Evaluation of six cold-season turfgrasses responses to lead phytotoxicity for screening tolerant species

Document Type: Research Paper

Authors

1 University of Guilan

2 University of Tehran

3 Soil and Water Research Institute Karaj

Abstract

Lead (Pb) is one of the most hazardous heavy metal that caused serious problems for ecosystem at recent years. In this study, the effect of Pb on seed germination, incipient growth and antioxidant enzymes of six common used turfgrasses was investigated. The results showed that after implementation of different concentrations (0, 1000, 2000 and 3000 µM) of Pb, Agropyron elongatum exhibited the highest germination percentage (90%) under 3000 µM Pb, followed by Lollium perenne L. (85.56%). The lowest germination parameters including germination vigor, rate, index and germination percentage was found in Festuca ovina, which means it is the most sensitive species to Pb than the other studied turfgrasses. In contrast, F. rubra, F. ovina, A. elongatum and L. perenne had the most significant aerial growth and the lowest chlorophyll degradation even at high lead concentration. An increase in L. perenne and A. elongatum antioxidant enzymes activities as well as a decrease in F. ovina observed after exposure to 3000 µM Pb which correlated to their tolerance and sensitivity to Pb respectively. However, such increase in antioxidant activity was also found in F. Rubra and Poa pratensis but could not inhibit chlorophyll degradation and seedling growth retardation. It might support that there is suggesting the probable existence of a lead-injury mechanism rather than the oxidative stress. Overall, A. elongatum is introduced as the most tolerant turfgrass among studied ones, which could be cultivated in Pb-polluted sites.

Keywords


[Research]

Evaluation of six cold-season turfgrasses responses to lead phytotoxicity for screening tolerant species

 

K. Gholamipour Fard1, M. Ghasemnezhad1*, H. Zakizadeh1, M. Kafi2, F. Rejali3

 

1- Department of Horticultural Sciences, Faculty of Agricultural Sciences, University of Guilan, Rasht, Iran

2- Department of Horticultural Sciences, Faculty ofAgriculture and Natural Resources, University of Tehran, Karaj, Iran

3- Department of Soil Science, Soil and Water Research Institute, Karaj, Iran

* Corresponding author’s E-mail: Ghasemnezhad@guilan.ac.ir

(Received: Oct. 05. 2015 Accepted: March. 17. 2016)

ABSTRACT

Lead (Pb) is one of the most hazardous heavy metal that caused serious problems for ecosystem at recent years. In this study, the effect of Pb on seed germination, incipient growth and antioxidant enzymes of six common used turfgrasses was investigated. The results showed that after implementation of different concentrations (0, 1000, 2000 and 3000 µM) of Pb, Agropyron elongatum exhibited the highest germination percentage (90%) under 3000 µM Pb, followed by Lollium perenne L. (85.56%). The lowest germination parameters including germination vigor, rate, index and germination percentage was found in Festuca ovina, which means it is the most sensitive species to Pb than the other studied turfgrasses. In contrast, F. rubra, F. ovina, A. elongatum and L. perenne had the most significant aerial growth and the lowest chlorophyll degradation even at high lead concentration. An increase in L. perenne and A. elongatum antioxidant enzymes activities as well as a decrease in F. ovina observed after exposure to 3000 µM Pb which correlated to their tolerance and sensitivity to Pb respectively. However, such increase in antioxidant activity was also found in F. Rubra and Poa pratensis but could not inhibit chlorophyll degradation and seedling growth retardation. It might support that there is suggesting the probable existence of a lead-injury mechanism rather than the oxidative stress. Overall, A. elongatum is introduced as the most tolerant turfgrass among studied ones, which could be cultivated in Pb-polluted sites.

