Exogenous 5-aminolevulinic acid alleviates the detrimental effects of UV-B stress on lettuce (<Emphasis Type="Italic">Lactuca sativa</Emphasis> L) seedlings

One of the abiotic stress factors affecting plant metabolism is ultraviolet-B (UV-B) radiation. 5-Aminolevulinic acid (ALA), a key precursor of porphyrin biosynthesis, promotes plant growth and crop yields. To investigate the alleviating effects of exogenous ALA on the damages caused by UV-B exposure, two different concentrations [10 ppm (ALA1) and 25 ppm (ALA2)] of ALA were applied to lettuce seedlings for 24 h and then they were exposed to 3.3 W m^?2 UV-B. Results showed that UV-B treatment significantly decreased chlorophyll a and b (Chl a and b) concentration, enhanced the activity of antioxidant enzymes, total phenolic concentration, soluble sugar contents, expression of phenylalanine ammonia lyase (PAL) and γ-tocopherol methyltransferase (γ-TMT) genes, the concentration of malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and the rate of superoxide radical (\({\text{O}}_{2}^{ - }\)) generation in the lettuce seedlings when compared to the control. Pre-treatment with exogenous ALA significantly enhanced UV-B stress tolerance in lettuce seedlings by decreasing the reactive oxygen species. On the other hand, ALA application caused more increases in the PAL and γ-TMT gene expression, antioxidant enzymes activities, Chl a and b concentration, total phenolic content, antioxidant capacity and the concentrations of soluble sugars. Obtained results indicated that UV-B radiation exerts an adverse effect on lettuce seedlings, and some of the negative effects of UV-B radiation can be alleviated by exogenous ALA.     

Use of the atLEAF+ chlorophyll meter for a nondestructive estimate of chlorophyll content

At present, chlorophyll meters are widely used for a quick and nondestructive estimate of chlorophyll (Chl) contents in plant leaves. Chl meters allow to estimate the Chl content in relative units - the Chl index (CI). However, using such meters, one can face a problem of converting CI into absolute values of the pigment content and comparing data acquired with different devices and for different plant species. Many Chl meters (SPAD-502, CL-01, CCM-200) demonstrated a high degree of correlation between the CI and the absolute pigment content. A number of formulas have been deduced for different plant species to convert the CI into the absolute value of the photosynthetic pigment content. However, such data have not been yet acquired for the atLEAF+ Chl meter. The purpose of the present study was to assess the applicability of the atLEAF+ Chl meter for estimating the Chl content. A significant species-specific exponential relationships between the atLEAF value (corresponding to CI) and extractable Chl a, Chl b, Chl (a+b) for Calamus dioicus and Cleistanthus sp. were shown. The correlations between the atLEAF values and the content of Chl a, Chl b, and Chl (a+b) per unit of leaf area was stronger than that per unit of dry leaf mass. The atLEAF value- Chl b correlation was weaker than that of atLEAF value-Chl a and atLEAF value-Chl (a+b) correlations. The influence of light conditions (Chl a/b ratio) on the atLEAF value has been also shown. The obtained results indicated that the atLEAF+ Chl meter is a cheap and convenient tool for a quick nondestructive estimate of the Chl content, if properly calibrated, and can be used for this purpose along with other Chl meters.     

Effects of drought stress on growth and chlorophyll fluorescence of Lycium ruthenicum Murr. seedlings

The present study aimed to determine effects of drought stress on Lycium ruthenicum Murr. seedlings. Our results showed that mild drought stress was beneficial to growth of L. ruthenicum seedlings. Their height, basal diameter, crown, leaf number, stem dry mass, leaf and root dry mass increased gradually when the soil water content declined from 34.7 to 21.2%. However, with further decrease of the soil water content, the growth of L. ruthenicum seedlings was limited. After 28 d of treatment, the seedlings were apparently vulnerable to drought stress, which resulted in significant leaf shedding and slow growth. However, growth was restored after rehydration. Drought treatments led to a decrease in contents of chlorophyll (Chl) a, b, and Chl (a+b) and increase in the Chl a/b ratio. After rewatering, the Chl content recovered to the content of the control plants. Under drought stress, minimal fluorescence and nonphotochemical quenching coefficient increased, thereby indicating that L. ruthenicum seedlings could protect PSII reaction centres from damage. Maximum fluorescence, maximum quantum yield, actual quantum yield of PSII photochemistry, and photochemical quenching decreased, which suggested that drought stress impacted the openness of PSII reaction centres. A comparison of these responses might help identify the drought tolerance mechanisms of L. ruthenicum. This could be the reference for the planting location and irrigation arrangements during the growing period of L. ruthenicum.     

