Zhongzheng Yan

Mangroves: obligate or facultative halophytes? A review

Salinity plays significant roles in regulating the growth and distribution of mangroves, and the salt tolerance mechanisms of mangroves have been the focus of research for several decades. There are contradictory views regarding the relationship between mangroves and salt: (1) Mangroves are facultative halophytes, i.e. freshwater is a physiological requirement and salt water is an ecological requirement for mangroves because they are capable of growing in freshwater. The former prevents excess respiratory losses while the latter prevents invasion and competition from non-halophytes. (2) Mangroves are obligate halophytes, i.e. salt is necessary for their growth. Mangroves cannot survive in freshwater permanently and salt water is a physiological requirement. Up to now, mangroves are usually considered as facultative halophytes. In this review, we provided five lines of evidence to evaluate these two contradictory views: (1) the results of laboratory culture experiments and field investigations; (2) the viviparous nature of mangroves; (3) the salt accumulation of mangroves under freshwater or low salinity; (4) the effect of salinity on the photosynthetic rate and in vitro enzyme activities, and (5) the effects of salinity fluctuation on mangrove growth and physiology. Contrary to widely accepted view, our evaluations of the aforementioned evidence suggest that mangroves are obligate halophytes. Mangroves can grow in freshwater for a limited time by drawing upon the nutrients and salt reserves in their hypocotyls while prolonged culture in freshwater is fatal to them. Mangroves have the ability to absorb Na+ and Cl− rapidly and preferentially under low-salinity conditions. Not all of the enzymes in mangroves are sensitive to salt. In fact, the activities of some enzymes are even stimulated by low or moderate salinity. Plants grown under constant salinity in a laboratory setting are unlikely to behave in the same way as those in their natural habitat with fluctuating salinity. Thus, studies on the effects of freshwater or low salinity and salinity fluctuation on mangroves, as well as the physiological mechanisms that allow maintenance of function under fluctuating salinity conditions should be strengthened in future research.

Effect of methyl jasmonate on cadmium uptake and antioxidative capacity in Kandelia obovata seedlings under cadmium stress.

This study investigated the effects of methyl jasmonate (MeJA) on chlorophyll concentration, lipid peroxidation, Cd uptake, antioxidative capacity, and type-2 metallothionein gene (KoMT2) expression in the leaves of Kandelia obovata seedlings exposed to Cd stress. Deleterious effects, including decreased chlorophyll content and increased malondialdehyde concentration, were observed in leaves of K. obovata after 9d of 200μmol L(-1) Cd treatment. Application of MeJA (0.1 to 1μmol L(-1)) increased the concentration of ascorbic acid and the activities of catalase and ascorbate peroxidase in the leaves of K. obovata, which helped alleviate the oxidative damage induced by Cd stress. The concentration of endogenous jasmonic acid in the leaves of K. obovata was decreased by Cd but was positively stimulated by exogenous MeJA. The expression of KoMT2 in the leaves was enhanced after 9d of 200μmol L(-1) Cd treatment, while the exogenous application of MeJA significantly restored the expression of KoMT2. Exogenous MeJA also inhibited the uptake of Cd to the aboveground part (leaves) of the seedlings, which helped reduce direct damages of Cd to the photosynthetic organ of the plant. The reduced uptake of Cd might be a result of stomatal closure and decreased transpiration by exogenous MeJA.

Methyl jasmonate as modulator of Cd toxicity in Capsicum frutescens var. fasciculatum seedlings

Methyl jasmonate (MeJA) elicits protective effects as form of plant response to abiotic stress. However, related studies on plant response to metal stress are insufficient. This study aimed to examine the effects of MeJA on growth and physiological responses of Capsicum frutescens seedlings exposed to cadmium (Cd) stress. The study was performed in an artificial climate chamber. Results showed that 50 mg L⁻¹ Cd significantly impaired the growth of the seedlings by increasing leaf MDA content and decreasing chlorophyll b. These effects were significantly mitigated by MeJA at low concentrations (0.1 µmol L⁻¹). The dry weights of different plant parts, chlorophyll content, and leaf catalase and ascorbate peroxidase activities were increased by a low MeJA concentration (0.1 µmol L⁻¹) but were decreased by a high MeJA concentration (1000 µmol L⁻¹). Significant increases in endogenous jasmonic acid were observed at 48 h after the samples were treated with Cd and 0.1 µmol L⁻¹ MeJA. These results suggested that low exogenous MeJA concentrations exhibited protective effects on the growth and physiology of C. frutescens seedlings under Cd stress.

