Characterization of photosystem I in rice (Oryza sativa L.) seedlings upon exposure to random positioning machine

To gain a better understanding of how photosynthesis is adapted under altered gravity forces, photosynthetic apparatus and its functioning were investigated in rice (Oryza sativa L.) seedlings grown in a random positioning machine (RPM). A decrease in fresh weight and dry weight was observed in rice seedlings grown under RPM condition. No significant changes were found in the chloroplast ultrastructure and total chlorophyll content between the RPM and control samples. Analyses of chlorophyll fluorescence and thermoluminescence demonstrate that PSII activity was unchanged under RPM condition. However, PSI activity decreased significantly under RPM condition. 77 K fluorescence emission spectra show a blue-shift and reduction of PSI fluorescence emission peak in the RPM seedlings. In addition, RPM caused a significant decrease in the amplitude of absorbance changes of P700 at 820 nm (A ₈₂₀) induced by saturated far-red light. Moreover, the PSI efficiency (Φ _I) decreased significantly under RPM condition. Immunoblot and blue native gel analyses further illustrate that accumulation of PSI proteins was greatly decreased in the RPM seedlings. Our results suggest that PSI, but not PSII, is down-regulated under RPM condition.     

Cyclic electron transport around photosystem I and its relationship to non-photochemical quenching in the unicellular green alga Dunaliella salina under nitrogen deficiency

Electron transport in photosystem II (PSII) and photosystem I (PSI) was estimated in terms of chlorophyll fluorescence and changes in P700 redox, respectively, in the unicellular green alga Dunaliella salina in the presence or absence of a nitrogen source in the culture medium. In a nitrogen-containing medium, the quantum yield of PSII (Φ_II) and that in PSI (Φ_I) were at the same level in low light, but cyclic electron transport around photosystem I (CET-PSI) was induced under high light as estimated from an increase in Φ_I/Φ_II. High light might further enhance the rate of electron transport in PSI by inducing the state 2 transition, in which the distribution of light energy is shifted to PSI at the expense of PSII. Nitrogen deficiency resulted in a decrease in Φ_II and an increase in Φ_I. As a consequence, the rate of CET-PSI was expected to increase. The high CET-PSI under N deficiency was probably associated with a high level of energy quenching (qE) formation in PSII.     

Effects of heat stress on photosynthetic electron transport in a marine cyanobacterium <Emphasis Type="Italic">Arthrospira</Emphasis> sp.

Arthrospira (Spirulina) is widely used as human health food and animal feed. In cultures grown outdoors in open ponds, Arthrospira cells are subjected to various environmental stresses, such as high temperature. A better understanding of the effects of high temperature on photosynthesis may help optimize the productivity of Arthrospira cultures. In this study, the effects of heat stress on photosynthetic rate, chlorophyll a fluorescence transients, and photosystem (PS) II, PSI activities in a marine cyanobacterium Arthrospira sp. were examined. Arthrospira cells grown at 25 °C were treated for 30 min at 25 (control), 30, 34, 37, or 40 °C in the dark. Heat stress (30–37 °C) enhanced net photosynthetic O₂ evolution rate. Heat stress caused over-reduction PSII acceptor side, damage of donor side of PSII, decrease in the energetic connectivity of PSII units, and decrease in the performance of PSII. When the temperature changed from 25 to 37 °C, PSII activity decreased, while PSI activity increased, the enhancement of photosynthetic O₂ evolution was synchronized with the increase in PSI activity. When temperature was further increased to 40 °C, it induced a decrease in photosynthetic O₂ evolution rate and a more severe decrease in PSII activity, but an increase in PSI activity. These results suggest that PSI activity was the decisive factor determining the change of photosynthetic O₂ evolution when Arthrospira was exposed to a temperature from 25 to 37 °C, but then, PSII activity became the decisive factor adjusting the change of photosynthetic O₂ evolution when the temperature was increased to 40 °C.     

