Photosynthetic Metabolism and Activity of Carboxylating Enzymes in Emergent,Floating, and Submersed Leaves of Hydrophytes

Radioisotope techniques were used to compare photosynthetic CO₂ fixation, activities of carboxylating enzymes, and the composition of photosynthates in 42 species of aquatic plants (emergent, floating, and submersed hydrophytes) collected from rivers Sysert' and Iset' in Sverdlovsk oblast (Russia). The submersed leaves, in comparison with the emergent and floating leaves, featured lower rates of potential photosynthesis (by 2.2 mg CO²/(dm² h) on average), low content of the fraction I protein, and low activity of Rubisco and phosphoenolpyruvate carboxylase (PEPC). The averaged activities of Rubisco and PEPC were diminished in submersed leaves by 10 and 1 mg/(dm² h), respectively. Different hydrophyte groups showed similar composition of assimilates accumulated after 5-min photosynthesis and did not differ in this respect from terrestrial plants. However, the incorporation of ¹⁴C into sucrose and starch in submersed leaves (30 and 9% of total labeling, respectively) was lower than in emergent and floating leaves (45 and 15%, respectively). At the same time, the incorporation of ¹⁴C into C₄ acids (malate and aspartate) was 1.5 times higher in submersed leaves than in other leaf types. Analysis of leaf differentiation, the Rubisco/PEPC activity ratio, the PEPC activity, and the composition of primary photosynthates in the pulse–chase experiments revealed no evidence of the C₄ effect in the submersed hydrophytes examined. The adaptation of hydatophytes to specific conditions of an aquatic environment was structurally manifested in the reduction (by a factor of 3–5) in the number of chloroplasts per 1 cm² leaf area. This small number of chloroplasts was responsible for low photosynthetic rates in submersed leaves, although metabolic activities of individual chloroplasts were similar for all three hydrophyte groups.     

Accumulation of heavy metals in leaves of submerged hydrophytes (<Emphasis Type="Italic">Elodea canadensis</Emphasis> Michx. and <Emphasis Type="Italic">Potamogeton perfoliatus</Emphasis> L.) and their responses to the effect of the wastewater of a metallurgical plant

The accumulation of heavy metals (Cu, Ni, Zn, Mn, and Fe), parameters of the photosynthetic apparatus, and flavonoid content in leaves of two widespread species of submerged hydrophytes (Elodea canadensis Michx. and Potamogeton perfoliatus L.) from habitats with different levels of pollutions have been studied. The investigations are carried out in the surroundings of the metallurgical plant in the town of Revda, Sverdlovsk oblast, Russia (Revda River, upstream and downstream the confluence of the sewage). It is shown that hydrophytes from polluted habitats (impact site) differ from the plants of a less polluted zone (background site) by a higher accumulation of heavy metals (HMs), greater leaf thickness, and larger cells and chloroplasts. Plants from the polluted site are characterized by a high content of photosynthetic pigments, while flavonoid content in the leaves is lower. The identified trends indicate a high adaptive potential of these species of plants and their ability to inhabit in an aquatic environment polluted with metals and other pollutants.     

Effect of heavy metals on photosynthetic apparatus and antioxidant status of elodea

Elodea plants (Elodea (Egeria) densa Planch.) were incubated in the presence of individual and mixed 1 μM sulfate salts of Ni, Cd, Cu, Zn, and Mn to study the influence of heavy metals (HM) on shoot growth, structural-and functional parameters of the photosynthetic apparatus, lipid peroxidation, enzymatic activities of the antioxidant defense system (superoxide dismutase and catalase), and the content of non-protein and protein thiols in leaves. The accumulation of HM in leaves decreased in a row: Mn > Cu > Cd > Zn > Ni. The largest reduction in chlorophyll content was caused by Mn and Cu, whereas the strongest reduction in carotenoid content was induced by Cu. The presence of Cu produced the largest decrease in the maximal quantum efficiency of photosystem II (PSII) (F _v/F _m). These changes were paralleled by the shift of the pro-/antioxidant balance towards the dominance of oxidative processes. The presence of Cd elevated the content of chlorophyll and carotenoids without altering the photochemical efficiency of PSII; Cd retarded the shoot growth but had no appreciable effect on leaf mesostructure. The addition of the second metal to the growth medium alleviated in most treatments the detrimental action of individual ions owing to the enhanced activities of SOD and catalase and because of the significant increase in the content of non-protein thiols. It is supposed that the observed antagonism of metal ions is related to their competitive interactions restricting the entry of HM into the cell.     

