was only possible when plant elongation was reduced. In addition re-acclimation of the photosynthetic apparatus to low temperature was impossible in fast growing plants. A possible relationship between PSII, growth rate and frost resistance during cold acclimation and de-acclimation is discussed.     

Midday depression in photosynthesis: Effect on sucrose-phosphate synthase and ribulose-1,5-bisphosphate carboxylase in leaves of Prosopis juliflora (Swartz) DC

The midday depression in net photosynthetic rate (PN) and stomatal conductance (gs) in Prosopis juliflora was studied in relation to two key enzymes of carbon metabolism. Diurnal gas exchange measurements carried out in autumn on P. juliflora showed a pronounced depression in PN and gs along with a decrease in apparent carboxylation efficiency (CE*) during midday. The activities of sucrose-phosphate synthase (SPS) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) also showed large diurnal fluctuations. Initial RuBPCO activity (that present in vivo) and total activity (fully carbamylated activity) increased gradually with increase in irradiance, in the morning reaching a maximum by 08:00 h. The RuBPCO activity declined sharply during midday due to decrease in initial activity. The drop in the % activation of RuBPCO indicated that the deactivation of RuBPCO was achieved via a decarbamylation mechanism. There was a marked similarity in the diurnal patterns of SPS activity and the PN. During a diurnal rhythm, SPS activity increased after irradiation, reaching a maximum at 08:00 h and then declined during midday. Diurnal fluctuations in SPS activity could be due to the changes in the amount of protein (changes in Vmax) as well as to the changes in kinetic properties (changes in Vlim). Hence the midday decline in photosynthesis is closely associated with the regulation of RuBPCO and SPS activities.     

Midday depression in photosynthesis: Effect on sucrose-phosphate synthase and ribulose-1,5-bisphosphate carboxylase in leaves of Prosopis juliflora (Swartz) DC

The midday depression in net photosynthetic rate (PN) and stomatal conductance (gs) in Prosopis juliflora was studied in relation to two key enzymes of carbon metabolism. Diurnal gas exchange measurements carried out in autumn on P. juliflora showed a pronounced depression in PN and gs along with a decrease in apparent carboxylation efficiency (CE*) during midday. The activities of sucrose-phosphate synthase (SPS) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) also showed large diurnal fluctuations. Initial RuBPCO activity (that present in vivo) and total activity (fully carbamylated activity) increased gradually with increase in irradiance, in the morning reaching a maximum by 08:00 h. The RuBPCO activity declined sharply during midday due to decrease in initial activity. The drop in the % activation of RuBPCO indicated that the deactivation of RuBPCO was achieved via a decarbamylation mechanism. There was a marked similarity in the diurnal patterns of SPS activity and the PN. During a diurnal rhythm, SPS activity increased after irradiation, reaching a maximum at 08:00 h and then declined during midday. Diurnal fluctuations in SPS activity could be due to the changes in the amount of protein (changes in Vmax) as well as to the changes in kinetic properties (changes in Vlim). Hence the midday decline in photosynthesis is closely associated with the regulation of RuBPCO and SPS activities. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of Chilling Temperature on the Activity of Enzymes of Sucrose Synthesis and the Accumulation of Saccharides in Leaves of Three Sugarcane Cultivars Differing in Cold Sensitivity

The effects of short-term exposure to chilling temperature (10 °C) on sucrose synthesis in leaves of the cold-tolerant sugarcane cultivars Saccharum sinense R. cv. Yomitanzan and Saccharum sp. cv. NiF4, and the cold-sensitive cultivar S. officinarum L. cv. Badila were studied. Plants were grown at day/night temperatures of 30/25 °C, and then shifted to a constant day/night temperature of 10 °C. After 52-h exposure to the chilling temperature, sucrose content in the leaves of NiF4 and Yomitanzan showed a 2.5- to 3.5-fold increase relative to that of the control plants that had been left on day/night temperatures of 30/25 °C. No such increase was observed in Badila leaves. Similarly, starch content in the leaves of NiF4 and Yomitanzan was maintained high, but starch was depleted in Badila leaves after the 52-h exposure. During the chilling temperature, sucrose phosphate synthase (SPS; E.C.2.4.1.14) activity was relatively stable in the leaves of NiF4 and Yomitanzan, whereas in Badila leaves SPS activity significantly decreased. There was no significant change in cytosolic fructose-1,6-bisphosphatase activity for the three cultivars at the chilling temperature. This supports the hypothesis that: (1) on exposure to chilling temperature, sucrose content in sugarcane leaves is determined by the photosynthetic rate in the leaves, and is not related to SPS activity; (2) SPS activity in sugarcane leaves at chilling temperature is to be determined by sugar concentration in the leaves.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