Light Intensity Alters the Extent of Arsenic Toxicity in Helianthus annuus L. Seedlings

The present study is aimed at assessing the extent of arsenic (As) toxicity under three different light intensities—optimum (400 μmole photon m^?2 s^?1), sub-optimum (225 μmole photon m^?2 s^?1), and low (75 μmole photon m^?2 s^?1)—exposed to Helianthus annuus L. var. DRSF-113 seedlings by examining various physiological and biochemical parameters. Irrespective of the light intensities under which H. annuus L. seedlings were grown, there was an As dose (low, i.e., 6 mg kg^?1 soil, As₁; and high, i.e., 12 mg kg^?1 soil, As₂)-dependent decrease in all the growth parameters, viz., fresh mass, shoot length, and root length. Optimum light-grown seedlings exhibited better growth performance than the sub-optimum and low light-grown seedlings; however, low light-grown plants had maximum root and shoot lengths. Accumulation of As in the plant tissues depended upon its concentration used, proximity of the plant tissue, and intensity of the light. Greater intensity of light allowed greater assimilation of photosynthates accompanied by more uptake of nutrients along with As from the medium. The levels of chlorophyll a, b, and carotenoids declined with increasing concentrations of As. Seedlings acquired maximum Chl a and b under optimum light which were more compatible to face As₁ and As₂ doses of As, also evident from the overall status of enzymatic (SOD, POD, CAT, and GST) and non-enzymatic antioxidant (Pro).     

Fluorescence F 0 of photosystems II and I in developing C3 and C4 leaves,and implications on regulation of excitation balance

This work addresses the question of occurrence and function of photosystem II (PSII) in bundle sheath (BS) cells of leaves possessing NADP-malic enzyme-type C₄ photosynthesis (Zea mays). Although no requirement for PSII activity in the BS has been established, several component proteins of PSII have been detected in BS cells of developing maize leaves exhibiting O₂-insensitive photosynthesis. We used the basal fluorescence emissions of PSI (F _0I) and PSII (F _0II) as quantitative indicators of the respective relative photosystem densities. Chl fluorescence induction was measured simultaneously at 680 and 750 nm. In mature leaves, the F _m(680)/F ₀(680) ratio was 10.5 but less in immature leaves. We propose that the lower ratio was caused by the presence of a distinct non-variable component, F _c, emitting at 680 and 750 nm. After F _c was subtracted, the fluorescence of PSI (F _0I) was detected as a non-variable component at 750 nm and was undetectably low at 680 nm. Contents of Chls a and b were measured in addition to Chl fluorescence. The Chl b/(a + b) was relatively stable in developing sunflower leaves (0.25–0.26), but in maize it increased from 0.09 to 0.21 with leaf tissue age. In sunflower, the F _0I/(F _0I + F _0II) was 0.39 ± 0.01 independent of leaf age, but in maize, this parameter was 0.65 in young tissue of very low Chl content (20–50 mg m^?2) falling to a stable level of 0.53 ± 0.01 at Chl contents >100 mg m^?2. The values of F _0I/(F _0I + F _0II) showed that in sunflower, excitation was partitioned between PSII and PSI in a ratio of 2:1, but the same ratio was 1:1 in the C₄ plant. The latter is consistent with a PSII:PSI ratio of 2:1 in maize mesophyll cells and PSI only in BS cells (2:1:1 distribution). We suggest, moreover, that redox mediation of Chl synthesis, rather than protein accumulation, regulates photosystem assembly to ensure optimum excitation balance between functional PSII and PSI. Indeed, the apparent necessity for two Chls (a and b) may reside in their targeted functions in influencing accumulation of PSI and PSII, respectively, as opposed to their spectral differences.     

Influence of sulphur on arsenic accumulation and metabolism in rice seedlings

The influence of sulphur on the accumulation and metabolism of arsenic in rice was investigated. Rice seedlings were grown in nutrient solutions with low sulphate (1.8 μM SO₄²⁻) or high sulphate (0.7 mM SO₄²⁻) for 12 or 14 d, before being exposed to 10 μM arsenite or arsenate for 2 or 1 d, respectively. In the arsenite exposure treatment, low sulphate-pretreated rice accumulated less arsenite than high sulphate pretreated plants, but the arsenite concentrations in shoots of low sulphate pretreated rice were higher than those of high sulphate pretreated. In the arsenate exposure treatment, the low sulphate pre-treatments also resulted in less arsenite accumulation in rice roots. Sulphur deprivation in nutrient solution decreased the concentrations of non-protein thiols in rice roots exposed to either arsenite or arsenate. The low sulphate-pretreated plants had a higher arsenic transfer factor than the high sulphate-pretreated plants. The results suggest that rice sulphate nutrition plays an important role in regulating arsenic translocation from roots to shoots, possibly through the complexation of arsenite-phytochelatins.     