Influence of copper and nickel on morphophysiological indicators of seedlings of coastal aquatic plants

The effect of copper and nickel ions at concentrations of 1, 10, 25, 50, and 100 mg/L on morphometric and pigment indicators of the seedlings of narrowleaf water-plantain Alisma gramineum and seashore dock Rumex maritimus was studied. It was found that the pigment characteristics were more sensitive to nickel and copper than the morphometric characteristics. It was noted that copper started to be toxic at relatively low concentrations and lead to a significant degradation of the pigment complex compared with nickel. An inverse correlation was found in seedlings between metal concentrations in the medium and chlorophyll a (Chl a). It was shown that the synthesis of chlorophyll a was inhibited by metal salts more strongly than by chlorophyll b and carotenoids, and seedlings of seashore dock were more resistant to the action of the factors studied than seedlings of the narrowleaf water-plantain.     

Not all anthocyanins are born equal: distinct patterns induced by stress in Arabidopsis

Main Conclusion

Different abiotic stress conditions induce distinct sets of anthocyanins, indicating that anthocyanins have different biological functions, or that decoration patterns of each anthocyanin are used for unique purposes during stress. The induction of anthocyanin accumulation in vegetative tissues is often considered to be a response of plants to biotic or abiotic stress conditions. Arabidopsis thaliana (Arabidopsis) accumulates over 20 anthocyanins derived from the anthocyanidin cyanidin in an organ-specific manner during development, but the anthocyanin chemical diversity for their alleged stress protective functions remains unclear. We show here that, when grown in various abiotic stress conditions, Arabidopsis not only often accumulates significantly higher levels of total anthocyanins, but different stress conditions also favor the accumulation of different sets of anthocyanins. For example, the anthocyanin patterns of seedlings grown at pH 3.3 or in media lacking phosphate are very similar and characterized by relatively high levels of the anthocyanins A8 and A11. In contrast, anthocyanin inductive conditions (AIC) provided by high sucrose media are characterized by high accumulation of A9* and A5 relative to other stress conditions. The modifications present in each condition correlate reasonably well with the induction of the respective anthocyanin modification enzymes. Taken together, our results suggest that Arabidopsis anthocyanin profiles provide ‘fingerprints’ that reflect the stress status of the plants.     

Salicylic acid and calcium-induced protection of wheat against salinity

Soil salinity is one of the important environmental factors that produce serious agricultural problems. The objective of the present study was to determine the interactive effect of salicylic acid (SA) and calcium (Ca) on plant growth, photosynthetic pigments, proline (Pro) concentration, carbonic anhydrase (CA) activity and activities of antioxidant enzymes of Triticum aestivum L. (cv. Samma) under salt stress. Application of 90 mM of NaCl reduced plant growth (plant height, fresh weight (FW) and dry weight (DW), chlorophyll (Chl) a, Chl b, CA activity) and enhanced malondialdehyde (MDA) and Pro concentration. However, the application of SA or Ca alone as well as in combination markedly improved plant growth, photosynthetic pigments, Pro concentration, CA activity and activities of antioxidant enzymes peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR) and ascorbate peroxidase (APX) under salt stress. It was, therefore, concluded that application of SA and Ca alone as well as in combination ameliorated the adverse effect of salinity, while combined application proved more effective to reduce the oxidative stress generated by NaCl through reduced MDA accumulation, Chl a/b ratio and Chls degradation and enhanced activities of antioxidant enzymes.     

