The effect of mycorrhizal symbiosis on metal accumulation and plant tolerance are not commonly studied in medicinal plants under metal stress. The objective of this study was to assess the impact of mycorrhiza on alfalfa plants with the increase of Zn and Cd toxicity. The experiment was conducted under controlled laboratory conditions. Zinc (Zn) and cadmium (Cd) uptake, some biochemical and physiological parameters were studied in eight-week-old alfalfa plants in response to inoculation or not with arbuscular mycorrhizal fungi (AMF) and with the increase of Zn (0, 100, 300, 900 mg·kg -1) and Cd concentrations (0, 100, 300, 600 mg·kg -1) in soil. The results showed that mycorrhizal (M) plants exhibited tolerance to Zn and Cd up to 300 mg·kg -1 in comparison to non-mycorrhizal (NM) plants which exhibited a significant growth reduction at the same soil Zn and Cd level. M inoculation reduced the Zn and Cd accumulation in shoot and showed higher Zn and Cd contents in roots which showed a different Zn and Cd distribution in AMF associated or non-associated plants. Mycorrhizal plants increased phosphorus (P) contents at all Zn and Cd concentrations except the highest (600 and 900 mg·kg -1) leading significant alterations in biochemical contents such as proline, antioxidant enzymes in leaves and also in nutrients (N, P, K, Cu, Ni, Fe, Mn). Zn and cadmium toxicity cause to increase the proline content in shoot of NM plants, however, proline contents are lower in M plants. Results confirmed that AMF protected alfalfa plants against Zn and Cd toxicity. Mycorrhizal colonization was able to form an efficient symbiosis with alfalfa plants in moderately contaminated Zn and Cd soils (300 mg·kg -1) and play an important role in food quality and safety.
Soil pollution by heavy metals due to anthropogenic activities is the most important problem nowadays because metals are noxious, persistent and non-biodegradable. They tend to accumulate readily in soils and organisms, mainly where effects of human activities are severe. Cadmium (Cd), a non-essential element, is considered as more toxic because it tends to accumulate more readily in the environment especially in biological organisms even at low concentrations in the environment, leading to harsh consequences [
Cd is rarely present alone in soil and is mostly linked to other heavy metals such as high levels of zinc (Zn). In polluted soils, Cd and Zn uptake in plants and soils are associated [
In soil microorganisms, arbuscular mycorrhizal (AM) fungi are commonly studied because of their capacity to develop plant strength under toxic and inappropriate conditions [
The non-essential elements such as Cd can also transfer by AM fungi towards plants and store them in roots [
In the study, we used alfalfa as a test plant because it is one of the most popular species used for perennial grazing and is widely cultured on the global scale for medicinal purposes. Medicago sativa (alfalfa) is a flowering plant in the pea family Fabaceae. It is a perennial legume from three to twelve years, depending upon climate and variety [
A pot culture experiment was installed under controlled laboratory conditions. The treatments were either inoculation or non-inoculation of the AM fungi and the addition of zinc (0, 100, 300 and 900 mg/kg) and cadmium concentrations (0, 100, 300, 600 mg/kg) to soil. The sample of soil and sand were collected from the top layer (0 - 20 cm) in the vicinity of Quaid-i-Azam University, Islamabad. The area has subtropical climate, with a mean temperature of 19˚C - 25˚C and an average rainfall of 31 mm. The soil and sand were air-dried and sieved with a 2-mm diameter sieve for analysis. Soil was chemically characterized with a pH (6.7), T. Phosphorus (4.3 mg∙kg−1), T. Potassium (19.5 mg∙kg−1), Calcium (34.45 mg∙kg−1), Magnesium (42.50 mg∙kg−1), Extractable nitrate nitrogen (1.04 mg∙kg−1), Extractable potassium (1.45 mg∙kg−1), Extractable phosphorus (1.53 mg∙kg−1), Zinc (1.50 mg∙kg−1), Nickel (1.33 mg∙kg−1), Copper (30.3 mg∙kg−1), Cadmium (1.60 mg∙kg−1), Iron (28.51 mg∙kg−1), Lead (1.6 mg∙kg−1), Chromium (4.25 mg∙kg−1) and Manganese (10.4 mg∙kg−1) respectively. The soil and sand were autoclaved-sterilized (121˚C, 2 h) in order to eliminate native AM fungal propagules and other microorganisms. The soil was manually mixed with sand in ratio of 1:3 (v/v). The mixture of soil and sand were used as growth medium of plants. ZnCl2 and CdCl2 were added to the growth medium as Zn and Cd stress respectively.