Key words:Agropyron, Antioxidant enzymes, Festuca, Lead pollution, Lollium, Poa


INTRODUCTION

Heavy metal-contaminated soil and water are serious worldwide problems at recent years that progressively increase. Lead (Pb) is the most common heavy metal contaminant which has been caused a great concern because of its wide spread occurrence, toxic nature, and long life in biological system. It originates from both natural sources and anthropogenic activities and causes noticeably annual reduction of crops productivity (Duruibe et al. 2007). Lead toxicity results in several damages from interception of germination to retardation of plant growth in herbages (Lamhamdi et al. 2011) and tree species  (Lin et al. 2005). Perturbations in cellular metabolism (Pandey & Sharma 2002), alteration in membrane permeability (Lukatkin et al. 2013), decrease in electron transport activity (Heyno et al. 2008), enzymes activities enhancement or inhibition(Shi & Zhu 2008), and particularly the elevated production of reactive oxygen species (ROS) (Steffens 2014) have been reported as consequences of plant exposure to excessive heavy metals. Moreover, seed germination, the initial event in the plant life, has been considerably influenced by the heavy metal stresses (Moosavi et al. 2012). Turf, a plant with high irrigation requirement, is commonly used in landscapes at widespread scales throughout the world (Romero & Dukes 2014). Several factors such as water deficiency, water pollution, and heavy metal-contaminated soils have strongly restricted the plant germination and even subsequent growth and development.  Bidar et al. (2007) Reported that L. perenne was sensitive to heavy metal toxicity than Trifolium repens. The tolerance of Poa annua to high concentrations of Cu in soil was shown by Brun et al. (2003). Vamerali et al. (2010) Considered Festuca spp. and Lollium spp. for phytoremediation of heavy metals-polluted soils. Moreover, Gąbka & Wolski (2011) showed that F. arundinacea Schreb. cv. Spirit increased its biomass yield under elevated Pb levels. However, they did not biochemically examine on them, nor introduced the most tolerant cultivar to the heavy metals.

The cessation of seed germination and the growth retardation have been illustrated as a response to elevated concentrations of  lead in L. perenne (Bidar et al. 2007), Elsholtzia argyi (Islam et al. 2007), Leucaena leucocephala (Shafiq et al. 2008), Triticum aestivum L. (Lamhamdi et al. 2013), Cicer arietenum (Goswami et al. 2013) and Sesamum indicum (Rao & Lakshmi 2014). However, the studies about the effects of lead on the seed germination of turfgrasses species are little.

Moreover, it has not been well understood to reveal the antioxidant enzymes profiles as determining factors in response of cold-season turfgrasses to various concentrations of lead. Bidar et al. (2007) evaluated only the superoxide dismutase (SOD) activity in L. perenne as affected by Cd, Zn, and Pb. Unfortunately, it has not been reported, to date, the analogy of tolerance/sensitivity of six commonly used cold-season turfgrasses, perennial ryegrass, tall fescue, blue sheep fescue, red fescue, kentucky bluegrass and agropyron, to lead-induced stress. Therefore, the present study was accomplished to identify the most sustainable vegetation cover under toxic levels of lead for the employing of not only heavy metal-contaminated soils, but also wastewater in cultivation and irrigation purposes of the grasses. So, the germination and incipient growth parameters, as well as the biochemical analyses of the six types of turfgrass were evaluated and monitored under lead-induced stress.

The results of this study will be very helpful in phytomanagement of metal-polluted soils, and provide a tendency to the employing of lead - contained wastewater for irrigation of the grass in low water regions.

 

MATERIALS AND METHODS

Plant material

In the present study, the seeds of six turfgrasses; perennial ryegrass (Lollium perenne), tall fescue (Festuca aurandinaceae), blue sheep fescue (Festuca ovina), red fescue (Festuca rubra), kentucky bluegrass (Poa pratensis) and agropyron (Agropyron elongatum) were purchased from Pakan Bazr Company, Esfahan, Iran.

 

Germination experiments

The firs seeds were surface-sterilized with 5% (v/v) sodium hypochlorite for 10 minutes to prevent any fungal contamination, thereafter seeds were thoroughly rinsed with deionized water to ensure that no residual Na adhered to the seed surface and allowed to imbibe for 3 h. After water imbibition, the seeds were placed into Petri dishes (90 mm diameter) containing sterile filter sheets (Whatman No. 42) moistened with either 6 ml of distilled water as a control or Pb solutions (1000, 2000, 3000 µM). The lead solutions were freshly prepared by dissolving Pb (NO3)2 in deionized water and adjusting their pH to 5.5 with HNO3. There were four replicates per each test concentrations and control. Petri dishes were kept in an incubator at 25 ± 2.5˚C and 70% RH under a light irradiance of 300 mmol m-2 s-1 (12:12 h light: dark conditions), and the various parameters of germination and growth indices were recorded.

 

Germination and seedling growth indices

The number of seeds germinated was determined in each treatment every 24 h intervals for 14 days using radicle protrusion (i.e., appearance of a radicle ≥2 mm in length) as a criterion (Gill et al. 2003), and the seed germination percent (GP) was calculated as follows: (number of seeds germinated/total number of seeds) × 100. Day 14th of experiment was considered as the final day, because no more germination was observed and root and shoot length was measurable for all dishes. Germination rate (GR) was determined  daily until the seedling stage(Ellis & Roberts 1980). Germination vigor (GV) and germination index (GI) were measured using following formulae: GV = seedling length (cm) × GP, and GI = (GP/DT), where GP is the germination percentage on the T day, and DT is the day of germination. The length of shoots and roots were also recorded after two weeks from culturing.