Photosynthetic response of wheat cultivar to long-term exposure to elevated temperature

Wheat (Triticum aestivum L. cv. HD 2285) was grown in control (C) and heated (H) open top chambers (OTCs) for entire period of growth and development till maturity. The mean maximum temperature of the entire period was 3 °C higher in H-compared to C-OTCs. Net photosynthetic rate (P _N) measured at different temperature (20–40 °C) of C-and H-grown plants showed greater sensitivity to high temperature in H-plants. P _N measured at respective growth temperature was lower in H-compared to C-plants. The CO₂ and irradiance response curves of photosynthesis also showed lesser response in H-compared to C-plants. The initial slope of P _N versus internal CO₂ concentration (P _N/C _i) curve was lower in H-than C-plants indicating ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) limitation. In irradiance response curve, the plateau was lower in H-compared to C-plants which is interpreted as RuBPCO limitation. RuBPCO content in the leaves of C-and H-plants, however, was not significantly different. Ribulose-1,5-bisphosphate carboxylase (RuBPC) initial activity was lower in H-plants, whereas activity of fully activated enzyme was not affected, indicating a decrease in activation state of the enzyme. This was further substantiated by the observed decrease in RuBPCO activase activity in H-compared to C-plants. RuBPCO activase was thus sensitive even to moderate heat stress. The decrease in P _N under moderate heat stress was mainly due to a decrease in activation state of RuBPCO catalysed by RuBPCO activase.     

去果河套蜜瓜源叶碳水化合物及其相关酶昼夜变化特征

以河套蜜瓜为试材,在果实迅速膨大期通过去果处理改变库源关系,研究源叶净光合速率,蔗糖、还原糖和淀粉含量及其代谢相关酶活性的昼夜变化规律。结果表明:(1)源叶的净光合速率为单峰曲线,无明显的"光合午休"现象,去果处理对其无影响。(2)源叶中蔗糖和还原糖含量的昼夜变化为单峰曲线,蔗糖磷酸合成酶和蔗糖合成酶合成方向活性的昼夜变化为双峰曲线,蔗糖合成酶分解方向、酸性转化酶和中性转化酶活性的昼夜变化无明显规律,改变库源关系对这些指标均无显著影响;蔗糖含量升高受蔗糖磷酸合成酶和蔗糖合成酶合成方向正调控,而蔗糖含量降低则受多种酶的共同调节。(3)源叶中淀粉含量和腺苷二磷酸葡萄糖焦磷酸化酶活性的昼夜变化为单峰曲线,去果处理可以显著提高淀粉含量和腺苷二磷酸葡萄糖焦磷酸化酶活性,淀粉含量升高受腺苷二磷酸葡萄糖焦磷酸化酶正调控。     

Hormonal Regulation of Photosynthetic Enzymes in Cotton under Water Stress

Activities of ribulose bisphosphate carboxylase/oxygenase (RuBPCO), phosphoenolpyruvate carboxylase (PEPC), and carbonic anhydrase (CA) were determined in leaves of cotton (Gossypium hirsutum L. cv. H-777) subjected to 8-d waterlogging (WL) at the vegetative stage, or to drought (D) at the reproductive stage, or to interaction of both stresses. The soil moisture of control plants was kept at field capacity. One day prior to stress various growth hormones (5 μM) were sprayed up to runoff. WL reduced RuBPCO and CA activities, while PEPC activity increased. Upon D, RuBPCO and PEPC activities were reduced while CA activity was increased. Imposition of both stresses increased activities of all three enzymes. Effect of stresses on enzyme activity was alleviated by benzylaminopurine (BAP), but indol-3-yl-acetic acid was more promoting under interactive stress. No CA activity with BAP was observed during interactive stress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differential inhibition of photosynthesis under drought stress in <Emphasis Type="Italic">Flaveria</Emphasis> species with different degrees of development of the C<Subscript>4</Subscript> syndrome