The alleviation of zinc toxicity by silicon is related to zinc transport and antioxidative reactions in rice

The objective of this study is to elucidate the roles of silicon (Si) in enhancing tolerance to excess zinc (Zn) in two contrasting rice (Oryza sativa L.) cultivars: i.e. cv. TY-167 (Zn-resistant) and cv. FYY-326 (Zn-sensitive). Root morphology, antioxidant defense reactions and lipid peroxidation, and histochemical staining were examined in rice plants grown in the nutrient solutions with normal (0.15 μM) and high (2 mM) Zn supply, without or with 1.5 mM Si. Significant inhibitory effects of high Zn treatment on plant growth were observed. Total root length (TRL), total root surface area (TRSA) and total root tip amount (TRTA) of both cultivars were decreased significantly in plants treated with high Zn, whereas these root parameters were significantly increased when Zn-stressed plants were supplied with 1.5 mM Si. Supply of Si also significantly decreased Zn concentration in shoots of both cultivars, indicating lower root-to-shoot translocation of Zn. Moreover, superoxide dismutase (SOD), catalase (CAT), and asorbate peroxidase (APX) activities were increased, whereas malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) concentrations were decreased in Si-supplied plants of both Zn-sensitive and Zn-resistant rice cultivars exposed to Zn stress. These alleviative effects of Si, further confirmed by the histochemical staining methods, were more prominent in the Zn-resistant cultivar than in the Zn-sensitive one. Taken together, all these results suggest that Si-mediated alleviation of Zn toxicity is mainly attributed to Si-mediated antioxidant defense capacity and membrane integrity. The possible role of Si in reduction of root-to-shoot translocation of Zn can also be considered.     

Stoichiometries of Photosystem I and Photosystem II in Rice Mutants Differently Deficient in Chlorophyll b

Stoichiometries of photosystem I (PSI) and photosystem II (PSII)reaction centers in a cultivar of rice, Norin No. 8, and threechlorophyll b-deficient mutants derived from the cultivar wereinvestigated. Quantitation of PSI by photooxidation of P-700and chromatographic assay of vitamin K₁ showed that, on thebasis of chlorophyll, the mutants have higher concentrationsof PSI than the wildtype rice. Greater increases were observedin the PSII contents measured by photoreduction of Q_A, bindingof a radioactive herbicide and atomic absorption spectroscopyof Mn. Consequently, the PSII to PSI ratio increased from 1.1–1.3in the wild-type rice to 1.8 in chlorina 2, which contains noChl b, and to 2.0–3.3 in chlorina 11 and chlorina 14,which have chlorophyll a/b ratios of 9 and 13, respectively.Measurement of oxygen evolution with saturating single-turnoverflashes revealed that, whereas at most 20% of PSII centers areinactive in oxygen evolution in the wildtype rice, the non-functionalPSII centers amount to about 50% in the three mutant strains.The fluorescence induction kinetics was also analyzed to estimateproportions of the inactive PSII in the mutants. The data obtainedsuggest that plants have an ability to adjust the stoichiometryof the two photosystems and the functional organization of PSIIin response to the genetically induced deficiency of chlorophyllb. (Received July 29, 1994; Accepted February 7, 1996)     

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

Changes in the photosynthetic characteristics and photosystem stoichiometries in wild-type and Chl <Emphasis Type="Italic">b</Emphasis>-deficient mutant rice seedlings under various irradiances

By using a wild-type rice (Oryza sativa L. cv. Norin No. 8) and the chlorophyll (Chl) b-deficient mutant derived from Norin No. 8 (chlorina 11), the present study monitored the oxygen evolution, contents of Chl a and b, β-carotene, and lutein in leaf and the contents of cytochrome f, and the reaction centres of photosystem I (PSI) and photosystem II (PSII) in thylakoids. The oxygen evolution, maximal quantum yield of PSII (F_v/F_m) and Chl concentration remained constant in both Norin No. 8 and chlorina 11 under 5 and 2% of full sunlight for six days. On the other hand, on the thylakoid level, the PSII reaction centre of chlorina 11 was more stable even under high irradiance, while approximately 40% decrease in levels of the PSII reaction centre occurred under 2% of full sunlight for six days. However, under such conditions, by regulating the stoichiometry of active PSII and PSI centres, the light absorption balance in both rice types was adjusted between the two photosystems. The present study attempted to examine whether the light absorption balance between PSII and PSI is altered to effectively conduct photosynthesis in the wild-type and Chl b-deficient mutant rice seedlings.     