 

Biochemical analysis

Chlorophyll content

Weighed 0.5 g of leaf samples from controls and the different Pb treatments were homogenized in 20 ml 80% acetone. The absorbance of the supernatant measured at 663 and 645 nm using a Spectrophotometer (Ltd T80 + UV/VIS; PG Instruments, Leicestershire, UK). It was finally calculated according to Wellburn & Lichtenthaler (1984) and expressed as mg.g-1 fresh weight (FW).

 

Preparations and assays of antioxidant enzymes

The leaves of the turfgrasses (200 mg) were homogenized in 1 ml ice-cold extraction buffer containing 50 mM phosphate buffer (pH 7), 1 mM EDTA and 1.5% (w/v) of PVP. The homogenate was centrifuged at 9000 g for 15 min. The supernatant was used as the crude extract for determination of enzyme activities (Zhang & Nan 2007).

The SOD activity was measured spectrophotometrically as described by Beyer & Fridovich (1987). The reaction solution (1 ml) contained 50 mM phosphate buffer (pH  7), 12 mM riboflavin, 13 mM methionine, 0.1 mM EDTA , 7 mM nitro blue tetrazolium (NBT) and 10 µL of extracted enzyme solution. A solution with no enzyme was used as the control. Test tubes were irradiated under fluorescent lights at 100 mM m-2 s-1 for 20 min.

The absorbance of each solution was measured at 560 nm using a spectrophotometer. One unit of enzyme activity was defined as the amount of enzyme that would inhibit 50% of NBT photo reduction.

The catalase (CAT) activity was assayed as reported by (Brouwer & Brouwer 1998). The reaction solution (0.5 ml) contained 25 mM phosphate buffer (pH = 7), 10 mM H2O2 and 10 µL of extracted enzyme solution. The reaction was initiated by adding the enzyme solution. Changes in absorbance at 240 nm were read every 10s for 60s using a spectrophotometer. One unit of CAT activity was defined as the absorbance change of 0.01 units per minute. The peroxidase (POD) activity was determined by the method of Chance & Maehly (1955). POD activity in leaves was assayed by the oxidation of guaiacol in the presence of H2O2. The increase in absorbance was recorded at 470 nm.

The reaction mixture contained 100 µL of crude enzyme extract, 500 µL of 5 mM H2O2, 500 µL of 28 mM guaiacol, and 1900 µL of 50 mM potassium phosphate buffer (pH 7.0). POD activity of the extract was expressed as activity U/g FW min.

 

Statistical analysis

Statistical analysis was performed using SAS software (Version 9.0). The significance of the differences among treatments was tested by One-Way ANOVA at p ˂ 0.01. Values reported here are means of four replicates.

 

RESULTS

Seeds germination characteristics

The effect of Pb concentrations on seed germination characteristics of different turfgrasses are summarized in Table 1. Germination index (GI) of studied turfgrasses was significantly affected by Pb concentrations. GI was significantly decreased when Pb concentration increased in the solution except for A. elongatum. Among the lead concentrations, No significant difference was found for GI in A. elongatum. The lowest GI was observed in F. ovina when exposed to 3000µM Pb (Table 1). Increasing Pb concentration in the solution caused a significant decreasing in germination percentage (GP) in the all turfgrass species. However no significant decreasing was found among Pb concentrations in A. elongatum and L. perenne (Table 1). The GP of the seeds of F. ovina was considerably affected by Pb treatments, so as it reduced from 93.6% in control to 8.9% in plants treated by 3000 µM Pb concentration. A. elongatum and L. perenne had the maximum GP about 90 and 85.56% respectively, whereas F. ovina showed only 8.9% GP. As shown in Table 1, by increasing Pb concentration in the solution, germination vigor (GV) also decreased in all studied turfgrass species, however, no significant difference was found between Pb concentrations in A. elongatum. The maximum and minimum GV were observed in A. elongatum and F. ovina as compared to their controls, with 57% and 97% reduction, respectively. The germination rate (GR) of F. ovina was more sensitive than that of other studied turfgrasses in response to the high Pb concentration in the solution, as shown by the 97% reduction at 3000 µM Pb solution.

