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Seagrasses play an important role in coastal marine ecosystems, but they have been increasingly threatened by human activities. In recent years, seagrass communities have rapidly degenerated in the coastal marine ecosystems of China. To identify the reasons for the decline in seagrasses, the phytotoxic effects of trace metals (Cu, Cd and Zn) on the seagrass Thalassia hemprichii were investigated, and the environmental contents of the metals were analyzed where the seagrass grows. The results showed that leaf necrosis in T. hemprichii exposed to 0.01-0.1 mg L -1 of Cu 2+ for 5 days was more serious than that in plants exposed to the same concentrations of Cd 2+ and Zn 2+ . The chlorophyll content in T. hemprichii declined in a concentration-dependent manner after 5 days of exposure to Cu 2+ , Cd 2+ and Zn 2+ . The evident reduction in ΔF/F m ' in T. hemprichii leaves was observed at day 1 of exposure to 0.01-1.0 mg L -1 of Cu 2+ and at day 3 of exposure to 0.1-1.0 mg L -1 of Cd 2+ . The antioxidant enzyme activities (SOD, POD and CAT) in T. hemprichii leaves exposed to the three metal ions also showed significant changes. In seawater from Xincun Bay (Hainan, China), where T. hemprichii grows, Cu had reached a concentration (i.e., 0.01 mg L -1 ) that could significantly reduce chlorophyll content and ΔF/F m ' in T. hemprichii leaves. Our results indicate that Cu influences the deterioration of seagrasses in Xincun Bay.

The value of intrinsic chlorophyll fluorescence polarization, and the intensity in emission spectrum were investigated in leaf segments of Alocasia macrorrhiza under several stress conditions including different temperatures (25-50 degrees C), various concentrations of NaCl (0-250 mM), methyl viologen (MV, 0-25 microM), SDS (0-1.0%) and NaHSO(3) (0-80 microM). Fluorescence emission spectrum of leaves at wavelength regions of 500-800 nm was monitored by excitation at 436 nm. The value of fluorescence polarization (P value), as result of energy transfer and mutual orientation between chlorophyll molecules, was determined by excitation at 436 nm and emission at 685 nm. The results showed that elevated temperature and concentrations of salt (NaCl), photooxidant (MV), surfactant (SDS) and simulated SO(2) (NaHSO(3)) treatments all induced a reduction of fluorescence polarization to various degrees. However, alteration of the fluorescence spectrum and emission intensity of F(685) and F(731) depended on the individual treatment. Increase in temperature and concentration of NaHSO(3) enhanced fluorescence intensity mainly at F(685), while an increase in MV concentration led to a decrease at both F(685) and F(731). On the contrary, NaCl and SDS did not cause remarkable change in fluorescence spectrum. Among different treatments, the negative correlation between polarization and fluorescence intensity was found with NaHSO(3) treatments only. We concluded that P value being measured with intrinsic chlorophyll fluorescence as probe in leaves is a susceptible indicator responding to changes in environmental conditions. The alteration of P value and fluorescence intensity might not always be shown a functional relation pattern. The possible reasons of differed response to various treatments were discussed.

Many studies have investigated the photoprotective and photosynthetic capacity of plant leaves, but few have simultaneously evaluated the dynamic changes of photoprotective capacity and photosynthetic maturation of leaves at different developmental stages. As a result, the process between the decline of photoprotective substances and the onset of photosynthetic maturation during plant leaf development are still poorly understood, and the relationship between them has not been quantitatively described. In this study, the contents of photoprotective substances, photosynthetic pigment content and photosynthetic capacity of leaves at different developmental stages from young leaves to mature leaves were determined by spatio-temporal replacement in eight dominant tree species in subtropical evergreen broadleaved forests. The correlation analysis found that the data sets of anthocyanins, flavonoids, total phenolics and total antioxidant capacity were mainly distributed on one side of the symmetry axis (y  =  x), while the data sets of flavonoids, total phenolics and total antioxidant capacity were mainly distributed on both sides of the symmetry axis (y  =  x). In addition, the content of photoprotective substances in plant leaves was significantly negatively correlated with photosynthetic pigment content and photosynthetic capacity but was significantly positively correlated with dark respiration rate (Rd). When chlorophyll accumulated to ~50% of the final value, the photoprotective substance content and Rd of plant leaves reached the lowest level, and anthocyanins disappeared completely; in contrast, the photosynthetic capacity reached the highest level. Our results suggest that anthocyanins mainly play a light-shielding role in the young leaves of most plants in subtropical forests. In addition, 50% chlorophyll accumulation in most plant leaves was the basis for judging leaf photosynthetic maturity. We also believe that 50% chlorophyll accumulation is a critical period in the transition of plant leaves from high photoprotective capacity (high metabolic capacity, low photosynthetic capacity) to low photoprotective capacity (low metabolic capacity, high photosynthetic capacity). [ABSTRACT FROM AUTHOR]

