HEAVY METALS AND SEED GERMINATION IN SOME MEDICINAL AND AROMATIC PLANTS

Ekaterina A. Jeliazkova*, Lyle E. Craker (Department of Plant and Soil Sciences, University of Massachusetts, Amherst, MA, 01003, USA Tel: (902) 895-6317, e-mail: Jeliazkov@auracom.com)

 

Experiments were conducted to test the effect of heavy metals (Cd, Cu, Pb, Zn) on seed germination and root growth in Alyssum sp., Cuminum cyminum L., and Salvia officinalis L.  Randomly selected samples of fifty seeds in three replicates from each test species were used.  Test solutions were prepared from each metal at two concentrations (the critical concentration of the metal in the soil and approximately two times the critical concentration).  Distilled water was used as a control treatment.  The objective of the research was to screen selected species for heavy metal tolerance.  Root growth was more readily affected by the heavy metals than was seed germination.  The lower concentrations of Cd (6 mg/l) and Pb (100 mg/l) stimulated root growth of Cuminum cyminum L. by over 30 percent as compared with the control.  Tested concentrations of Pb (100 and 500 mg/l) and zinc (400 and 800 mg/l) stimulated seed germination in Salvia officinalis L. by more than 50 percent as compared with the control.  Cuminum cyminum L. showed tolerance to cadmium and lead.

 

Introduction

Heavy metals are naturally present in the environment.  Their occurrence, however, has gradually been increasing with the increase of industrialization.  Agricultural soils, as an essential part of the environment, are no exception of this phenomenon.  Cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn) are among the most abundant heavy metals in the agricultural soils (Förstner, 1995).  Copper and Zn, when present in low concentrations, are important micronutrients, while in high concentrations, these two metals become toxic to plants.  Although Cd and Pb have no known role as nutrients, plants readily accumulate them in their system.  The ability of plants to accumulate heavy metals is used in the process of phytoremediation where the green plants are employed to cleanse contaminated soils. 

Medicinal and aromatic plants appear to be a good choice for phytoremediation since these species are mainly grown for secondary products (essential oil) thus the contamination of the food chain with heavy metals is eliminated.  Aromatic and medicinal plants also have a demonstrated ability to accumulate heavy metals (Schneider and Marquard, 1996; Scora and Chang, 1997; Zheljazkov and Nielsen, 1996).  Research has shown that heavy metals accumulated by aromatic and medicinal plants do not appear in the essential oil (Scora and Chang, 1997; Zheljazkov and Nielsen, 1996) and that some of these species are able to grow in metal contaminated sites without significant yield reduction. 

In this study alyssum (Alyssum sp.) family Brassicaceae, anise (Pimpinela anisum L.) family Apiaceae, and sage (Salvia officinalis L.) family Lamiaceae, were tested for heavy metal tolerance.  Alyssum has closely related species (A. lesbiacum L., A. murale L., A. bertolonii, A. pintosadilvae L.) known as nickel hyperaccumulators (Kabata-Pendias and Pendias, 1992).  Anise has shown ability to accumulate heavy metals when present in the medium (Shalaby et al., 1996) and is an aromatic plant grown for essential oil.  Sage (Salvia officinalis L.) belongs to the same genera as clary sage (Salvia sclarea L.), was able to grow on heavy metal contaminated soils (Zheljazkov and Nielsen, 1996) and the obtained essential oil had no heavy metals. 

Although most aromatic and medicinal plants are directly sown in the field, the impact of the heavy metals on seed germination or the metal tolerance of seedlings during the early stages of development is not known.  The objectives of this research were to: test the ability of seeds from the selected aromatic and medicinal plants to germinate in a heavy metal contaminated environment; screen the selected aromatic and medicinal plants for heavy metal tolerance.

 

Methodology 

To determine the effect of heavy metals on seed germination, randomly selected samples of fifty seeds in three replications from each test species were used.  The selected seeds were placed on two sheets of filter paper (Whatman No. 1) contained in Petri plates (9 cm diameter).  Solutions of heavy metals (15 mL) were subsequently added to the seed-containing Petri plates to wet the filter paper and contaminate the samples (Table 1). 