The maximum GR was observed in A. elongatum, therefore this species could be less sensitive to Pb (Table 1).

 

Seedling shoot and root growth

Pb significantly affected different turfgrasses seedling shoot and root length (Fig. 1a).

The root growth of all studied turfgrasses was significantly inhibited by Pb solutions. The highest suppressing effect was found in the F. ovina roots with 2 mm in length, while the lowest one was found in those of A. elongatum with 28mm in length (Fig 1a).

So that, it was suppressed in all turfgrasses as 50% as control at 1000 µM, except for F. ovina, which remained nearly unchanged.

This inhibition reached over 90% for the treatment with 3000 µM Pb. Unlike roots, the seedling shoots was less affected by Pb concentrations (Fig 1 b). No significant difference was found between 1000 µM Pb treated plants in comparison with control. But, the F. rubra and F. ovina shoots length significantly reduced after treatment with 2000 µM Pb solution. All turfgrass species (except for L. perenne) showed a significant reduction in their shoot length when exposure to 3000 µM Pb. The highest shoot growth was observed in A. elongatum with 96 mm in length comprising about 85% of control, whereas the minimum was in F. rubra with only 26 mm about 50% of control.

 

Chlorophyll content

The chlorophyll content of all turfgrass species was significantly decreased with increasing Pb concentration in the solution, except for A. elongatum (Table 2). However, no significant difference was found between control and 1000 µM Pb from point view of chlorophyll content. The lowest chlorophyll content was found in F. ovina, when treated with 3000 µM Pb. It was declined as 45.8% as control in F. ovina. Therefore, this species could be the most sensitive turfgrass to the Pb contamination with respect to chlorophyll content.

 

Antioxidant enzymes activities

Pb - induced oxidative stress was further confirmed by the dose-dependent increase in the antioxidant enzyme activities, as shown in Fig. 2A about the SOD activity in all Pb-treated seedlings, but F. ovina, tended to ascend when the lead concentration increased from 1000 to 3000 µM, in comparison with control. The SOD activity in F. ovina was significantly inhibited by the increasing of Pb concentration. The highest SOD activity was observed in L. Perenne with 263 U.g-1 FW. As shown in Fig. 2B, except for F. ovina, the CAT activity continued to increase with the elevation of Pb concentration in all turfgrass species and peaked at the exposure of 3000 µM. So that, the maximum CAT activity was observed in L. perenne and P. pratensis (109 and 101 U.g-1 FW, respectively), whereas the minimum enzyme activity was shown in F. ovina (20.2 U. g-1 FW). Moreover, when Pb concentration reached 3000 µM, CAT activity in F. Rubra and A. elongatum became approximately 5.2 and 3.2-fold higher than in their controls respectively.

Effects of Pb on POD activity are shown in Fig. 2C.

 

 

 

Fig. 1. Percentage decrease in shoot length (A) and root length (B) of turfgrass seedlings at different concentrations of Pb (NO3)2 (1000, 2000 and 3000 µM). Bars indicated mean ± SE (n = 3).

 

Table 1. Germination index, percentage, vigor and rate of six turfgrass species in response to different concentrations of Pb.

 

 

 

 

 

 

 

 

Indices

Pb (NO3)2   concentration (µM)

A. elongatum

F. rubra

F.  aurandiance

F. ovina

L. perenne

P.pratensis

 

0

35.1aA

24.76cA

22.7cA

27.26bA

32.86abA

28.06cA

Germination

1000

36.4aA

23.1cA

22.8cA

25.9cA

30.8bA

22.03cB

Index

2000

33.1aA

21.9bA

20.33bA

6.3cB

29.46aA

15.26bC

 

3000

34.4aA

17.7cB

16.16cB

2.7eB

25.5bB

11.03dC

 

0

88.33aA

84.11aA

87.13aA

93.36aA

91.1aA

88.23aA

Germination

1000

88.86aA

81.86aA

69.46bB

89.96aA

89.9aA

64.43bB

percentage

2000

86.46aA

59.03cB

65.56bcBC

38.86dB

85.33aB

58.9cC

 

3000

88aA

49.13bC

60bC

8.9cC

83.56aB

47.8bD

 

0

26111aA

10176cA

13084bA

10753cA

14069bA

10473cA

Germination

1000

17823aB

7162.2cB

7088.86cB

7843.3cB

10227.2bB

6771.1cB

vigor

2000

14421aC

6766.6cBC

7102.2cB

1410eC

8858.6bBC

3420dC

 