Effect of Chelate-Assisted Hexavalent Chromium on Physiological Changes,Biochemical Alterations,and Chromium Bioavailability in Crop Plants—An In Vitro Phytoremediation Approach

This study revealed heavy metal–induced physiological and biochemical alterations in crop seedlings by supplementing chelating agents in the nutrient solution. Hexavalent chromium (Cr⁺⁶) induces several toxic effects in hydroponically grown rice, wheat, and green gram seedlings. A noticeable decrease was observed in root length, shoot length, biomass content, and chlorophyll biosynthesis of the seedlings grown in the nutrient solutions supplemented with Cr⁺⁶ at 100 μM. The seedling growth was stimulated with supplement of chelating agents such as EDTA, DTPA, and EDDHA. An increase in proline content was noticed with the application of Cr⁺⁶ (100 μM) in nutrient solutions. Stimulated activities of antioxidant enzymes such as catalase and peroxidase were noticed with increasing concentrations of chromium. Cr bioaccumulation was significantly high in roots of seedlings treated with Cr⁺⁶ at 100 μM in nutrient solution. Shoot translocation of Cr as depicted by transportation index (Ti) values for different crops were enhanced with the application of chelating agents. The total accumulation rate (TAR) for Cr was enhanced with the supplementation of DTPA in rice and wheat, whereas the application of EDDHA was found effective for increasing the accumulation rate of Cr in green gram seedlings. This study demonstates the role of chelating agents in lessening the toxic effects of Cr⁺⁶. The chelating agents supplemented with Cr⁺⁶ in the culture medium enhanced the Cr bioavailability in plants.     

水稻永绿色基因突变和超表达对其叶片类囊体蛋白降解的影响

应用蛋白质免疫杂交技术分析了永绿色基因(Stay-green Rice,SGR)突变和超表达对水稻(Oryza sativa)叶片类囊体蛋白质降解的影响.结果表明,在正常生长条件下,SGR超表达降低了光系统Ⅱ(PSⅡ)、光系统Ⅰ(PS Ⅰ)和电子传递链等的蛋白质含量.暗诱导衰老处理时,SGR突变延缓了PSⅠ和PSⅡ的蛋...     

Salinity improves chilling resistance in Suaeda salsa

Suaeda salsa L., a C3 euhalophytic herb, is native to saline soils, demonstrates high resistance to salinity stress. The effect of chilling stress on S. salsa under high salinity, particularly the change in unsaturated fatty acid content within membrane lipids, has not been investigated. After a 12 h chilling treatment (4 °C) performed under low irradiance (100 μmol m^?2 s^?1), the chlorophyll contents, maximal photochemical efficiency of photosystem II (F _v/F _m) and actual PSII efficiency (ΦPSII) were determined. These measurements were significantly decreased in S. salsa leaves in the absence of salt treatment yet there were no significant changes with a 200 mM NaCl treatment. Chlorophyll contents, F _v/F _m and ΦPSII in S. salsa under 200 mM NaCl were higher than those without salt treatment. The unsaturated fatty acid content and the double bond index (DBI) of major membrane lipids of monogalactosyldiacylglycerols, digalactosyldiacylglycerols (DGDG), sulphoquinovosyldiacylglycerols and phosphatidylglycerols (PG) significantly increased following the chilling treatment (4 °C) (with 12 h of low irradiance and 200 mM of NaCl). The DBI of DGDG and PG was decreased in the absence of the salt treatment. These results suggest that in the euhalophyte S. salsa, a 200 mM NaCl treatment increases chilling tolerance under conditions of low irradiance (100 μmol m^?2 s^?1).     

Sulfur stress-induced antioxidative responses in leaves of Triticum aestivum L.

Antioxidative responses were investigated in leaves of wheat (Triticum aestivum L.) grown at varying S levels ranging from deficiency to excess (1, 2, 4, 6 and 8 mM S). Optimum yield was observed in plants supplied with 4 mM S. Wheat responded to S deficiency and excess supply by decreasing growth of root and shoot. Chlorosis in young leaves was observed after 15 days of deficient S supply. The biomass and concentration of photoassimilatory pigments decreased in plants grown at 1, 2, 6 and 8 mM S supply. The concentration of thiobarbituric acid reactive substances (TBARS), cysteine, nonprotein thiol and hydrogen peroxide (H₂O₂) increased in plants grown under S stress. Accumulation of TBARS and H₂O₂ in leaves indicated oxidative damage in S-deficient and S-excess plants. Deficient and excess levels of S showed an increase in the activities of antioxidative enzymes superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2).     