Growth and photosynthetic responses of soybean seedlings to maize shading in relay intercropping system in Southwest China

Intercropping, a traditional and worldwide cropping practice, has been considered as a paradigm of sustainable agriculture based on complementary mechanisms among different crop species. Soybean (Glycine max) is widely relay intercropped about 60 days before maize (Zea mays) harvest in Southwest China. However, shade caused by tall maize plants may be a limiting factor for soybean growth at a seedling stage. In field research, we studied the ecophysiological responses of two widely cultivated soybean varieties [Gongqiudou494-1 (GQD) and Gongxuan 1 (GX)] to maize shading in a relay intercropping system (RI) compared with monocropped soybean plants (M). Our results showed that soybean seedlings intercropped with maize exhibited significantly downregulated net photosynthetic rate (P _N) (?38.3%), transpiration rate (?42.7%), and stomatal conductance (?55.4%) due to low available light. The insignificant changes in intercellular CO₂ concentration and the maximum efficiency of PSII photochemistry suggested that the maize shading-induced depressions in P _N were probably caused by the deficiency of energy for carbon assimilation. The significantly increased total chlorophyll (Chl) content (+27.4%) and Chl b content (+52.2%), with lowered Chl a/b ratios (?20.5%) indicated soybean plants adjusted their light-harvesting efficiency under maize shading condition. Biomass and leaf area index (LAI) of seedlings under RI decreased significantly (?78.7 and ?71%, respectively) in comparison with M. Correlation analysis indicated the relative reduction in biomass accumulation was caused by the decline in LAI rather than P _N, it affected negatively the final yields of soybean (32.8%). Cultivar-specific responses to maize shading were observed in respects of LAI, biomass, and grain yield. It indicated that GX might be a better cultivar for relay intercropping with maize in Southwest China.     

Effects of different levels of water stress on leaf photosynthetic characteristics and antioxidant enzyme activities of greenhouse tomato

Two greenhouse experiments were conducted in order to investigate the effects of different levels of water stress on gas exchange, chlorophyll fluorescence, chlorophyll content, antioxidant enzyme activities, lipid peroxidation, and yield of tomato plants (Solanum lycopersicum cv. Jinfen 2). Four levels of soil water content were used: control (75 to 80% of field water capacity), mild water stress (55 to 60%), moderate water stress (45 to 50%), and severe water stress (35 to 40%). The controlled irrigation was initiated from the third leaf stage until maturity. The results of two-year trials indicated that the stomatal conductance, net photosynthetic rate, light-saturated photosynthetic rate, and saturation radiation decreased generally under all levels of water stress during all developmental stages, while compensation radiation and dark respiration rate increased generally. Water stress also declined maximum quantum yield of PSII photochemistry, electron transfer rate, and effective quantum yield of PSII photochemistry, while nonphotochemical quenching increased in all developmental stages. All levels of water stress also caused a marked reduction of chlorophyll a, chlorophyll b, and total chlorophyll content in all developmental stages, while activities of antioxidant enzymes, such as superoxide dismutase, peroxidase, and catalase, and lipid peroxidation increased.     

Effect of selenium on growth and antioxidant capacity of <Emphasis Type="Italic">Triticum aestivum</Emphasis> L. during development of lead-induced oxidative stress

Effects of two selenium concentrations—0.4 and 0.8 mg Se⁶⁺ per kilogram of soil (treatments Se0.4 and Se0.8)—on seedling growth, chlorophyll content (Chl (a + b)), the content of thiobarbituric acidreactive substances (TBARs) indicative of peroxidation rates, and the activities of antioxidant enzymes (ascorbate peroxidase, AsP; glutathione reductase, GR; and guaiacol peroxidase, GPX) were studied in roots and leaves of wheat (Triticum aestivum L., cv. Triso) plants that were exposed for 14 days to oxidative stress induced by 50 and 100 mg Pb²⁺ per kilogram of soil (treatments Pb50 and Pb100, respectively). The pollution of soil with Pb²⁺ inhibited growth, lowered Chl (a + b) content, and intensified peroxidation. The content of TBARs increased by 44 and 72% in leaves and by 25 and 45% in roots for treatments Pb50 and Pb100, respectively. Activities of the antioxidant enzymes GR and GPX were higher in Pb²⁺-treated than in untreated (control) plants. The introduction of Se⁶⁺ into Pb²⁺-free soil (treatment Se0.4) was found to promote growth, stimulate AsP activity by 40% in leaves, and enhance AsP, GR, and GPX activities in roots by 38, 33, and 74%, respectively. The content of TBARs was reduced in Se⁶⁺-treated plants. By contrast, the treatment Se0.8 suppressed growth, elevated TBARs content, and stimulated activities of antioxidant enzymes in roots and leaves. The addition of 0.4 mg Se⁶⁺/kg to Pb²⁺-contaminated soil alleviated the negative influence of lead on plant growth, whereas the addition of 0.8 mg Se⁶⁺/kg aggravated the effect of Pb²⁺ contamination, especially in treatment (Pb100+Se0.8). Thus, the effectiveness of exogenous Se⁶⁺ application on growth and adaptive potential of plants cultivated under optimal pollutant-free conditions and on soils contaminated with lead depended on the concentration of Se⁶⁺ supplemented to soil and on the content of the stressor agent.     