The AMF used was the mixture of different Glomus sp with dry soil substrates obtained from the AMF collection maintained by the company (Agrauxine) in France. Spores and dried sand-soil mixture (growth medium) were used in mycorrhizal inoculated treatments. Each pot (10 cm diameter and 12 cm height) contained 2 kg growth medium plus 50 g of AM fungal inoculum to mycorrhizal treatments, while the same amounts of growth medium were added to non-mycorrhizal treatments. Each pot received approximately 2500 spores at the time of sowing. AMF inoculation was performed during the transplantation process and was not provided in non-mycorrhizal treatments.
Seeds of alfalfa (Medicago sativa L.) were obtained from Department of Crop Science, National Agriculture Research Centre, Islamabad. Seeds were surface sterilized (10 min, 3% Chlorox) and gently washed by deionized water for several times at room temperature and then put them on the sterile moist filter papers (Xin Hua No. 101, China) in Petri dishes at 28˚C for 48 hours for germinating. These were selected for uniformity before sowing. Five pre-germinated seeds were sown per pot and the plants were allowed to grow for 8 weeks. Seedlings were grown in the growth chamber with 12 h light per day at 25˚C - 35˚C. Water lost was replaced daily by top watering with deionized water and to maintain the moisture of the soil at about 60% until the end of the experiment. Each pot was irrigated with long Ashton’s nutrient solution (20 ml) every week. Six pots per treatment were used and seedlings were randomly harvested 60 days after sowing.
Root mycorrhizal colonization was estimated after clearing and staining [
The growth performance including stem diameter, shoot and root height, breadth and area were recorded. Height and diameter were measured by precision straight edge (Sword fish, China) and vernier caliper (ECV150C, China).
At harvest, roots and shoots were separated. Subsamples of fresh roots were taken to assess mycorrhizal colonization. Fresh weights of total roots and sub-samples were measured. Leaves and remaining roots were rinsed with tap water and then with deionized water. Tissues were weighed after oven drying at 60˚C for 72 h and then ground to <0.25 mm in a stainless mill. The percentage of water content in remaining roots and total root fresh weight were used to estimate total root dry weight.
After dry weight determination, the oven dried tissue samples (shoots and roots) were ground and digested in HNO3 (70%) and H2O2 using the microwave digestion system (CEM-MDS 2000). The digest was filtered using Whatman No. 42 filter paper and made up to 50 ml by using deionized water. The metal contents (Na, K, Ca, Mg, Co, Cr, Cu, Fe, Ni, Pb, Mn, Cd, Zn) in plant tissues (shoot and roots) were determined by using atomic absorption spectrophotometer (Varian FAAS-240). Total Phosphorus (P) in plant digest was determined by ammonium-vanadomolybdate method [
Chlorophyll content in the fresh leaves (50 mg) of the plant was measured in 10 cm3 dimethylsulfoxide (DMSO) by using the method [
For enzyme analysis, fresh samples of leaves (300 mg each) were ground in a chilled mortar and extracted with 3 ml of 100 mM potassium phosphate buffer (pH 7.5). The homogenate was centrifuged at 12,000 rpm for 15 min. The supernatant was used for the estimation of antioxidant enzyme activities. Superoxide dismutase (SOD) activity was assessed spectrophotometrically at 560 nm based on the inhibition of the photochemical reduction of nitroblue tetrazolium (NBT) as described by method [
The chemicals used were analytical grade and obtained from Sigma, Aldrich and Merck. All the analyses were performed in triplicates under standard optimizing conditions. Analytical data quality of metals in soil and plant samples was ensured through repeated analysis (n = 6) of roots and shoot samples. The blank reagent and standard reference soil (NIST, 2709 San Joaquin) and plant materials (NIST, 1547 Peach leave) of National Institute of science and Technology were included in each sample batch to verify the accuracy and precision of the digestion procedure. Recoveries of metals from the plant tissues were found to be 99%. The blanks were run after five samples.