Ceratophyllum demersum L. and Potamogeton alpinus Balb. from Iset’ River,Ural Region,Russia Differ in Adaptive Strategies to Heavy Metals Exposure – A Comparative Study

We examined the uptake of five heavy metals (Cu, Fe, Ni, Zn, and Mn) in Ceratophyllum demersum L. (hornwort) and Potamogeton alpinus Balb. (pondweed) from Iset’ river, Ural region, Russia. This study was conducted in a territory that is highly urbanized where the surface waters are contaminated by a wide spectrum of pollutants. The environmental situation in this territory drastically deteriorated due to anthropogenic activity. The water quality in most of the water bodies in the Ural region is rather poor. In a comparative study ofC. demersum and P. alpinus, differential accumulation pattern was noted for heavy metals (HMs). Higher amounts of HMs accumulated in C. demersum compared to P. alpinus. Also it was shown that in leaves of C. demersum there were high amount of total phosphorus, nitrogen, organics acids and ash; high activity of guaiacol peroxidase; high content of non-enzymatic antioxidants viz., flavonoids, ascorbate, glutathione and proline; high amount of thiols (soluble and membrane bound) compared to P. alpinus.     

Responses of Lemna trisulca L. (Duckweed) exposed to low doses of cadmium: thiols, metal binding complexes, and photosynthetic pigments as sensitive biomarkers of ecotoxicity

Lemna species are reported to accumulate a variety of metals from contaminated/polluted sites. Cadmium is a nonessential element for plant metabolism. In this work, we aimed to investigate physiological responses to low doses of cadmium (up to 100 μM). From exposure to the lowest Cd concentration (1 µM) to the highest (100 µM), photosynthetic pigments (Chl a, b, carotenoids) and the ratios of Chl a/b, Chl (a + b)/carotenoids decreased as a function of the Cd dose. The content of soluble proteins decreased in a dose-dependent manner, while total soluble thiols drastically increased. In Cd-treated fronds, the dose-dependent accumulation of a polypeptide with an apparent molecular weight of 24 kDa, as well as the appearance of two smaller polypeptides with molecular weights <6.5 kDa, was observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. Our results show that in Lemna trisulca, different adaptative mechanisms may be involved in counterbalancing low and high doses of a particular toxicant (cadmium). This feature makes this plant potentially useful material in biomonitoring and phytotoxicity testing.     

Thiols as biomarkers of heavy metal tolerance in the aquatic macrophytes of Middle Urals,Russia

Aquatic macrophytes, viz. Sagittaria sagittifolia L., Lemna gibba L., Elodea canadensis Michx., Batrachium trichophyllum (Chaix.) Bosch., Ceratophyllum demersum L. and Potamogeton sp. (P. perfoliatus L., P. alpinus Balb., P. crispus L., P. berchtoldii Fieber, P. friesii Rupr., P. pectinatus L.) were collected from 11 sites for determining their metal accumulation and thiols content. Cu²⁺, Ni²⁺, Mn²⁺, Zn²⁺, and Fe³⁺ exceeded maximum permissible concentrations in chosen sites. Significant transfer of metals from water to leaves is observed in the order of Ni²⁺ < Cu²⁺ < Zn²⁺ < Fe³⁺ < Mn²⁺. The maximum variation of bioconcentration factor was noticed for manganese. The accumulation of heavy metals in leaves was correlated with non-protein and protein thiols, confirming their important role in metal tolerance. The largest contribution was provided by Cu²⁺ (on the average r = 0.88, p < 0.05), which obviously can be explained as an important role of these ions in thiols synthesis. Increased synthesis of thiols in the leaves allows the usage of SH-containing compounds as biomarkers of metal tolerance. Considering accumulation of metals and tolerance, B. trichophyllum, C. demersum and L. gibba are the most suitable species for phytoremediation of highly multimetal contamination, while E. canadensis and some species of Potamageton are suitable for moderately metal-polluted sites.