3000

12262.8aC

4344.4cC

3935.5cC

360dC

6090.9bC

2957.7cC

 

0

9.2aA

4.93cA

4.46cA

5.26bcA

6.86bA

4.06cA

Germination

1000

11aA

4.56cA

4.2cA

5cA

6.76bA

4.63cA

rate

2000

8.7aA

3.6cAB

3.83cAB

1.46dB

6.1bAB

3.3cB

 

3000

10aA

2.6cB

2.5cB

0.66dB

5.03bB

2.6cB

                 

Means in each row and column followed by different letters were significantly different (P< 0.01 level). Capital and small letters show statistical differences for data in columns and rows, respectively. 


 

 

 

Fig. 2. The activities of SOD (A), CAT (B) and POD (C) of six turfgrass seedlings treated with 0, 1000, 2000 and 3000 µM Pb (NO3)2. Data are expressed as mean ± SE (n = 4).

 

Table 2. Shoot chlorophyll content of six turfgrass species (mg.g-1 fresh weight) when exposed to different Pb concentrations.

 

 

 

 

 

 

 

Pb (NO3)2  concentration (µM)

A. elongatum

F.  rubra

F. aurandiance

F. ovina

L. perenne

P. pratensis

0

4.25a

4.38a

5.49a

3.72a

3.91a

3.35a

1000

4.21a

4.34a

5.67a

3.69a

3.92a

3.31a

2000

4.12a

3.75b

4.97b

2.67b

3.66ab

2.73b

3000

4.01a

3.19c

4.28c

2.02c

3.39b

2.16c

               

Values within the same column with different letters are significantly different at P<0.01.

 

 

As found in the results, POD activity increased significantly in the most of turfgrass species when exposure to the higher Pb concentrations. For P. pratensis and F. rubra seedlings, POD activity significantly increased in 3000 µM (276.65 and 214.27% of controls, respectively) in comparison with that in the other Pb concentrations. But, by increasing the Pb concentration from 2000 to 3000 µM, POD activity was significantly declined in F. ovina and F. aurandinaceae.

 

DISCUSSION

The plant seed germination and seedling growth can be regarded as general responses associated with heavy metal toxicity (Kopyra & Gwóźdź 2003; Radić et al. 2010). In the present study, seed germination-related parameters of all tested turfgrasses, except for A. elongatum and L. perenne were significantly affected by Pb-induced stress. Sengar et al. (2009) have reported that inhibition of seed germination may result from the interference of Pb with germination-involved enzymes. In contrast, in A. elongatum and L. perenne, both seed GP and GV as well as the incipient growth of roots and shoots were not considerably affected by Pb toxicity. The F. ovina seed germination was notably inhibited more than that of other turfgrasses at higher Pb concentrations; its root growth was not as reduced as that of P. pratensis, F. aurandinaceae and F. rubra. Gąbka & Wolski (2011) Have showed that F. ovina “Noni” was able to accumulate heavy metals in their tissues, but its general aspect (aesthetic value) became worse than that of non-heavy metal - accumulated plants.

 According to finding of the present study, the terrestrial parts of all Pb-treated turfgrasses were more strictly affected by Pb than the aerial ones. It has been well established that the roots growth is more sensitive to metal toxicitythan seed germination, because they are the primary targets of metal ions. Hence, root testing is widely used for evaluating the toxicity levels of heavy metals (Márquez-García et al. 2013). This is confirmed by the results of the present study, so the roots of all turfgrasses were affected particularly under the highest Pb concentration.

The inhibition of root growth by Pb has also been shown to affect nitrogen assimilation through the enzymes nitrate reductase and glutamine synthetase in Cucumis sativus and Glycine max, respectively (Sharma & Dubey 2005).

A. elongatum and L. perenne had most appropriate growth even under high lead concentrations which may be resulted from less chlorophyll degradation. In contrast, the other turfgrass such as F. ovina and F. rubra showed enhanced chlorophyll decomposition at elevated level of Pb. So, their shoots were eventually dried. Reduction in chlorophyll contents by excess Pb has been reported in Oryza sativa L. (Lou et al. 2007) and Zea mays L. (Ekmekçi et al. 2009). It has been revealed that Pb can displace metal cofactors from metalloenzymes, which includes Mg in the chlorophyll porphyrin ring,  resulting in  interference with photosynthesis by reducing chlorophyll levels (Patra et al. 2004). Sharma & Dubey (2005) have also found that the key chlorophyll biosynthesis enzymes were also strongly inhibited by Pb, as well as many enzymes in the Calvin cycle followed by  reducing the rate and efficiency of CO2 fixation.