The effect of drought stress (DS) on photosynthesis and photosynthesis-related enzyme activities was investigated in F. pringlei (C₃), F. floridana (C₃–C₄), F. brownii (C₄-like), and F. trinervia (C₄) species. Stomatal closure was observed in all species, probably being the main cause for the decline in photosynthesis in the C₃ species under ambient conditions. In vitro ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and stromal fructose 1,6-bisphosphatase (sFBP) activities were sufficient to interpret the net photosynthetic rates (P _N), but, from the decreases in P _N values under high CO₂ (C _a = 700 μmol mol^{− 1}) it is concluded that a decrease in the in vivo rate of the RuBPCO reaction may be an additional limiting factor under DS in the C₃ species. The observed decline in the photosynthesis capacity of the C₃–C₄ species is suggested to be associated both to in vivo decreases of RuBPCO activity and of the RuBP regeneration rate. The decline of the maximum P _N observed in the C₄-like species under DS was probably attributed to a decrease in maximum RuBPCO activity and/or to decrease of enzyme substrate (RuBP or PEP) regeneration rates. In the C₄ species, the decline of both in vivo photosynthesis and photosynthetic capacity could be due to in vivo inhibition of the phosphoenolpyruvate carboxylase (PEPC) by a twofold increase of the malate concentration observed in mesophyll cell extracts from DS plants.     

Photosynthetic acclimation to CO2 enrichment related to ribulose-1,5-bisphosphate carboxylation limitation in wheat

Net photosynthetic rate (P _N) measured at the same CO₂ concentration, the maximum in vivo carboxylation rate, and contents of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (RuBPCO) and RuBPCO activase were significantly decreased, but the maximum in vivo electron transport rate and RuBP content had no significant change in CO₂-enriched [EC, about 200 μmol mol⁻¹ above the ambient CO₂ concentration (AC)] wheat leaves compared with those in AC grown wheat leaves. Hence photosynthetic acclimation in wheat leaves to EC is largely due to RuBP carboxylation limitation.     

Differences in the Activation State of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase in Barley,Pea, and Wheat at Two Altitudes

Activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) is an important parameter determining the rate of net photosynthesis (P _N) in situ for which no information is available with reference to altitude. We analyzed activation state along with P _N in three plant species and their cultivars grown at low (LA, 1 300 m) and high (HA, 4 200 m) altitudes. No significant change in P _N and the initial activity of RuBPCO was obtained with reference to altitude. However, activation state of RuBPCO was reduced significantly in the HA plants as compared to the LA ones. Hence low partial pressure of CO₂ prevailing at HA might be responsible for the lower activation state of RuBPCO.     

Seasonal Changes of Selected Parameters of CO2 Fixation Biochemistry of Norway Spruce Under the Long-Term Impact of Elevated CO2

Twelve-year-old Norway spruce (Picea abies [L.] Karst.) trees were exposed to ambient (AC) or elevated (EC) [ambient + 350 μmol(CO₂) mol^-1] CO₂ concentrations in open-top-chamber (OTC) experiment under the field conditions of a mountain stand. Short-term (4 weeks, beginning of the vegetation season) and long-term (4 growing seasons, end of the vegetation season) effects of this treatment on biochemical parameters of CO₂ assimilation were evaluated. A combination of gas exchange, fluorescence of chlorophyll a, and application of a mathematical model of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity was used. The analysis showed that the depression of photosynthetic activity by long-term impact of elevated CO₂ was mainly caused by decreased RuBPCO carboxylation rate. The electron transport rate as well as the rate of ribulose-1,5-bisphosphate (RuBP) formation were also modified. These modifications to photosynthetic assimilation depended on time during the growing season. Changes in the spring were caused mainly by local deficiency of nitrogen in the assimilating tissue. However, the strong depression of assimilation observed in the autumn months was the result of insufficient carbon sink capacity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Coarse control of sucrose-phosphate synthase in leaves: Alterations of the kinetic properties in response to the rate of photosynthesis and the accumulation of sucrose