Physiological parameters, biochemical contents, antioxidant enzymes and root colonization were analyzed with two way analysis of variance (ANOVA) technique using statistix (version 8.1) software. For significant F value, Tukey test was used for mean comparison at 5% level.
The highest colonization was observed in control plants where no Cd concentration was applied. The trend observed was decreasing as the concentration of Cd increased from 100 to 300 mg∙kg−1. In general, the results showed that Zn and Cd addition negatively affects mycorrhizal root colonization and decreasing trend was observed with the increase of metal concentration in soil.
Experiment (Zn, mg∙kg−1) | N (g∙kg−1) | K (g∙kg−1) | Ca (g∙kg−1) | Mg (g∙kg−1) | Na (g∙kg−1) | |
---|---|---|---|---|---|---|
0 | NM | 0.26 ± 0.0458 ab | 26.89 ± 4.2547 ab | 17.747 ± 2.8582 abc | 13.047 ± 1.5734 a | 17.509 ± 5.1792 ab |
M | 0.34 ± 0.0635 ab | 29.783 ± 2.0910 a | 22.997 ± 1.6896 a | 15.747 ± 0.8079 a | 29.135 ± 0.4642 a | |
100 | NM | 0.25 ± 0.0624 ab | 17.863 ± 1.2801 bc | 15.967 ± 1.2731 bcd | 16.770 ± 2.2291 a | 17.217 ± 1.3811 ab |
M | 0.3667 ± 0.0318 a | 22.187 ± 1.8409 abc | 19.343 ± 0.5625 ab | 17.337 ± 1.4834 a | 23.307 ± 6.4599 ab | |
300 | NM | 0.2067 ± 0.0593 ab | 16.07 ± 1.4586 c | 11.217 ± 0.7254 cd | 14.027 ± 1.0267 a | 15.590 ± 1.8537 ab |
M | 0.3267 ± 0.0353 ab | 18.673 ± 0.7576 bc | 14.747 ± 0.8079 bcd | 16.897 ± 0.7442 a | 26.696 ± 1.8572 ab | |
900 | NM | 0.116 ± 0.0296 b | 12.343 ± 1.0002 c | 10.45 ± 0.5021 d | 11.467 ± 0.4378 a | 12.477 ± 1.3603 b |
M | 0.236 ± 0.0291 ab | 15.887 ± 1.5258 c | 13.927 ± 0.9324 bcd | 12.443 ± 1.4404 a | 15.033 ± 2.3617 ab | |
0 | NM | 0.18 ± 0.0346 abc | 10.917 ± 0.7294 bcd | 9.480 ± 0.4246 ab | 10.583 ± 0.5605 ab | 12.007 ± 0.9444 b |
M | 0.2867 ± 0.0318 a | 13.52 ± 1.3718 abc | 11.45 ± 0.5700 a | 11.443 ± 2.0367 a | 16.673 ± 0.6343 a | |
100 | NM | 0.23 ± 0.0289 ab | 14.157 ± 0.8920 abc | 9.817 ± 1.1249 ab | 9.043 ± 1.1767 ab | 10.04 ± 0.8271 bc |
M | 0.2767 ± 0.0233 a | 18.193 ± 1.0045 a | 10.027 ± 0.8233 ab | 10.443 ± 1.1970 ab | 11.217 ± 1.1799 bc | |
300 | NM | 0.13 ± 0.0173 bc | 12.657 ± 1.0397 bcd | 8.09 ± 0.5516 ab | 5.443 ± 0.6451 ab | 8.43 ± 0.6409 bc |
M | 0.2467 ± 0.0410 ab | 14.74 ± 0.8087 ab | 9.433 ± 0.4902 ab | 8.703 ± 0.9034 ab | 9.11 ± 1.2677 bc | |
900 | NM | 0.0833 ± 0.0203 c | 8.583 ± 0.6648 d | 7.347 ± 0.9700 b | 5.110 ± 1.2303 b | 7.437 ± 0.5305 c |
M | 0.1667 ± 0.0328 abc | 9.447 ± 0.3163 cd | 8.180 ± 0.9905 ab | 4.633 ± 0.8287 b | 8.083 ± 0.6759 c |
Experiment (Zn, mg∙kg−1) | Mn (mg∙kg−1) | Fe (mg∙kg−1) | Ni (mg∙kg−1) | Cu (mg∙kg−1) | |
---|---|---|---|---|---|
Shoot | |||||