Lanthanum improves salt tolerance of maize seedlings

In this study, the effects of lanthanum were investigated on contents of pigments, chlorophyll (Chl) fluorescence, antioxidative enzymes, and biomass of maize seedlings under salt stress. The results showed that salt stress significantly decreased the contents of Chl and carotenoids, maximum photochemical efficiency of PSII (F_v/F_m), photochemical quenching (q_P), and quantum efficiency of PSII photochemistry (Φ_PSII), net photosynthetic rate (P_N), and biomass. Salt stress increased nonphotochemical quenching (q_N), the activities of ascorbate peroxidase, catalase, superoxide dismutase, glutathione peroxidase, and the contents of malondialdehyde and hydrogen peroxide compared with control. Pretreatment with lanthanum prior to salt stress significantly enhanced the contents of Chl and carotenoids, F_v/F_m, q_P, q_N, Φ_PSII, P_N, biomass, and activities of the above antioxidant enzymes compared with the salt-stressed plants. Pretreatment with lanthanum also significantly reduced the contents of malondialdehyde and hydrogen peroxide induced by salt stress. Our results suggested that lanthanum can improve salt tolerance of maize seedlings by enhancing the function of photosynthetic apparatus and antioxidant capacity.     

Effects of paclobutrazol on cultivars of Chinese bayberry (<Emphasis Type="Italic">Myrica rubra</Emphasis>) under salinity stress

Salt stress is one of the most critical factors hindering the growth and development of plants. Paclobutrazol (PBZ) is widely used to minimize this problem in agriculture because it can induce salt stress tolerance in plants. This study investigated the effects of PBZ on salt tolerance of seedlings from two Chinese bayberry cultivars (i.e., Wangdao and Shenhong). Plants were treated with three salt concentrations (0, 0.2, and 0.4 % NaCl) and two PBZ concentrations (0 and 2.0 μmol L^–1). Application of PBZ increased a relative water content, proline content, chlorophyll (a+b) content, and antioxidant enzyme activities in both cultivars, resulting in a better acclimation to salt stress and an increase in dry matter production. We concluded that PBZ ameliorated the negative effects of salt stress in Chinese bayberry seedlings.     

Divinyl Chlorophyll(ide) a Can Be Converted to Monovinyl Chlorophyll(ide) a by a Divinyl Reductase in Rice