The heavy metal test solutions were made with Cd, Pb, Cu, and Zn, using CuCl2.2H2O, PbCl2, CdCl2.2 ˝ H2O, and ZnCl2, respectively, and were prepared at two concentrations, the critical concentration of the metal in the soil and approximately two times the critical concentration (Alloway, 1990; Beckett and Davis, 1977).  Distilled water was added to the Petri plates in place of the heavy metal solution as a control.  After the test solutions were added, the Petri plates with seeds were randomly positioned in a controlled environmental chamber on a 24 h temperature cycle 24±1oC and 18±1oC (for 12 h each) for germination.  Seed germination was measured every 24 h.  Seeds were considered to have germinated at radical emergence of 1 mm.  Root length of germinated seedlings was measured at the end of the experimental trial.

Table 1.  The heavy metal solutions used as treatments in testing the potential of some medicinal and aromatic plants for phytoremediation.

 

Elements                                Concentrations (mg/l)                                         ph

 

Cd                                                           6                                                              5.53

Cd                                                           10                                                            5.62

Cu                                                           60                                                            6.10

Cu                                                           150                                                          5.80

Pb                                                           100                                                          6.85

Pb                                                           500                                                          5.70

Zn                                                           400                                                          7.30

Zn                                                           800                                                          6.50

Dist. H2O                                               0                                                              5.79

 

1 Lower value = the critical concentration of the metal in the soil (Alloway, 1990; Beckett and

    Davis, 1979).  Upper value = the approximate concentration at which plants have been

  observed to grow.

 

Statistical analysis of data was done using two-way analysis of variance and the general linear models procedure in SAS software program (SAS, Institute, 1989-96).  Means were separated using Duncan’s multiple range test procedure and were also compared against the control using Dunnett’s procedure.  Significant differences were defined at the 0.05 level. 

 

Results and discussion

In an environment with Cd seed germination among the selected species was affected only in cumin (88.6 %) with Cd 10 mg/L (Table 2).  Both tested Cd concentrations (6 and 10 mg/L) had a similar effect on germination in all tested species.  Root growth was apparently promoted by low levels of Cd in cumin, as root growth was 30 percent higher in seedlings treated with Cd 6 mg/L as compared with control.  Under all other tested conditions, root growth was significantly reduced by Cd.  In alyssum root growth was reduced by over 50 percent. 

Copper concentration of 60 mg/L had no effect on seed germination in cumin and sage (Table 3) while Cu concentration of 150 mg/L greatly reduced seed germination in these two species, 11.9 and 46.8 percent of control, respectively.  Root growth in all species was significantly reduced by both of the tested Cu concentrations. 

Seed germination was greatly promoted in sage by both Pb levels (100 and 500 mg/L), as seed germination was 70 percent higher as compared with controls (Table 4).  In alyssum, seed germination was not affected by Pb.  Root growth among the tested species was highest in cumin (130.6 %) with Pb 100 mg/L.  In all other tested species, root growth was significantly reduced by both tested Pb levels.  The most sensitive species to Pb regarding root growth was sage. 

Zinc at both tested concentrations (400 and 800 mg/L) significantly promoted seed germination in sage (Table 5) and reduced seed germination in cumin, as compared with controls.  Root growth in all species was reduced by both of the tested Zn concentrations.  No differences in species sensitivity to Zn regarding root growth were observed. 

Stimulation of seed germination by low levels of Cd and Pb has been observed in other studies (Scherbatskoy et al., 1987) in forest tree species, but the phenomenon was not explained.  In the present study, both tested levels of Pb and Zn significantly promoted seed germination in sage (Table 4 and Table 5) and we did not know why.  Results from the conducted research indicated that root growth was more readily affected by the tested heavy metals than seed germination was.  Roots are the primary plant organs that sense, become in contact with, and accumulate heavy metal(s) from the substrate.  Root growth has been proven to be an indicator of metal tolerance in plants (Wilkins, 1978).  Considering seed germination and root growth together, results from the present study suggested that cumin could be used for phytoremediation of soil contaminated with Cd or Pb if concentration of the metal in the soil solution does not exceed 6 mg/L and 100 mg/L, respectively. 

 

References:

Alloway BJ (1990), Heavy metals in soils. Glasgow, B. J. Alloway.

Beckett PHT, Davis RD (1977), New Phytol. 79: 95-106.

Förstner U (1995), In: Metal speciation and contamination of soil. (AE Herbert, CP Huang, GW Bailey, AR Bowers), London, Lewis, pp.1-33.

Kabata-Pendias A, Pendias H (1992), Trace elements in soils and plants. London, CRC press.

Shalaby MH, Omran MS, Raslan MI (1996), Egyptian J. Soil Sci. 36:1-4: 133-43.