0 | NM | 179.35 ± 18.715 a | 73.030 ± 4.3985 abc | 12.813 ± 1.6108 ab | 8.040 ± 0.7744 a |
M | 133.87 ± 6.5347 ab | 92.547 ± 2.5647 a | 8.740 ± 0.8173 b | 8.953 ± 0.8108 a | |
100 | NM | 164.25 ± 17.651 ab | 61.837 ± 3.1418 bc | 14.090 ± 0.6005 ab | 10.147 ± 0.8714 a |
M | 113.14 ± 16.125 ab | 70.483 ± 4.2766 abc | 9.103 ± 1.1201 ab | 11.453 ± 0.5167 a | |
300 | NM | 109.79 ± 16.967 ab | 56.30 ± 8.0698 bc | 14.983 ± 1.9024 ab | 10.473 ± 2.0368 a |
M | 95.06 ± 7.9648 b | 83.567 ± 7.4396 ab | 9.4 ± 0.5541 ab | 15.577 ± 4.0707 a | |
900 | NM | 118.22 ± 10.116 ab | 48.640 ± 8.1033 c | 15.363 ± 1.2835 a | 10.873 ± 0.7902 a |
M | 125.48 ± 16.293 ab | 52.807 ± 5.6784 c | 10.857 ± 0.7449 ab | 11.763 ± 1.0327 a | |
Roots | |||||
0 | NM | 81.890 ± 8.0254 ab | 38.263 ± 10.026 a | 8.77 ± 0.8260 ab | 9.377 ± 0.4824 ab |
M | 57.25 ± 9.4086 ab | 45.68 ± 5.8473 a | 5.813 ± 0.9385 b | 11.177 ± 0.8312 ab | |
100 | NM | 63.573 ± 12.077 ab | 54.93 ± 4.9003 a | 8.48 ± 0.5522 ab | 9.413 ± 0.5820 ab |
M | 46.923 ± 3.1745 b | 65.51 ± 4.4623 a | 6.373 ± 0.5500 ab | 13.82 ± 0.9789 a | |
300 | NM | 96.073 ± 4.3756 a | 48.93 ± 4.2158 a | 10.067 ± 0.7166 a | 10.85 ± 1.5069 ab |
M | 45.513 ± 7.0815 b | 52.62 ± 8.9798 a | 5.78 ± 0.9777 b | 13.143 ± 1.4814 a | |
900 | NM | 82.947 ± 13.586 ab | 46.227 ± 10.878 a | 9.88 ± 0.7850 ab | 7.54 ± 0.6116 b |
M | 92.56 ± 9.3940 a | 47.403 ± 5.5555 a | 7.36 ± 0.7900 ab | 9.397 ± 0.5174 ab |
Means (n = 3) with the different letters are significantly different (P <0.05) by the Tukey test.
The statistical significance was obtained for K, P, Na, N, Ca, Mn in M and NM plants but not significant results obtained in Cu and Mg at 0, 100, 300 and 900 mg∙kg−1 Zn. The detrimental effect of highest Zn concentration (900 mg∙kg−1) was recorded on the concentration of the analyzed nutrients as there was a significant decrease in both inoculated (M) and non inoculated (NM) plants.
The result of the experiment indicated that mycorrhizal inoculation significantly affects the mineral nutrition of alfalfa plants. In M inoculated plants, the increase in K, N, Ca, Mg, Na, Cu, Ni was recorded in shoot part of plants but decrease in Mn and Fe contents was recorded. In M roots, the Fe, Ni, Cu contents was increased, while reduction of K, N, Ca, Mg contents was also observed. In NM inoculated plants, the increase of soil Zn concentrations caused reductions in K, P, N, Mn, Ni and Fe contents in the shoots except Cu and Na in which the increase in concentration was observed in shoots of NM plants. The increase of nutrient contents were observed in N, Ca, Na, K and Ni concentrations in roots of the alfalfa plants with increasing soil Zn concentration.