Effects of growth irradiance levels on the ratio of reaction centers in two species of marine phytoplankton

Cells of two species of single-celled marine algae, the diatom Skeletonema costatum (Greve), Cleve, and the chlorophyte Dunaliella tertiolecta Butcher, were cultured in white light of high (500-600 microeinsteins per square meter per second) and low (30 microeinsteins per square meter per second) intensity. For both algal species, cells grown at low light levels contained more chlorophyll a and had a lower ratio of chlorophyll a to chlorophylls b or c than did cells grown at high light levels. When photosynthetic unit sizes were measured on the basis of either oxygen flash yields or P₇₀₀ photooxidation, different results were obtained with the different species. In the chlorophyte, the cellular content of photosystem I (PSI) and photosystem II (PSII) reaction centers increased in tandem as chlorophyll a content increased so that photosynthetic unit sizes changed only slightly and the ratio PSI:PSII reaction centers remained constant at about 1.1. In the diatom, as the chlorophyll content of the cells increased, the number of PSI reaction centers decreased and the number of PSII reaction centers increased so that the ratio of PSI:PSII reaction centers decreased from about unity to 0.44. In neither organism did photosynthetic capacity correlate with changes in cellular content of PSI or PSII reaction centers. The results are discussed in relationship to the physical and biological significance of the photosynthetic unit concept.     

The physiological links of the increased photosystem II activity in moderately desiccated Porphyra haitanensis (Bangiales, Rhodophyta) to the cyclic electron flow during desiccation and re-hydration

Photosynthetic electron flow changed considerably during desiccation and re-hydration of the intertidal macroalgae Porphyra haitanensis. Activities of both photosystem (PSI) and photosystem (PSII) increased significantly at moderate desiccation levels. Whereas PSII activity was abolished at an absolute water content (AWC) <24 %, PSI remained active with progressive decreases in AWC to values as low as 16 %. This result suggested that cyclic electron flow around PSI was still active after inactivation of linear electron flow following severe desiccation. Moreover, the PSI activity was restored more rapidly than that of PSII upon re-hydration. Pretreatment of the blades with 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) suppressed PSII activity following desiccation to an AWC of ~16 % AWC. Cyclic electron flow around PSI decreased markedly in blades pretreated with DCMU than in blades without pretreatment of DCMU during re-hydration in seawater containing DCMU. All results suggested that the activity of PSII under desiccation conditions plays an important role in the operation of cyclic electron flow during desiccation and its recovery during re-hydration. Therefore, we proposed the PSII activity during desiccation could eventually lead to the accumulation of NADPH, which could serve as electron donor for P700⁺ and promote its recovery during re-hydration, thereby favoring the operation of cyclic electron flow.     

Different responses of photosystem I and photosystem II in three tropical oilseed crops exposed to chilling stress and subsequent recovery

Key message

Different responses of photosystem I and II to chilling.

Abstract

Tropical crops are sensitive to chilling stress, but the underlying physiological mechanisms are unclear. We investigated the maximum quantum yield of PSII (F _v/F _m), the maximum photo-oxidizable P700 (P _m), the energy distribution in PSII, and the redox state of P700 in leaves of seedlings of three promising oilseed crops originating from tropical regions, Plukenetia volubilis, Jatropha curcas and Ricinus communis, during chilling treatment and subsequent recovery under a photon flux density of 450 μmol m^?2 s^?1. Our results showed that F _v/F _m decreased progressively and significantly to about 44.7, 62.2 and 77.0 % of the control after chilling treatment for 3 days in P. volubilis, J. curcas and R. communis, respectively, mainly due to the decrease in F _m (maximum fluorescence of PSII). After recovery under 18 °C for 6 days, F _v/F _m recovered to 81.4 and 94.9 % of the control in J. curcas and R. communis, but only to 26.3 % in P. volubilis. Under chilling stress and subsequent recovery, P _m remained stable in J. curcas and R. communis, whereas it decreased slightly in P. volubilis. These results indicated that PSII was more sensitive to chilling stress than PSI under moderate light for all three species, and that P. volubilis was the most susceptible. Cyclic electron flow around PSI and effective quantum yield of photosystem II [Y _(CEF)/Y _(II)] ratio were stimulated much more in J. curcas and R. communis compared with that in P. volubilis under chilling conditions, resulting in more severe injury as indicated by higher accumulation of hydrogen peroxide and malondialdehyde. There was a significantly negative relationship between F _v/F _m and Y _(CEF)/Y _(II), suggesting that stimulation of Y _(CEF)/Y _(II) plays a pivotal role in protecting PSI and PSII from photoinhibition caused by chilling stress.     

