Fungal and Bacterial Dynamics in the Lettuce Rhizosphere Responding to Successive Additions of Cd and Zn.
A. M. I. D. Amarakoon * and R. M. C. P. Rajapaksha
Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka.
Introduction
Results and Discussion
• Response of mycorrhizal fungi to elevated metals varied depending upon their sensitivity to metals,
bioavailability and threshold concentrations of metals in soil. 2 Thus, heavy metal uptake by food
crops grown in contaminated soils may be governed by mycorrhizal infection to a greater extent. 3
• Plant uptake of Zn and Cd increased
continuously with increasing metal
dose and maximum uptake
remained lower than 10% of the
contamination (Fig. 2).
100
Zn (Control)
80
Zn (Zn)
80
70
Zn (Cd)
Zn (Cd)
Zn -(Cd+Zn)
60
Cd (Cd)
Cd (Cd+Zn)
40
60
Zn -(Cd+Zn)
50
Cd (Cd)
Cd (Cd+Zn)
40
30
20
20
10
0
• Presence of Zn reduced Cd uptake
by lettuce and vice versa.
Objectives
1. To assess the response of arbuscular mycorrhizal fungi (AMF) inhabiting lettuce rhizosphere
to successive additions of Cadmium and Zinc
• Cadmium, zinc and a combination of both were introduced to one month old plants and then two
successive additions were made as given below:
At 6 weeks
Dose 2
kg-1 soil
12.5 mg
250 mg
2.5 mg
50 mg
2.5 mg Cd + 50 mg Zn
Control
12.5 mg Cd + 250 mg Zn
-
Dose 3
Dose 1
Figure 1: DTPA extractable Zn and Cd in soil
60
Control
Control -R
Zn
Zn-R
Cd
Cd-R
40
Cd+Zn
(Cd+Zn)-R
-
At 8 weeks
Dose 3
25 mg
500 mg
25 mg Cd + 500 mg Zn
-
All treatments replicated three times. The experimental design was complete randomized design.
• One week after imposing each metal dose, plants were uprooted along with their rhizosphere soil.
• Soils were analyzed for
DTPA extractiable Zn and Cd
• Plants were analyzed for
Root mycorrhizal colonization, Cd and Zn uptake
Dose 2
Dose 3
Figure 2: Tissue Zn and Cd contents
Control
50
a
D
u…
8
R2 = 0.86
6
Cd
30
Root colonization %
• Lettuce (Lactuca sativa) plants were grown in pots for a period of 10 weeks on limed or non-limed
tropical Entisol.
Zn
Cd
Zn + Cd
Dose 2
10
CFU x 105 g-1 soil
Methodology
Treatment
Dose 1
0
Zn
2. To assess the Cd and Zn uptake by lettuce plants
At 4 weeks
Dose 1
Zn (Control)
Zn (Zn)
mg kg-1 soil
• Elevated concentrations of metals alter soil microbial community structures towards a more metal
resistant populations eliminating sensitive species.1 Such changes in microbial communities affect
soil quality and thereby plant growth.
• Application of 500 mg of Zn and 25
mg of Cd resulted in about 90 and
10 mg of DTPA extractable Zn and
Cd, in experimental soil respectively
(Fig. 1).
mg kg-1 soil
• Cadmium and Zinc are introduced to soil ecosystems mainly through agricultural inputs (e.g.
fertilizers, manures, pesticides etc.) and industrial emissions. Metals in soils are known to persist
long-run and eventually introduce to biological systems and aquatic environments.
40
30
20
4
Cd+Zn
R2 = 0.94
0
a
20
b
b
0
b
3
6
9
12
Figure 5:
Relationship between
tissue and soil Cd contents
10
10
2
a
b
b
c
b
b
b
0
Dose 1
Dose 2
Dose 3
0
Dose 1
Treatment
Figure 3: Dynamics in total and Cd resistant bacterial population
Dose 2
Dose 3
Treatment
Figure 4: Dynamics in mycorrhizal root colonization
• The dual metal treatment showed the highest bacterial population and AMF infection percentage, with a 19%
increase, following application of the first metal dose (Figs. 3 and 4). The second metal dose reduced bacterial
population but enhanced AMF infection in single metal treatments. The third metal dose affect bacteria and AMF
adversely but enhanced growth of Cd-resistant bacteria population.
• About 90 and 10 mg of DTPA extractable Zn and Cd kg-1 soil, respectively were critical for AMF when they were
added alone. The level of tolerance declined to 75 and 1 mg of DTPA extractable Zn and Cd in the presence both
metals. The observed critical levels were lower than those reported previously suggesting that AMF of
experimental soils are sensitive to metals.
• Plant uptake of Cd increased with increasing metal dose (Fig. 5) despite a significant reduction in the AMF
infection from dose 2 to 3. This may indicate that plant protective mechanisms of AMF do not operate at those
metal levels.
Conclusions
References
1. Landi, L., Renella, G., Moreno, J. L., Falchini, L., Nanniperi, P., 2000. Influence of cadmium on the metabolic quotient, L-:D-glutamic
acid respiration ratio and enzyme activity: microbial biomass ratio under laboratory conditions. Biology and Fertility of Soils 32:8-16
2. Val, D., C., Barea J., M., Aguilar, C., 1999. Diversity of Arbuscular Mycorrhizal Populations in Heavy Metal Contaminated Soils.
Applied and Environmental Microbiology 65: 718-723.
3. Chen, B., D., Liu, Y., Shen, H., Christie, X., L., Li., P., 2004. Uptake of Cadmium from an experimentally Contaminated Calcareous
soil by Arbuscular Mycorrhizal Maize (Zea mays L.). Mycorrhiza 14:347–354.
DTPA extractable Zn and Cd concentrations of 90 and 10 mg
kg -1 soil, respectively were critical for bacteria and AMF and lead to
a development of metal resistant bacterial population.
Zinc and Cd levels beyond these levels resulted in metal
accumulation in lettuce at a toxic level that retard crop growth.
* Present affiliation
Department of Soil Science, Faculty of Food and
Agriculture, University of Manitoba, Winnipeg R3T 2N2
[email protected] –