Combined toxicity of cadmium and copper in Avicennia marina seedlings and the regulation of exogenous jasmonic acid

Seedlings of Avicennia marina were exposed to single and combined metal treatments of cadmium (Cd) and copper (Cu) in a factorial design, and the combined toxicity of Cu and Cd was tested. The effects of the exogenous jasmonic acid (JA) on chlorophyll concentration, lipid peroxidation, Cd and Cu uptake, antioxidative capacity, endogenous JA concentration, and type-2 metallothionein gene (AmMT2) expression in seedlings of A. marina exposed to combined metal treatments were also investigated. A binary mixture of low-dose Cd (9 µmolL(-1)) and high-dose Cu (900 µmolL(-1)) showed toxicity to the seedlings, indicated by the significant augmentation in leaf malondialdehyde (MDA) and reduction in leaf chlorophylls. The toxicity of the combined metals was significantly alleviated by the addition of exogenous JA at 1 µmolL(-1), and the chlorophyll and MDA contents were found to be restored to levels comparable to those of the control. Compare to treatment with Cd and Cu only, 1 and 10 µmolL(-1) JA significantly enhanced the ascorbate peroxidase activity, and 10 µmolL(-1) JA significantly decreased the uptake of Cd in A. marina leaves. The relative expression of leaf AmMT2 gene was also significantly enhanced by 1 and 10 µmolL(-1) JA, which helped reduce Cd toxicity in A. marina seedlings.

Effects of lead stress on anti-oxidative enzymes and stress-related hormones in seedlings of Excoecaria agallocha Linn

Background and AimsThis study aimed to evaluate the responses of anti-oxidative enzymes and stress-related hormones in E. agallocha to different levels of Pb stresses at different exposure time.MethodsThe study was carried out in greenhouse, and the pot trials were conducted to investigate the stress responses of root and leaf to Pb exposure in seedlings of E. agallocha.ResultsPb stress posed higher toxic effects on root than leaf at day 49. At days 1, 7and 49, the activities of superoxide dismutase and peroxidases increased significantly, especially in leaves. Significant increases of malondialdehyde content were also observed at day 1 but significant increases of proline were only found at day 49 in leaf. Increases of salicylic acid and jasmonic acid were mainly observed in the leaves at day 1.ConclusionsE. agallocha was sensitive to Pb stress and damages, but tended to acclimate to low levels of Pb stresses by increasing and maintaining high levels of SOD and POD activities even at the later stage of exposure (day 49). Increases of endogenous SA and JA concentrations at day 1 might also involve in the plant’s tolerance to Pb stress.

Temporal changes of polyphenols and enzyme activities in seedlings of Kandelia obovata under lead and manganese stresses

The temporal responses of peroxidase (POD), superoxide dismutase (SOD), total polyphenols (TP), extractable condensed tannins (ECT) to different levels of lead (Pb) or manganese (Mn) stress, as well as the metal accumulation, in seedlings of Kandelia obovata were investigated. Both stress time and stress intensity had significant effects with significant accumulation of Pb and Mn in roots at Day 1. Pb and Mn showed no significant effects on root or leaf TP and ECT at Day 1. Prolonged exposure to metals caused significant drops of root TP and ECT but increased in leaves at Days 7 and 49. POD activities decreased in both roots and leaves at Day 1, while POD and SOD both increased under moderate levels of Pb and Mn at Day 7. The present study showed that antioxidative enzyme activities were more sensitive indicators to Pb or Mn stresses than phenolic compounds.