Water deficit and aluminum tolerance are associated with a high antioxidative enzyme capacity in Indica rice seedlings

Plant growth and productivity are greatly affected due to changes in the environmental conditions. In the present investigation, the interactive effects of two important abiotic stresses, i.e., water deficit and Al toxicity, were examined in the seedlings of two rice (Oryza sativa L.) cvs. Malviya-36 (water deficit/Al sensitive) and Vandana (water deficit/Al tolerant). When 15 days grown seedlings were exposed to water deficit (created with 15 % polyethylene glycol 6000) or Al (1 mM AlCl₃) treatment or both the treatments together for 48 h, the lengths of root/shoot, relative water content, and chlorophyll greatly declined in the seedlings of the sensitive cultivar, whereas in the tolerant seedlings, either little or insignificant decline in these parameters was observed due to the treatments. Seedlings subjected to water deficit or Al treatment alone or in combination showed increased intensity of the isoenzyme activity bands of superoxide dismutase (SOD), guaiacol peroxidase (GPX), and ascorbate peroxidase (APX) in in-gel activity staining studies. Water deficit caused decrease in intensity of catalase (CAT) activity bands; however, when seedlings were exposed to AlCl₃ alone or in combination with water deficit, the intensity of the CAT isoforms increased in both the rice cultivars. The level of expression of the activity bands of SOD, CAT, GPX, and APX was always higher in the seedlings of tolerant cv. Vandana compared to the sensitive cv. Malviya-36 under both controls as well as stress treatments. Higher intensity of isozymes representing higher activity levels of antioxidative enzymes in the rice seedlings and their further increase under water deficit, Al exposure, or in combination of both the stresses appears to serve as useful marker for specifying a combination of water deficit and Al tolerance in rice.     

Salt stress effects on the photosynthetic electron transport chain in two chickpea lines differing in their salt stress tolerance

The main objective of this study was to evaluate the effects of salt stress on the photosynthetic electron transport chain using two chickpea lines (Cicer arietinum L.) differing in their salt stress tolerance at the germination stage (AKN 87 and AKN 290). Two weeks after sowing, seedlings were exposed to salt stress for 2 weeks and irrigated with 200 ml of 200 mM NaCl every 2 days. The polyphasic OJIP fluorescence transient and the 820-nm transmission kinetics (photosystem I) were used to evaluate the effects of salt stress on the functionality of the photosynthetic electron transport chain. It was observed that a signature for salt stress was a combination of a higher J step (V_J), a smaller IP amplitude, and little or no effect on the primary quantum yield of PSII (φ_Po). We observed for AKN 290 a shorter leaf life cycle, which may represent a mechanism to cope with salt stress. For severely salt-stressed leaves, an inhibition of electron flow between the PQ pool and P700 was found. The data also suggest that the properties of electron flow beyond PSI are affected by salt stress.     

Effects of light quality on the chloroplastic ultrastructure and photosynthetic characteristics of cucumber seedlings

To understand how light quality influences plant photosynthesis, we investigated chloroplastic ultrastructure, chlorophyll fluorescence and photosynthetic parameters, Rubisco and chlorophyll content and photosynthesis-related genes expression in cucumber seedlings exposed to different light qualities: white, red, blue, yellow and green lights with the same photosynthetic photon flux density of 100 μmol m^?2 s^?1. The results revealed that plant growth, CO₂ assimilation rate and chlorophyll content were significantly reduced in the seedlings grown under red, blue, yellow and green lights as compared with those grown under white light, but each monochromatic light played its special role in regulating plant morphogenesis and photosynthesis. Seedling leaves were thickened and slightly curled; Rubisco biosynthesis, expression of the rca, rbcS and rbcL, the maximal photochemical efficiency of PSII (Fv/Fm) and quantum yield of PSII electron transport (Ф_PSII) were all increased in seedlings grown under blue light as compared with those grown under white light. Furthermore, the photosynthetic rate of seedlings grown under blue light was significantly increased, and leaf number and chlorophyll content of seedlings grown under red light were increased as compared with those exposed to other monochromatic lights. On the contrary, the seedlings grown under yellow and green lights were dwarf with the new leaves etiolated. Moreover, photosynthesis, Rubisco biosynthesis and relative gene expression were greatly decreased in seedlings grown under yellow and green light, but chloroplast structural features were less influenced. Interestingly, the Fv/Fm, Ф_PSII value and chlorophyll content of the seedlings grown under green light were much higher than those grown under yellow light.