The result of the experiment indicated that mycorrhizal inoculation significantly affects the mineral nutrition of alfalfa plants. In NM inoculated plants, the increase of soil Zn concentrations caused decrease in K, N, Ca, Na, Mg, Fe, Mn, Ni and Zn contents in the shoots except Cu in which the increase in contents was observed in shoots of NM plants at all Zn concentrations of 100, 300, 900 mg∙kg−1. While, the decrease of nutrient contents were observed in roots part of alfalfa plants with increasing soil Cd concentrations.
Experiment (Cd, mg∙kg−1) | N (g∙kg−1) | K (g∙kg−1) | Ca (g∙kg−1) | Mg (g∙kg−1) | Na (g∙kg−1) | |
---|---|---|---|---|---|---|
0 | NM | 0.4867 ± 0.0233 b | 15.967 ± 1.5458 ab | 24.997 ± 2.8266 a | 19.297 ± 2.5794 ab | 7.523 ± 0.5344 abc |
M | 1.03 ± 0.1127 ab | 17.18 ± 2.6684 a | 27.330 ± 3.7871 a | 22.253 ± 2.4095 a | 9.547 ± 0.5704 ab | |
100 | NM | 0.66 ± 0.1582 ab | 10.62 ± 1.1116 bcd | 25.623 ± 4.6407 a | 11.957 ± 1.1261 c | 8.303 ± 1.2822 abc |
M | 1.26 ± 0.2629 a | 13.82 ± 0.9771 abc | 20.033 ± 2.8451 ab | 13.52 ± 0.6393 bc | 12.150 ± 2.0924 a | |
300 | NM | 0.6567 ± 0.0649 ab | 7.627 ± 1.2209 d | 13.913 ± 0.7760 b | 9.36 ± 0.449 c | 5.410 ± 0.3635 bc |
M | 0.8133 ± 0.1870 ab | 8.92 ± 0.8346 cd | 12.453 ± 1.1903 b | 10.447 ± 0.4101 c | 6.993 ± 0.4668 bc | |
600 | NM | 0.5867 ± 0.0867 ab | 4.627 ± 0.5434 d | 9.873 ± 0.8239 b | 7.137 ± 0.9732 c | 4.417 ± 0.6567 c |
M | 0.6633 ± 0.0639 ab | 6.077 ± 0.7849 d | 10.950 ± 0.9100 b | 7.583 ± 0.7262 c | 4.383 ± 0.4822 c | |
0 | NM | 0.53 ± 0.1021 abc | 6.9533 ± 1.0552 ab | 6.7067 ± 0.4461 ab | 6.7067 ± 0.7216 ab | 5.82 ± 0.7744 a |
M | 0.8067 ± 0.0441 a | 7.8467 ± 1.2444 a | 9.2533 ± 0.8080 a | 9.2533 ± 1.4912 a | 6.5433 ± 1.3462 a | |
100 | NM | 0.5533 ± 0.1192 abc | 5.2133 ± 0.6744 ab | 5.33 ± 0.3500 b | 5.33 ± 1.2052 b | 5.62 ± 1.0283 a |
M | 0.6367 ± 0.1161 ab | 6.3667 ± 0.7002 ab | 5.5467 ± 0.4086 b | 5.5467 ± 0.4740 b | 6.14 ± 0.8114 a | |
300 | NM | 0.3367 ± 0.0260 bc | 4.5967 ± 0.4736 ab | 4.8467 ± 0.9995 b | 4.8467 ± 0.7882 b | 4.6567 ± 0.5161 a |
M | 0.4 ± 0.0458 bc | 5.1133 ± 0.4388 ab | 5.0267 ± 0.0845 b | 5.0267 ± 0.5851 b | 6.2567 ± 0.7213 a | |
600 | NM | 0.1967 ± 0.0498 c | 3.8567 ± 0.4160 b | 3.7067 ± 0.4677 b | 3.7067 ± 0.7361 b | 4.4867 ± 0.3886 a |
M | 0.2867 ± 0.0406 bc | 4.2333 ± 0.4532 ab | 4.5933 ± 0.7188 b | 4.5933 ± 0.6728 b | 3.96 ± 0.6243 a |
Experiment (Cd, mg∙kg−1) | Mn (mg∙kg−1) | Fe (mg∙kg−1) | Ni (mg∙kg−1) | Cu (mg∙kg−1) | |||