3,8-Divinyl (proto)chlorophyll(ide) a 8-vinyl reductase (DVR) catalyzes the reduction of 8-vinyl group on the tetrapyrrole to an ethyl group, which is indispensable for monovinyl chlorophyll (Chl) synthesis. So far, three 8-vinyl reductase genes (DVR, bciA, and slr1923) have been characterized from Arabidopsis (Arabidopsis thaliana), Chlorobium tepidum, and Synechocystis sp. PCC6803. However, no 8-vinyl reductase gene has yet been identified in monocotyledonous plants. In this study, we isolated a spontaneous mutant, 824ys, in rice (Oryza sativa). The mutant exhibited a yellow-green leaf phenotype, reduced Chl level, arrested chloroplast development, and retarded growth rate. The phenotype of the 824ys mutant was caused by a recessive mutation in a nuclear gene on the short arm of rice chromosome 3. Map-based cloning of this mutant resulted in the identification of a gene (Os03g22780) showing sequence similarity with the Arabidopsis DVR gene (AT5G18660). In the 824ys mutant, nine nucleotides were deleted at residues 952 to 960 in the open reading frame, resulting in a deletion of three amino acid residues in the encoded product. High-performance liquid chromatography analysis of Chls indicated the mutant accumulates only divinyl Chl a and b. A recombinant protein encoded by Os03g22780 was expressed in Escherichia coli and found to catalyze the conversion of divinyl chlorophyll(ide) a to monovinyl chlorophyll(ide) a. Therefore, it has been confirmed that Os03g22780, renamed as OsDVR, encodes a functional DVR in rice. Based upon these results, we succeeded to identify an 8-vinyl reductase gene in monocotyledonous plants and, more importantly, confirmed the DVR activity to convert divinyl Chl a to monovinyl Chl a.Chlorophyll (Chl) is the main component of the photosynthetic pigments. Chl molecules universally exist in photosynthetic organisms and perform essential processes of harvesting light energy in the antenna systems and by driving electron transfer in the reaction centers (Fromme et al., 2003). In higher plants, there are two Chl species, Chl a and Chl b. The photosynthetic reaction centers contain only Chl a, and the peripheral light-harvesting antenna complexes contain Chl a and Chl b (Grossman et al., 1995). Chl a is synthesized from glutamyl-tRNA, and Chl b is synthesized from Chl a at the last step of Chl biosynthesis (Beale, 1999). So far, genes for all 15 steps in the Chl biosynthetic pathway have been identified in higher plants, at least in angiosperms represented by Arabidopsis (Arabidopsis thaliana; Beale, 2005; Nagata et al., 2005). Analysis of the complete genome of Arabidopsis showed that it has 15 enzymes encoded by 27 genes for Chl biosynthesis from glutamyl-tRNA to Chl b (Nagata et al., 2005). However, only six genes encoding three enzymes involved in Chl biosynthesis have been identified in rice (Oryza sativa). Magnesium chelatase comprises three subunits (ChlH, ChlD, and ChlI) and catalyzes the insertion of Mg²⁺ into protoporphyrin IX, the last common intermediate precursor in both Chl and heme biosyntheses. Jung et al. (2003) characterized OsCHLH gene for the OsChlH subunit of magnesium chelatase, and Zhang et al. (2006) cloned Chl1 and Chl9 genes encoding the OsChlD and OsChlI subunits of magnesium chelatase. Chl synthase catalyzes esterification of chlorophyllide (Chlide), resulting in the formation of Chl a. Wu et al. (2007) identified the YGL1 gene encoding the Chl synthase. Chl b is synthesized from Chl a by Chl a oxygenase; Lee et al. (2005) identified OsCAO1 and OsCAO2 genes for Chl a oxygenase.According to the number of vinyl side chains, Chls of oxygenic photosynthetic organisms are classified into two groups: 3,8-divinyl Chl (DV-Chl) and 3-vinyl Chl (monovinyl Chl [MV-Chl]). Almost all of the oxygenic photosynthetic organisms contain MV-Chls, regardless of the variation in their indigenous environments (Porra, 1997). The exceptions are species of Prochlorococcus marinus, marine picophytoplanktons that contain DV-Chls as their photosynthetic pigments (Chisholm et al., 1992).Chl biosynthetic heterogeneity is assumed to originate mainly in parallel DV- and MV-Chl biosynthetic routes interconnected by 8-vinyl reductases that convert DV-tetrapyrroles to MV-tetrapyrroles by conversion of the vinyl group at position 8 of ring B to the ethyl group (Parham and Rebeiz, 1995; Rebeiz et al., 2003). Most of Chls carry an ethyl group or, less frequently, a vinyl group. For example, Chl a and b occur as the MV-derivatives in green plants, but Chl precursors sometimes accumulate as DV-intermediates, and the ratio between the two forms can vary depending on the species, tissue, and growth conditions (Shioi and Takamiya, 1992; Kim and Rebeiz, 1996). So far, five 8-vinyl reductase activities have been detected at the levels of DV Mg-protoporphyrin IX (Kim and Rebeiz, 1996), Mg-protomonomethyl ester (Kolossov et al., 2006), protochlorophyllide (Pchlide) a (Tripathy and Rebeiz, 1988), Chlide a (Kolossov and Rebeiz, 2001; Nagata et al., 2005), and Chl a (Adra and Rebeiz, 1998). What is not clear at this stage is whether the various 8-vinyl reductase activities are catalyzed by one enzyme of broad specificity or by a family of enzymes of narrow specificity encoded by one gene or multiple genes, as is the case for NADPH Pchlide oxidoreductases (Rebeiz et al., 2003). The issue could be settled by purification of the various putative reductases and comparison of their properties.Nagata et al. (2005) and Nakanishi et al. (2005) independently identified the AT5G18660 gene of Arabidopsis as a divinyl reductase (DVR) that has sequence similarity to isoflavone reductase. Chew and Bryant (2007) demonstrated that BciA (CT1063), which is an ortholog of the Arabidopsis gene, encodes a DVR of the green sulfur bacterium Chlorobium tepidum TLS. They also considered that BchJ, which had been reported to be a vinyl reductase (Suzuki and Bauer, 1995), is not the enzyme, but it may play an important role in substrate channeling and/or regulation of bacteriochlorophyll biosynthesis. Islam et al. (2008) and Ito et al. (2008) independently identified a novel 8-vinyl reductase gene (Slr1923) in DVR-less cyanobacterium Synechocystis sp. PCC6803. However, no DVR gene has yet been identified in monocotyledonous plants.In this study, we isolated a spontaneous mutant, 824ys, from indica rice cv 824B. The mutant exhibited a yellow-green leaf phenotype throughout the growth stage, reduced level of Chls, arrested development of chloroplasts, and retarded growth rate. Map-based cloning of the mutant resulted in the identification of the OsDVR gene, showing sequence similarity to the DVR gene of Arabidopsis. In the 824ys mutant, nine nucleotides were deleted at residues 952 to 960 in the open reading frame (ORF), resulting in three amino acid deletion in the encoded protein. HPLC analysis of Chls indicated the mutant accumulates only DV-Chls. Enzymatic analysis demonstrated that the recombinant protein expressed in Escherichia coli is able to catalyze the conversion of DV-Chl(ide) a to MV-Chl(ide) a. Therefore, this study has confirmed that the OsDVR gene encodes a functional DVR in rice.     