It has been investigated whether diurnal rhythms of sucrose-phosphate synthase (SPS) are involved in controlling the rate of photosynthetic sucrose synthesis. Extracts were prepared from spinach (Spinacia oleracea L.) and barley (Hordeum vulgare L.) leaves and assayed for enzyme activity. The activity of SPS increased in parallel with a rising rate of photosynthesis, and was increased by feeding mannose and decreased by supplying inorganic phosphate. In leaf material where sucrose had accumulated during the photoperiod or when sucrose was supplied exogenously, SPS activity decreased. During a diurnal rhythm, SPS activity increased after illumination, declined gradually during the light period, decreased further after darkening and then recovered gradually during the night. These changes did not involve an alteration of the maximal activity, but were caused by changes in the kinetic properties, revealed as a change in sensitivity to inhibition by inorganic phosphate. In experiments which modelled the response of SPS to changing metabolite concentrations, it was shown that these alterations of kinetic properties would strongly modify the activity of SPS in vivo. It is proposed that SPS can exist in kinetically distinct forms in vivo, and that the distribution between these forms can be rapidly altered. As the rate of photosynthesis increases there is an activation of SPS, which may be directly or indirectly linked to changes in the availability of P_i. This activation can be modified by factors related to the accumulation of sucrose. Under normal conditions there is a balance between these factors, and the leaf contains a mixture of the different forms of SPS.Abbreviations Chl chlorophyll - Frul,6bisP fructose-1,6-bisphosphate - Fru2,6bisP fructose-2,6-bisphosphate - Fru6P fructose-6-phosphate - Fru1,6bisPase fructose-1,6-bisphosphatase - Fru6P 2kinase fructose-6-phosphate, 2kinase - Fru2,6bisPase fructose-2,6-bisphosphatase - Glc6P glucose-6-phosphate - P_j inorganic phosphate - SPS sucrose-phosphate synthase - UDPGLc uridine 5-diphosphate glucose     

Extent of ipt gene expression and resulting amount of cytokinins affect activities of carboxylation enzymes in transgenic plants

Three types of transgenic plants of Solanum tuberosum cvs. Kamyk and Oreb, and Nicotiana tabacum cvs. Maryland Mammoth and Trapezond were selected according to intensity of introduced ipt gene expression and resulting amount of synthesised cytokinins (CKs). In comparison with controls, original transgenic regenerants grown in vitro showed a massive increase of CK contents, in tobacco by 379 % and in potato by 159 % (MAS). Potato grown in soil from tubers of transgenic plants demonstrated a moderate increase (44 %) of CK contents (MOD). Transgenic tobacco grown from seeds in vitro did not show any significant change in CK contents (NOT). Initial (RuBPC_i and RuBPO_i) and total (RuBPC_t) activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and the activity of phosphoenolpyruvate carboxylase (PEPC) were not significantly affected by the transformation in the NOT plants. In the MOD plants, the RuBPCO activities were stimulated by up to 34 % whereas the PEPC activity was decreased by 17 %. On the other hand, all the measured enzyme activities were 32 – 91 % lower in the MAS. Leaf area, fresh and dry masses, and chlorophyll and soluble protein contents also went down with increasing CK amounts in the transformants. Dependence of RuBPC_i/RuBPO_i and RuBPC_t/PEPC ratios on the relative CK amounts in transgenic plants revealed that the individual enzyme activities were not affected uniformly. Endogenous CK contents in the MAS thus apparently exceeded an optimum needed for positive effects on many physiological traits and became a stress factor for such plants.     

Changes of Photosynthetic Characteristics in Relation to Leaf Senescence in Two Maize Hybrids with Different Senescent Appearance

A field experiment was conducted to investigate the changes in chlorophyll (Chl) and nitrogen (N) contents, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) contents and PEPC activity, and the photon-saturated net photosynthetic rate (P _Nsat), and their relationships with leaf senescence in two maize hybrids with different senescent appearance. One stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55) hybrid were used in this study, and we found that Chl and N contents and the P _Nsat in individual leaves of P3845 were greater than those in corresponding leaves of Hokkou 55 at the successive growth stages. In addition, larger contents of RuBPCO and PEPC, and a greater activity of PEPC were observed in P3845. Due to the lower rates of decrease of Chl, RuBPCO, and PEPC amounts per unit of N, and the lower net C translocation rate per unit of N in the stay-green hybrid, leaf senescence was delayed in comparison to the earlier senescent hybrid.     