---|---|---|---|---|---|---|---|
Shoot | |||||||
0 | NM | 71.733 ± 0.0350 a | 46.543 ± 4.0441 ab | 6.29 ± 0.9585 a | 9.507 ± 0.6093 a | ||
M | 54.213 ± 0.3947 a | 50.853 ± 4.3645 a | 5.7467 ± 0.7115 a | 5.027 ± 0.5155 b | |||
100 | NM | 57.060 ± 4.7509 a | 31.807 ± 5.9041 b | 4.4133 ± 0.5190 a | 11.45 ± 0.7044 a | ||
M | 49.323 ± 2.2645 a | 46.747 ± 1.7822 ab | 4.8467 ± 0.7568 a | 9.983 ± 0.1235 a | |||
300 | NM | 55.14 ± 5.5078 a | 42.257 ± 3.7790 ab | 3.9233 ± 0.2200 a | 13.447 ± 1.2788 a | ||
M | 52.033 ± 9.3103 a | 50.86 ± 3.2926 a | 3.6933 ± 0.1642 a | 11.587 ± 1.4921 a | |||
600 | NM | 66.657 ± 11.871 a | 32.843 ± 2.0503 ab | 6.0367 ± 0.7799 a | 13.817 ± 1.2432 a | ||
M | 73.693 ± 0.9930 a | 40.813 ± 3.0488 ab | 4.5633 ± 0.3148 a | 10.143 ± 1.4313 a | |||
Roots | |||||||
0 | NM | 57.557 ± 0.3463 ab | 26.523 ± 2.0653 ab | 7.433 ± 0.9616 ab | 19.033 ± 3.1755 ab | ||
M | 49.573 ± 2.1903 bc | 29.61 ± 3.2810 a | 6.88 ± 0.8600 ab | 14.847 ± 1.1866 b | |||
100 | NM | 64.85 ± 3.9822 a | 17.953 ± 1.7402 cde | 10.483 ± 1.0721 a | 16.697 ± 1.6717 b | ||
M | 43.747 ± 4.2026 bc | 21.517 ± 2.1262 bcd | 7.517 ± 0.5279 ab | 18.597 ± 0.6569 ab | |||
300 | NM | 50.353 ± 3.1850 abc | 16.817 ± 0.8762 cde | 5.463 ± 0.4869 b | 21.290 ± 1.4043 ab | ||
M | 38.180 ± 1.9500 c | 23.437 ± 2.6793 abc | 4.783 ± 0.8417 b | 24.89 ± 0.9469 a | |||
600 | NM | 58.177 ± 2.4011 ab | 14.51 ± 0.6416 e | 9.74 ± 1.0761 a | 14.557 ± 1.2846 b | ||
M | 47.737 ± 0.6273 bc | 16.287 ± 1.2858 de | 7.857 ± 0.8027 ab | 13.677 ± 0.3877 b | |||
Means (n = 3) with the different letters are significantly different (P <0.05) by the Tukey test.
Slight but statistically significant increase in leaf POD activity was observed in alfalfa plants after treatment with Zn concentration. The highest POD activity was observed at 100 mg∙kg−1 Zn concentration in both M and NM plants. However, the decrease in POD activity was recorded at 300 and 900 mg∙kg−1 Zn concentration. The CAT activity induced in the similar manner as the SOD activity. The activity was enhanced as the Zn concentration increased in the soil except at the highest Zn concentration (900 mg∙kg−1) in both M and NM plants. The APX activity was decreased as the Zn concentration increased in soil except at 100 mg∙kg−1 in which the increased activity was recorded. In both M and NM plants, the trend of APX was same as the POD activity. The highest APX activity was observed at 100 mg∙kg−1 in both M and NM plants.