It has been established that Pb toxicity results in enhanced ROS generation (Verma & Dubey 2003). ROS are dangerous primarily due to reaction with lipids, proteins, and nucleic acids (Halliwell & Gutteridge 2011). In the present study, the POD, CAT and SOD activities in A. elongatum were less than those in F. rubra, which had the lowest growth. It could indicate that the antioxidant enzymes activity manner was not in accordance with F. rubra responses to exogenous Pb toxicity. Contrary to F.rubra and F. ovina, A. elongatum and L. perenne had either the most aerial growth or the less chlorophyll degradation even at high Pb concentration. An increase in the L. perenne and A. elongatum antioxidant enzymes as well as a decrease in those of F. ovina observed after exposure to 3000 µM Pb, were in accordance with their tolerance and sensitive, respectively, indicating a strong relationship between Pb tolerance and antioxidant systems in the grasses. Inversely, such increase in F. rubra and P. protensis could not prevent from chlorophyll degradation and growth reduction. It may be due to the probable existence of a Pb-injury mechanism rather than the oxidative stress. A. elongatum and L. perenne tolerance mechanism to high Pb concentrations could be widely attributed to high efficient of antioxidant enzymes, particularly CAT and SOD. According to the finding of Malecka et al. (2001), although Pb is not a redox metal and cannot generate ROS directly, it cause oxidative stress indirectly as shown by the increased lipid peroxidation in rice and pea plants exposed to the metal. According to our finding, SOD and CAT activities are more strongly related to the Pb tolerance capabilities than to the POD activity. The increased antioxidant activities particularly in L. perenne, A. elongatum after exposure to the high concentration of Pb may indicate that the turfgrasses cells presented endogenous protective effects and the antioxidant enzymes were induced to prevent against Pb-induced ROS. However, our results showed that the efficiency of antioxidant enzymes activities is species-dependent in counteracting the adverse effects of Pb phytotoxicity. It was not in parallel to that of wheat seedlings reported by Yang et al. (2010). They have reported that appropriate growth of wheat seedlings exposed to lead toxicity was only due to the ROS scavenging by highly activated antioxidant enzymes.

According to its tolerance under Pb stress condition, A. elongatum seems to have a strong defense system during both seed germination and subsequent seedling growth. It may be due to the perturbation in Pb ion entrance into the tissues resulted from more thickness of the turfgrass seed coat in comparison with that of any other studied turfgrasses. Kranner & Colville (2011) have found that the  influences  of metals  on seed  germination in different  plants  depend mainly on interspecies differences  in  seed  structure,  particularly the seed  coat anatomy. Jian-Ling & Li-Qiang (2014) have also reported that seed coat plays a crucial role in protecting seed  to access the unique ability  for obstruction  and retention of Pb. In addition, it has been reviewed that Pb inhibited germination widely in species with Pb-permeable seed coats (Kranner & Colville 2011). It is in consistent with our seed germination results which indicated that the seeds of A. elongatum, apart from thick coat, may have a substantially higher threshold for toxicity than other turfgrasses species. Overall, A. elongatum and L. perenne not only had much seed germination under high Pb concentrations, but also showed better growth during subsequent stages. It may be hypothesized that there was a relationship between seed germination sensitivity/ tolerance and that of subsequent plant growth to lead stress.

 

CONCLUSION

In conclusion, Pb treatment reduces seed germination parameters and seedling growth, especially root growth of the six tested turfgrasses. The results also showed that those species having much seed germination under high Pb concentrations are more tolerant during subsequent growth. Root growth was more sensitive to Pb than seedlings shoot. An increase in L. perenne and A. elongatum antioxidant enzymes after exposure to higher Pb concentrations followed by both less chlorophyll degradation and less growth reduction could indicate that Pb tolerance was associated with antioxidant enzymes in the grasses. Vice versa, such increase in F. rubra and P. protensis could not prevent chlorophyll degradation and growth reduction. In our opinion, either Pb toxicity threshold or its mechanism was species-dependent. Among the six turfgrasses, A. elongatum was the most tolerant, while F. ovina was the most sensitive to Pb as indicated by their seed germination, growth, chlorophyll content and antioxidant enzymes activities. Therefore, A. elongatum could be considered as a promising species in Pb - contaminated sites for both aesthetic and phytoremediation purposes.

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