Differences in lead tolerance between Kandelia obovata and Acanthus ilicifolius seedlings under varying treatment times.

The effects of short-term (1 day) and long-term (49 days) of lead (Pb) stress on growth and physiological responses in the leaves and roots of two mangrove plants, Kandelia obovata and Acanthus ilicifolius, were compared. The growth of both species was affected by Pb at Day 49, whereas the root to shoot ratio of K. obovata remained unchanged. Compared with A. ilicifolius, less Pb accumulated in leaves of K. obovata, which indicates that this species is a typical Pb-excluder. Significant linear relationships were observed between the Pb concentrations in the roots and leaves and the Pb treatment concentrations in the sediments in A. ilicifolius but not in K. obovata. The proline concentration increased in both mangrove species at Day 49, especially in A. ilicifolius, but no changes were observed at Day 1. The tolerant species K. obovata tended to acclimate to metal stress by restricting the translocation of toxic metals and by increasing and/or maintaining high superoxide dismutase (SOD) activity, minimizing lipid peroxidation, and exhibiting prolonged unaltered growth (49 days) under Pb treatment. The non-tolerant species, A. ilicifolius, did not acclimate to metal stress, its leaves were seriously damaged with significant increased MDA content, and its SOD activity was decreased. An increase of endogenous jasmonic acid concentration was observed only in K. obovata, both at Day 1 and at Day 49, which suggests that this hormone plays an important role in metal tolerance under short-term and long-term metal treatment.

Effect of Stubble Heights and Treatment Duration Time on the Performance of Water Dropwort Floating Treatment Wetlands (FTWS)

Effect of Stubble Heights and Treatment Duration Time on the Performance of Water Dropwort Floating Treatment Wetlands (FTWS) Floating treatment wetlands (FTWs) with Water Dropwort (Oenanthe javanica) were established in winter to investigate their potential role in the purification of eutrophicated water, and to identify the effects of different stubble heights of the Water Dropwort on the performance of the FTWs. The results of the experiments demonstrated: The Water Dropwort FTWs were effective in buffering the pH of the experimental water. The Water Dropwort FTWs were efficient in purifying eutrophicated water, with removal rate for total nitrogen (TN), total phosphorus (TP), ammonium nitrogen (NH4+ - N), and nitrate nitrogen (NO3- - N) at 91.3, 58.0, 94.6, and 95.5% in the 15-day experiment, respectively. No significant difference in the purification effect was found among different stubble heights of Water Dropwort FTWs. Significant differences between the zero control and the FTWs were found for the removal of TP in the first 11 days; and for the removal of NH4+ - N in the first 4 days. No significant difference was found between the zero control and the FTWs for NO3- - N in the first 4 days, but significant difference was detected after day 4. The optimum treatment duration time for the FTWs with Water Dropwort will depend on the nutrients to be removed. These results will provide basis for further application of the FTWs at large scale, as well as for future studies on the mechanism of nutrient removal process. Wpływ czasu i wysokości ścierniska na proces oczyszczania ścieków z wykorzystaniem ruchomych mokradeł Badano proces oczyszczania wód z wykorzystaniem ruchomych mokradeł (FTW) obsadzonych Oenanthe javanica. Celem prowadzonych w zimie badań była ocena ich potencjalnej roli w oczyszczaniu wód zeutrofizowanych oraz określenie wpływu różnych wysokości ściernisk roślinnych na wydajność procesu. Wyniki eksperymentów wykazały, że systemy FTW skutecznie buforowały pH badanej wody. Za pomocą FTW ze zeutrofizowanej wody usunięto azot ogólny (TN), fosfor ogólny (TP), azot amonowy (NH4+ - N) i azot azotanowy (NH3- - N) odpowiednio w ilościach: 91,3, 58,0, 94,6 i 95,5%, w czasie trwania 15-dniowego eksperymentu. Nie wykazano istotnych różnic w efekcie oczyszczania przy stosowaniu różnych wysokości ściernisk roślinnych. Stwierdzono wpływ czasu prowadzenia eksperymentu na usuwanie TP, którego usunięto najwięcej w pierwszych 11 dniach, a NH4+ - N w ciągu pierwszych 4 dni trwania procesu. Nie stwierdzono istotnej różnicy między kontrolą i FTW dla NH3- - N w ciągu pierwszych kilku dni, ale znacząca różnica pojawiła się po 4 dniu. Optymalny czas trwania procesu z wykorzystaniem FTW zależy od składników odżywczych, które mają być usunięte. Opisane wyniki stanowią podstawę zarówno do rozwinięcia zastosowania FTW na dużą skalę, jak i dla przyszłych badań nad mechanizmem procesu usuwania składników odżywczych.