The leaf POD activity was increased in alfalfa plants as the concentration of Zn increased in soil. The lowest POD content was observed at 900 mg∙kg−1 Zn concentration. The highest POD activity was observed at 300 mg∙kg−1 Zn concentration in both M and NM plants. In M plants, the POD content was increased as compared to NM inoculated plants at all Zn concentration. The CAT activity was decreased as the Zn concentration increased in the soil except at the highest Zn concentration (100 mg∙kg−1) in both M and NM plants. The decreased APX activity was recorded as the Cd concentration increased in soil. The Cd concentrations (0, 100, 300, 600 mg∙kg−1) caused to decrease the APX activity in both M and NM inoculated plants.
The results of the present study indicated increased alfalfa growth and biomass in the presence of AM fungi under Zn and Cd toxicity. However previous studies have shown that AMF has resistance to toxic metals found in the soil [
The benefits of the mycorrhizal symbiosis on plant growth and nutrition are well known and have been extensively studied for many plants. The application of Vesicular Arbuscular Mycorrhiza (VAM) fungi at contaminated sites increased plants biomass even at elevated levels of Zn and Cd in the soil [
decreased by increasing the HMs additions to soil [
In the present study, the presence of AMF contributed more to the retention of Cd and Zn in alfalfa roots and also to soil stabilization. The reason of plant protection against Zn and Cd toxicity in plants inoculated with AMF may occur indirectly by enhancing plant nutrition and increasing plant growth therefore resulting in a diluting effect of Cd and Zn in plant tissues [
The present study reported that plants inoculation with AMF improved growth and shoot P, N, Fe, Mn and Zn uptake of plants in M inoculated plants polluted with Zn and Cd in comparison with only metals polluted soils. The beneficial effects of inoculation of plants and AM fungi on nutrients uptake may act as a protection mechanism that decreases Zn and Cd toxicity. The primary mechanism by which mycorrhizal fungi improve P uptake is through more extensive soil exploration rather than a unique capacity to mobilize sources of P not available to plants [
Mycorrhizal plants alleviate the severe effects of Zn and Cd by changing the translocation of metals and sequestering it in their hypha, so the toxic effects of Zn and Cd on photosynthesis and carbohydrate metabolism might decrease. The reduce amount of phosphorus observed in non-AM plants may be due to interference of toxic concentrations of Zinc and Cd with phosphorus uptake by alfalfa plants. The great amount of phosphorous in M plants emphasizes the enhancement of P uptake from the soil and its translocation to plants by the extra-radical mycelium of AM fungi [
The results of the study indicated that AMF associated alfalfa plants had better biochemical activities than non AMF plants under high Zn and Cd concentrations. However, the decreased chlorophyll and carotene content was observed at toxic concentrations of Zn (900 mg∙kg−1) and Cd (600 mg∙kg−1). [
It is concluded from the result of the present study that mycorrhizal association with alfalfa plants has beneficial positive effects on growth, biochemical contents and antioxidant enzymatic activity. The plant grew faster, exhibited improved mineral nutrition and had higher yields than non-mycorrhizal seedlings. AMF protect the alfalfa plants against metal toxicity and also benefit for nutrient uptake. AM fungi immobilize heavy metals such as Zn and Cd in moderately polluted soils. The decrease Zn and Cd uptake in mycorrhizal plants could be associated with the decline of Zn and Cd availability resulting from the increase in soil pH caused by the AM fungi. The obtained results indicate the importance of mycorrhization for alfalfa especially when it grows in soils with high levels of heavy metals. As some common agricultural practices and the increasing use of sewage sludge in agriculture may cause the accumulation of toxic metals in soils. Furthermore, experiments under field conditions should be performed to study the extent to which mycorrhizal fungi can alleviate Zn and Cd plant toxicity.
The authors would like to thank Higher Education Commission (HEC) for the financial support of this project.
SadiaKanwal,AsmaBano,Riffat NaseemMalik, (2015) Effects of Arbuscular Mycorrhizal Fungi on Metals Uptake, Physiological and Biochemical Response of Medicago Sativa L. with Increasing Zn and Cd Concentrations in Soil. American Journal of Plant Sciences,06,2906-2923. doi: 10.4236/ajps.2015.618287