Chlorophyll fluorescence and antioxidative capability in detached leaves of the wild type Arabidopsis thaliana L. ecotype Landsberg erecta ( Ler) and three mutants deficient in anthocyanins biosynthesis ( tt3, tt4, and tt3tt4) were investigated during treatment with temperatures ranging 25–45 °C. In comparison with the wild type, chlorophyll fluorescence parameters Fv/Fm, ΦPSII, electron transport rate (ETR), Fv/Fo and qP in three anthocyanin-deficient mutants showed a more rapidly decreasing rate when the temperature was over 35 °C. Non-photochemical quenching (NPQ) in these mutants was almost completely lost at 44 °C, whereas the content of heat stable protein dropped and the rate of the membrane leakage increased. Fo-temperature curves were obtained by monitoring Fo levels with gradually elevated temperatures from 22 °C to 72 °C at 0.5 °C/min. The inflexion temperatures of Fo were 45.8 °C in Ler, 45.1 °C in tt3, 44.1 °C in tt4 and 42.3 °C in tt3tt4, respectively. The temperatures of maximal Fo in three mutants were 1.9–3.8 °C lower than the wild type plants. Meanwhile, three mutants had lower activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) and an inferior scavenging capability to DPPH (1.1-diphenyl-2-picrylhy.drazyl) radical under heat stress, and in particular tt3tt4 had the lowest antioxidative potential. The results of the diaminobenzidine-H2O2 histochemical staining showed that H2O2 was accumulated in the leaf vein and mesophyll cells of mutants under treatment at 40 °C, and it was significantly presented in leaf cells of tt3tt4. The sensitivity of Arabidopsis anthocyanins-deficient mutants to high temperatures has revealed that anthocyanins in normal plants might provide protection from high temperature injury, by enhancing its antioxidative capability under high temperature stress. [ABSTRACT FROM AUTHOR]

Previous studies have suggested that high light (HL) stress causes photoinhibition in plants, while anthocyanins could protect the photosynthetic apparatus against photoinhibition. However, the photoprotection mechanism of anthocyanins is still ambiguous. We studied physiological responses and molecular changes for CHS -overexpression lines ( CHS1 , CHS2 , CHS3 ), Arabidopsis thaliana ecotype Columbia ( Col ), and T-DNA insertion lines of CHS ( tt4 ) under HL (200 μmol m −2  s −1 ) to explore the photoprotection mechanism of anthocyanins. The results showed that HL induced anthocyanin synthesis and accumulation. The leaves of CHS -overexpression lines turned reddest and the genes, including CHS, DFR, ANS, were expressed at highest levels. Thus, the CHS -overexpression lines maintained the highest photosynthetic capacity and suffered the least damage from HL of the three phenotypes. However, the CHS enzyme and anthocyanins were undetectable in tt4 during the experiment. Correspondingly, chlorophyll fluorescence parameters of tt4 declined greatly. The photosynthetic apparatus and cell membranes were also impaired dramatically. The physiological characteristics of Col were compared between CHS -overexpression lines and tt4 . Together, the results suggest that over-expression of CHS gene enhances HL resistance by synthesizing more anthocyanins, that anthocyanins enhance the adaptability of plants to HL and that they maintain photosynthetic capacity via both antioxidation and attenuation of light. [ABSTRACT FROM AUTHOR]

• Different photoprotection strategies for mid- and late-successional tree species. • Mid- and late-successional tree species response patterns to high-light were different. • The mid-successional tree species better tolerate high-light. • Late-successional tree species has greater restoration ability after high-light stress. Light adaptability is a major driver of succession and evolution in natural forest plant communities. However, the relationship between plants and light is contradictory; light provides energy for plant growth, but excessive light negatively affects plant growth. To reveal the pattern of plant adaptation to diurnal variation in light, photoprotection strategies were analyzed in young and mature leaves of six dominant species at two successional stages. Mature leaves of both mid- and late-successional dominant species exhibited a higher net photosynthetic rate (Pn) than young leaves. In particular, the mature leaves of the mid-successional tree species had higher high-light tolerance because of a higher Pn, water use efficiency (WUE) and electron transfer rate (ETR). In contrast, the mature leaves of the late-successional tree species had a higher restoration ability after high-light stress because of higher thermal energy dissipation and faster stomatal regulation. Young leaves of both the mid- and late-successional dominant species exhibited a higher photoprotection capability than mature leaves. The young leaves of the mid-successional tree species had higher high-light tolerance because of their highest WUE and anthocyanin/chlorophyll (Anth/Chl) and higher flavonoid/chlorophyll (Flav/Chl), total phenolics/chlorophyll (Phen/Chl) and total antioxidant capacity/chlorophyll (TAC/Chl) ratios. The young leaves of the late-successional tree species had a stronger restoration ability due to highest Flav/Chl, Phen/Chl, and TAC/Chl ratios and a higher moisture content and non-photochemical quenching coefficient (qN, NPQ). This study shows that in a high-light environment, when leaves are at the same developmental stage, mid-successional tree species better tolerate high-light, while late-successional tree species have a greater restoration ability after high-light stress. [ABSTRACT FROM AUTHOR]