Scherbatskoy T, Klein RM, Badger GJ (1987), Environ. Exp. Bot. 27:2: 157-64.

Schneider M, Marquard DrR (1996), Acta Hort. 426: 435-41.

Scora RW, Chang AC (1997), J. Environ. Qual. 26:975-79.

Wilkins DA (1978), New Phytol. 80: 623-33.

Zheljazkov VD, Nielsen N (1996), Acta Hort. 426: 309-28.

 

Table 2.  Effect of cadmium on seed germination and root growth of some medicinal and aromatic plants

________________________________________________________________________

                                            Cadmium

Species                         concentration1                               Germination                        Root growth

                                               (mg/l)                            (% control)2                         (% control)3

________________________________________________________________________

Alyssum                                   6                                     100.0  a                                  62.2*c

                                                  10                                    100.0  a                                 44.5*d

 

Cumin                                         6                                      89.6  b                               132.6*a

                                                  10                                      88.6*b                                77.4*b

 

Sage                                           6                                    102.1  a                                 63.7*c

                                                  10                                    101.4  a                                 52.5*cd

________________________________________________________________________

Significance                                                                  significant                           significant

________________________________________________________________________

 

Table 3.  Effect of copper on seed germination and root growth of some medicinal and aromatic plants

_______________________________________________________________________

                                              Copper

Species                         concentration1                               Germination                        Root growth

                                               (mg/l)                            (% control)2                         (% control)3

_______________________________________________________________________

Alyssum                                  60                                84.7*b                                      12.0*ab

                                                150                                59.2*c                                         6.8*b

 

Cumin                                       60                                92.5  b                                       20.5*a

                                                150                                11.9*e                                         8.6*b

 

Sage                                         60                              106.4  a                                         3.7*b

                                                150                                46.8*d                                        1.8*b

_______________________________________________________________________

Significance                                                              significant                                 significant

_______________________________________________________________________

 

1 Lower value = the critical concentration of the metal in the soil (Alloway, 1990; Beckett and Davis, 1979).  Upper value = the approximate concentration at which plants have been observed to grow.

 

2 Seed germination for control was: alyssum 98 %, cumin 67 %, and sage 47 %. 

 

3 Root growth for control was: alyssum 14.6 mm, cumin 25.2 mm, and sage 56.8 mm.

 

  Means with the same letter were not significantly different at 0.05 level (Duncan’s test).  Means

  followed by * were significantly different from the control at 0.05 level (Dunnett’s test).

 

Table 4.  Effect of lead on seed germination and root growth of some medicinal and aromatic plants

_______________________________________________________________________

                                                Lead

Species                         concentration1                               Germination                        Root growth

                                               (mg/l)                            (% control)2                         (% control)3

_______________________________________________________________________

Alyssum                                100                                  100.0  b                                   61.6*b

                                                500                                    94.9  bc                                 16.6*d

 

Cumin                                     100                                    89.6  c                                 130.6*a

                                                500                                    34.3*d                                  40.3*c

 

Sage                                       100                                  170.2*a                                   47.2*c

                                                500                                  168.2*a                                     7.0*d

_______________________________________________________________________

Significance                                                                significant                             significant

_______________________________________________________________________

 

Table 5.  Effect of zinc on seed germination and root growth of some medicinal and aromatic plants

_______________________________________________________________________

                                                Zinc

Species                         concentration1                               Germination                        Root growth

                                               (mg/l)                            (% control)2                         (% control)3

_______________________________________________________________________

Alyssum                                400                                  94.9  c                                     20.0*

                                                800                                  84.7*d                                    18.3*

 

Cumin                                     400                                  37.3*e                                     18.5*

                                                800                                  29.9*e                                     15.5*

 

Sage                                       400                                148.2*b                                    12.9*

                                                800                                163.1*a                                       6.7*

_______________________________________________________________________

Significance                                                              significant                          not significant

_______________________________________________________________________

 

1 Lower value = the critical concentration of the metal in the soil (Alloway, 1990; Beckett and Davis, 1979).  Upper value = the approximate concentration at which plants have been observed to grow.

 

2 Seed germination for control was: alyssum 98 %, cumin 67 %, and sage 47 %. 

 

3 Root growth for control was: alyssum 14.6 mm, cumin 25.2 mm, and sage 56.8 mm.

 

  Means with the same letter were not significantly different at 0.05 level (Duncan’s test).  Means

  followed by * were significantly different from the control at 0.05 level (Dunnett’s test).