Use of a SPAD meter to estimate chlorophyll content in Eugenia uniflora L. leaves as affected by contrasting light environments and soil flooding

In three separate experiments, the effectiveness of a SPAD-502 portable chlorophyll (Chl) meter was evaluated for estimating Chl content in leaves of Eugenia uniflora seedlings in different light environments and subjected to soil flooding. In the first experiment, plants were grown in partial or full sunlight. In the second experiment plants were grown in full sunlight for six months and then transferred to partial sunlight or kept in full sunlight. In the third experiment plants were grown in a shade house (40% of full sunlight) for six months and then transferred to partial shade (25–30% of full sunlight) or full sunlight. In each experiment, plants in each light environment were either flooded or not flooded. Non-linear regression models were used to relate SPAD values to leaf Chl content using a combination of the data obtained from all three experiments. There were no significant effects of flooding treatments or interactions between light and flooding treatments on any variable analyzed. Light environment significantly affected SPAD values, chlorophyll a (Chl a), chlorophyll b (Chl b), and total chlorophyll [Chl (a+b)] contents in Experiment I (p≤0.01) and Experiment III (p≤0.05). The relationships between SPAD values and Chl contents were very similar among the three experiments and did not appear to be influenced by light or flooding treatments. There were high positive exponential relationships between SPAD values and Chl (a+b), Chl a, and Chl b contents.     

Physiological and growth characteristics of Ginkgo biloba L. exposed to open field and shade enclosure during the reproductive stage