Accumulation and Tolerance of Mangroves to Heavy Metals: a Review

Purpose of reviewMangroves are under increasing heavy metal (HM) pollution pressure from human activities because of the rapid industrialization and urbanization in coastal areas. Field and laboratory experiments showed that the tolerance of mangrove plants to HM stress is normally a mixture of metal avoidance and scavenging of reactive oxygen species (ROS). In this review, related studies during the past few decades on the accumulation and tolerance of mangrove to HMs have been synthesized.Recent findingsIn mangroves, metal accumulation mainly occurred at the root level with restricted transport to the aerial portions of the plant. The common founded HMs, such as copper, zinc, cadmium, chromium, and mercury, generally showed high bioconcentration factor in roots, while the concentration factors for these metals in leaves were usually much lower than one. The limited translocation of the toxic metals to the aerial parts renders the mangrove plants a high endurance ability to high levels of HM stress. To protect the cellular components from oxidative damage by HMs, mangroves have developed both enzymatic and nonenzymatic antioxidant mechanisms to scavenge the ROS. In some circumstances, the changes of antioxidative enzyme activity were usually in accordance with the changes of toxic metal concentrations in plant tissues. However, the responses of antioxidative enzymes in mangroves to HM stress varied with plant species, metal type, and concentration, as well as the duration of the treatment time.SummaryMore toxicity tests are needed with early life stages of mangroves to determine threshold effect concentrations under more realistic conditions.

Combined effect of water inundation and heavy metals on the photosynthesis and physiology of Spartina alterniflora

The frequency and duration of tidal flooding significantly influence the bioavailability of heavy metals (HMs) in sediment and hence exert toxicological effects on coastal wetland plants. In this study, the combined effects of different water inundation times (3, 6, 9, and 12 h) and HMs (Cu, Zn, Pb, and Cr) on the photosynthesis and physiology of Spartina alterniflora were investigated under greenhouse conditions. Results showed that S. alterniflora was somehow tolerant to the combined HMs treatments, and only the highest level of HM treatment decreased leaf chlorophyll content. Furthermore, the plants did not show any signs of victimization. Different times of water inundation with HMs did not exert any significant effect on the malonaldehyde (MDA) and chlorophyll contents in the leaves of S. alterniflora at day 20. Prolonged water inundation time at day 60 significantly reduced leaf chlorophyll content with the decrease in leaf photosynthetic rate, which was accompanied by a significant increase in the intercellular concentration of CO2. At day 60, abscisic acid dose-dependently increased along the different water inundation times, indicating that this phytohormone is involved in plant responses to flooding stress. Peroxidase (POD), superoxide dismutase (SOD), and ascorbate peroxidase (APX), showed different responses to the combined treatment of water inundation and HMs at different times. At day 20, the long duration of water inundation and HMs treatments (9 h+HMs and/or 12 h+HMs) significantly increased enzyme activity in the leaves compared with the control group (6 h). At day 60, the POD and SOD activities in the leaves of S. alterniflora decreased with prolonged water inundation time, and root APX activity significantly decreased compared with the 6 h water inundation treatment.