Turnover of D1 Protein Encoded by psbA Gene in Higher Plants and Cyanobacteria Sustains Photosynthetic Efficiency to Maintain Plant Productivity Under Photoinhibitory Irradiance

The photosynthesis and related plant productivity aspects of plants and cyanobacteria depend upon the functioning of photosystem 2 (PS2), associated with D1 and D2 heterodimer reaction centre core proteins. The D1 protein is encoded by psbA gene, genetically localized on the plastid genome (cpDNA), contains functional cofactors of PS2 in association with D2 protein, and also functions for radiant energy transformation through oxidation of water and reduction of plastoquinone. Surprisingly, D1 protein accounts for even less than 1% of the total thylakoid membrane protein content. In spite of that, its rate of turnover is very much comparable to ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) large subunit, most abundantly present in green tissue. The normal functioning of PS2 possesses damage-repair cycles of D1 protein. Generally, rate of photodamage does not exceed the rate of repair under optimal growth conditions, therefore, no adverse effect on photosynthetic efficiency is manifest. However, under strong irradiance coupled with elevated temperature, level of photodamage exceeds the rate of repair, resulting in photoinhibition, photodegradation of D1 protein, and lowering photosynthetic efficiency linked with plant productivity eventually. The features of D1 turnover process are reviewed, particularly with respect to molecular mechanisms.     

Physiological Effects of Plant Hormones in Cotton Under Drought

Effects of plant hormones indole-3-yl-acetic acid (IAA), gibberellic acid (GA), benzylaminopurine (BAP), abscisic acid (ABA) and ethrel (ETH) in 5 M concentration on gas exchange, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO, EC 4.1.1.39) activity, pigment content and yield in cotton (Gossypium hirsutum L. cv. H-777) under drought were studied. At reproductive stage (55 – 60 d after sowing) these hormones were sprayed on shoots one day prior to stress imposition by withholding irrigation. The soil moisture of control plants was kept at field capacity. Net photosynthetic rate (P_N), stomatal conductance (g_s), transpiration rate (E), carboxylation efficiency (CE), water use efficiency (WUE), RuBPCO activity, boll number per plant, seed number per plant and lint mass per plant significantly decreased at drought while chlorophyll (Chl) b content and flower number per plant increased. ABA and ETH significantly reduced gas exchange parameters, Chl a and Chl b content. Detrimental drought effect on P_N, g_s, E, CE, RuBPCO and lint mass per plant was significantly alleviated by BAP and also its effect on seed number and lint mass per plant was significantly alleviated with the ABA treatment.     

Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Specific Proteolysis in Barley Chloroplasts During Dark Induced Senescence

Intact chloroplasts were isolated from dark-senescing primary barley (Hordeum vulgare L.) leaves in order to study selective ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) degradation by the stromal and membrane fractions. RuBPCO specific degradation was estimated and characterised applying sensitive avidin-biotin ELISA method with non-modified or oxidatively modified biotinylated RuBPCO (BR) as substrates. Distinct proteolytic activities were detected. They differed in ATP and divalent metal ion dependence, protease inhibitory profile, and dynamics in the time-course of dark-induced senescence. The results supported involvement of ATP- and metal ion-dependent serine type proteolytic activity against non-modified BR early in induced senescence and appearance of ATP-independent activity at later stage. Active oxygen-modified BR was degraded by ATP-independent serine-type protease probably containing essential SH-groups and requiring divalent metal ions.     

Mulberry Leaf Metabolism under High Temperature Stress

Effects of high temperature on the activity of photosynthetic enzymes and leaf proteins were studied in mulberry (Morus alba L. cv. BC2-59). A series of experiments were conducted at regular intervals (120, 240 and 360 min) to characterize changes in activities of ribulose-1,5-bisphosphate carboxylase (RuBPC) and sucrose phosphate synthase (SPS), photosystem 2 (PS 2) activity, chlorophyll (Chl), carotenoid (Car), starch, sucrose (Suc), amino acid, free proline, protein and nucleic acid contents in leaves under high temperature (40 °C) treatments. High temperature markedly reduced the activities of RuBPC and SPS in leaf extracts. Chl content and PS 2 activity in isolated chloroplasts were also affected by high temperature, particularly over 360 min treatment. Increased leaf temperature affected sugar metabolism through reductions in leaf starch content and sucrose-starch balance. While total soluble protein content decreased under heat, total amino acid content increased. Proline accumulation (1.5-fold) was noticed in high temperature-stressed leaves. A reduction in the contents of foliar nitrogen and nucleic acids (DNA and RNA) was also noticed. SDS-PAGE protein profile showed few additional proteins (68 and 85 kDa) in mulberry plants under heat stress compared to control plants. Our results clearly suggest that mulberry plants are very sensitive to high temperature with particular reference to the photosynthetic carbon metabolism.