The current study compares responses to open field and shade enclosure condition (plastic shading nets were used to imitate a natural shading rate) to test the possible benefit of shading in terms of physiological and growth characteristics in Ginkgo biloba L. during the reproductive stage in summer. Compared with the net shade treated plants (NS-plants), the open-field plants (O-plants) contained lower chlorophyll (Chl) a + b content and Chl a/b ratio, and exhibited a decreased ratio of Chl/Car. Results showed that the chlorophyll fluorescence characteristics including maximum PSII photochemical efficiency (F _v /F _m ), potential electron transport per excited leaf cross-section (ET₀/CS₀), potential electron transport per PSII reaction center (ET₀/RC), dissipation per excited leaf cross-section (DI₀/CS₀), dissipation per PSII reaction center (DI₀/RC), and overall performance index of PSII photochemistry on absorbtion basis (PI_ABS) were altered by the net shade treatment. It was observed that the grana were illegible and difficult to distinguish by transmission electron microscopy, especially, in the cells of O-plants in which phenols were observed in the vacuole. The phenomenon of photoinhibition induced by excessive irradiance was confirmed by the abnormally high levels of the reactive oxygen species. Moreover, antioxidant enzymes activities were induced by high irradiance in the ginkgo leaves. In addition, significant differences were observed in the fresh weight and dry weight of leaves and seeds. Comparison of the variation of underlying physiological and biochemical mechanisms suggested that there was a better efficiency of ginkgo plants under artificial net shade conditions. Therefore, ginkgo plant would be best grown at 30–35 % of natural irradiance in summer months to be more profitably harvested and then meet the increasing demand of leaves and seeds.     

Extinction coefficients of chlorophyll a and B in n,n-dimethylformamide and 80% acetone

We found inconsistencies in the commonly used data for chlorophyll analysis in 80% acetone. Recently developed extinction coefficients for chlorophyll b in N,N-dimethylformamide (DMF) based on values from 80% acetone are low as a result of these inconsistencies. We determined extinction coefficients of chlorophyll a (Chl a) and chlorophyll b (Chl b) in DMF for wavelengths of 618 to 665 nanometers. The simultaneous equations necessary for quantifying Chl a, Chl b, or total Chl in DMF in the absence of other chlorophyllous pigments are: Chl a = 12.70A_664.5 - 2.79A₆₄₇; Chl b = 20.70 A₆₄₇ - 4.62A_664.5; total Chl = 17.90A₆₄₇ + 8.08A_664.5, where A = absorbance in 1.00 centimeter cuvettes and Chl = milligrams per liter.

N,N-Dimethylformamide is a very convenient solvent for Chl extraction since it is effective on intact plant parts and Chl is quite stable in DMF. There was no difference in the amount of Chl extracted when plant tissue was stored for 1 or 3 days at three temperatures, with or without solvent added.

     

Inhibition of root respiration induces leaf senescence in <Emphasis Type="Italic">Alhagi sparsifolia</Emphasis>

Leaf senescence can be induced by numerous factors. In order to explore the relationship between root respiration and leaf senescence, we utilized different types of phloem girdling to control the root respiration of Alhagi sparsifolia and its physiological response. Our results showed that both girdling and inhibition of root respiration led to a decline of stomatal conductance, photosynthesis, transpiration rate, chlorophyll (Chl) a, Chl b, carotenoid (Car) content, Chl a/b, Chl/Car, water potential, and Chl a fluorescence, as well as to an increase of abscisic acid (ABA), proline, and malondialdehyde content in leaves and to upregulation of senescence-associated gene expression. Our present work implied that both inhibition of root respiration and girdling can induce leaf senescence. In comparison with phloem girdling, the leaf senescence caused by inhibition of root respiration was less significant. The reason for girdling-induced senescence was ABA and carbohydrate accumulation. Senescence induced by inhibition of root respiration occurred due to leaf water stress resulting from inhibition of water absorption.     

Contrasting patterns of sun-red and shade-green leaves of <Emphasis Type="Italic">Buxus microphylla</Emphasis> in response to gradients of excess light during winter acclimation

Leaf reddening in overwintering evergreens largely restricts their application in landscapes and is generally triggered in response to excess light. To explore how leaves respond to excess light and examine the potential relevance of leaf reddening in this process, a comparative field study was conducted on the sun leaves (SUL), shade leaves (SHL) and three levels of artificially shaded sun leaves (SSUL) of Buxus microphylla ‘Wintergreen’. The seasonal changes in leaf colorations, chlorophyll (Chl) and carotenoid contents, leaf absorbance and chlorophyll fluorescence characteristics were investigated. The results showed that SUL upregulated Chl a/b with increased reductions in Chl b compared with Chl a, accumulated red pigments in the upper palisade mesophyll with reduced absorption in blue and red light but increased absorption in green light, and additionally, significantly downregulated photochemical activities through the sustained enhancement of energy dissipation in PSII antenna (ΦD) from fall to midwinter. In the SSUL, as the light intensity decreased, all of the above processes were mitigated except that the SSUL maintained constant absorptions in blue light region and whose levels were similar to those of the SUL and SHL. In contrast, the SHL maintained relatively high levels of Chl a and Chl b, remained completely green and showed regulated ΦD and ΦE (energy dissipation in PSII reaction centers) to maintain relatively high photochemical activity in the winter. We conclude that the sun leaves downregulate Chl contents to reduce the light absorption and simultaneously enhance sustained ΦD to dissipate most of the light energy, whereas shade leaves maintain relatively high Chl contents and demonstrate regulated proportions of ΦD and ΦE to match the extent to which the absorbed light can be utilized through photochemical reactions. The accumulated red pigments in sun phenotypes may provide a shading effect on Chls by directing energy to non-photosynthetic reaction centers in the blue light region where the absorption is offset by the reduced Chls.     

The combined effects of acidification stress and kinetin on chlorophyll content,dry matter accumulation and transpiration coefficient inSorghum bicolor plants

Increasing soil acidity (from pH 6.5 to pH 2.0) decreased chlorophyll (Chl)a andb contents, dry matter accumulation by plants and the transpiration coefficient. Chl stability to heat significantly increased with increased soil acidity. The Chla/b ratio was increased significantly at pH 5 and 4 and decreased at pH 3 and 2. SprayingSorghum shoots with kinetin solutions counteracted the above adverse effects on Chl content and dry matter accumulation. Kinetin-treated plants showed a lower transpiration coefficient than the untreated plants.     

Foliar application of the triterpene derivative 24-methylen-elemo-lanosta-8,24-dien-3-one alleviates salt toxicity in grapevine

This work focused on the effect triterpene derivative 24-methylen-elemo-lanosta-8,24-dien-3-one (F3) on the induction of salt stress tolerance of the Moroccan grapevine cv. “Doukkali”. Hardwood cuttings of the grapevine from a homogeneous plant material collected in the field were grown in hydroponic medium under different salt concentrations and treated with 50 or 100 µg ml^?1 of F3. Salt stress affected several physiological and biochemical parameters including relative water content, chlorophyll a and b content, peroxidase, and polyphenol oxidase activities, which decreased along with time. Meanwhile, proline, proteins, soluble sugars, H₂O₂, and carotenoid content, as well as phenolic compound content increased, suggesting an evidence of tolerance of this local variety to salinity. An exogenous supply of the triterpenic product increased all these parameters under normal conditions. In addition, F3 at low dose was found to be successful in lowering Na⁺ content and alleviating the inhibitory effects of salt stress on relative water content as well as on chlorophyll a and b.     

Activity of Photosystem 2, Lipid Peroxidation,and the Enzymatic Antioxidant Protective System in Heat Shocked Barley Seedlings

The impact of heat shock on minimising the activity of photosystem 2 (PS2) initiating high lipid peroxidation (POL) level and consequently changes in the enzymatic-antioxidant protective system was studied in seedlings of two Egyptian cultivars of barley (Giza 124 and 125). Heat doses (35 and 45 °C for 2, 4, 6, and 8 h) decreased chlorophyll (Chl) contents coupled with an increase in Chl a/b ratio, diminished Hill reaction activity, and quenched Chl a fluorescence emission spectra. These parameters reflect the disturbance of the structure, composition, and function of the photosynthetic apparatus as well as the activity of PS2. POL level, as dependent on the balance between pro- and anti-oxidant systems, was directly correlated with temperature, exposure time, and their interaction. Heat shock caused an increase in the electric conductivity of cell membrane, and malonyldialdehyde content (a peroxidation product) coupled with the disappearance of the polyunsaturated linolenic acid (C_18:3), reflecting the peroxidation of membrane lipids which led to the loss of membrane selective permeability. Moreover, it induced distinct and significant changes in activities of antioxidant enzymes. Superoxide dismutase and peroxidase activities have been progressively enhanced by moderate and elevated heat doses, but the most elevated one (45 °C for 8 h) showed a decrease in activities of both enzymes. In contrast, catalase activity was reduced with all heat shocks.