CORN (ZEA MAYS L.) GROWTH AS AFFECTED BY SOIL COMPACTION AND ARBUSCULAR MYCORRHIZAL FUNGI

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1 Journal of Plant Nutrition, 36: , 2013 Copyright C Taylor & Francis Group, LLC ISSN: print / online DOI: / CORN (ZEA MAYS L.) GROWTH AS AFFECTED BY SOIL COMPACTION AND ARBUSCULAR MYCORRHIZAL FUNGI M. Miransari Department of Soil Science, College of Agricultural Sciences, Shahed University, Tehran, Iran A two-year research work was performed under greenhouse conditions to evaluate: 1) the effects of soil compaction on corn growth, and 2) if using different species of mycorrhizal fungi under nonsterilized and sterilized conditions can alleviate the stress of soil compaction on corn growth. After sieving the field soil and sterilizing half of it using an autoclave, it was compacted in 2-kg pots. The soil was then planted with corn seeds and inoculated with different species of AM species including Glomus etunicatum, G. mosseae, andg. intraradices. While high levels of soil compaction decreased corn growth, AM inoculation significantly enhanced root and shoot growth and hence plant growth under compaction and G. etunicatum was the most efficient one. These results are important complementary to the previously little documented results regarding the effects of AM fungi on corn growth under compaction stress and are of agricultural and ecological significance. Keywords: corn (Zea mays L.), root and shoot growth, soil compaction, arbuscular mycorrhizal fungi INTRODUCTION Soil compaction has some important effects on plant growth and crop production and is the result of using agricultural machinery in the field, especially at high soil moisture. Under such conditions soil bulk density increases and altered soil (physical, chemical and biological) properties create a new environment in the soil affecting plant growth and crop production. Moreover, compaction stress increases soil resistance, and hence the rate of soil micropores, which eventually reduce plant growth. This is because under such conditions roots growth and hence water and nutrients uptake decrease (Nadian et al., 1997, 1998; Miransari et al., 2007, 2008). Received 23 November 2010; accepted 23 May Address correspondence to M. Miransari, Department of Soil Science, College of Agricultural Sciences, Shahed University, Tehran, 18151/159, Iran. miransari1@gmail.com 1853

2 1854 M. Miransari The following soil properties, affecting plant growth and crop production are highly affected by soil compaction: 1) soil available moisture and aeration, 2) soil reduction and oxidation conditions and hence soil nutrient availability, and 3) soil microbial activities including the mineralization of soil organic matter and the availability of soil nutrients. Decreased soil oxygen results in the emission of greenhouse gases including nitrogen, nitrogen oxide and methane (CH 4 ) from the soil surface. Such important effects influence soil fertilization as well as the amount and combination of greenhouse gases in the atmosphere. These all indicate the great importance of soil compaction in agriculture and environment (Miransari et al., 2007, 2008; Saha and Mandal, 2010; Becerra et al., 2010; Besso et al., 2010). Accordingly, it is pertinent to evaluate methods, which can alleviate the stress of soil compaction on plant growth and crop production, while economically and environmentally friendly. There are very little data on the use of arbuscular mycorrhizal (AM) fungi as a biological method on plant growth, specifically corn (Miransari et al., 2007, 2008) under compaction stress. Hence, the results of this research work can be importantly complementary to the previously documented results. Arbuscular mycorrhiza are soil fungi, developing mutual symbiosis with their host plant, resulting in the exchange of carbon and soil nutrients between the two partners. For the development of such a symbiosis the presence of the host plant is necessary, although in its absence the fungal spores may germinate. It has been previously documented that AM fungi are able to alleviate different stresses on plant growth including salinity (Subramanian et al., 2006; Daei et al., 2008), different chemicals like fungicides (Samarbakhsh et al., 2009), heavy metals (Hildebrandt et al., 1999; Aude and Charest, 2006), and drought (Auge, 2001; Subramanian et al., 2006). However, data related to the effects of soil compaction on plant growth like corn using AM fungi is little (Miransari et al., 2007, 2009a). According to Nemec and Meredith (1981) and Gupta (2003) AM fungi can alter plant metabolism. In addition, AM fungi are able to adjust plant physiology through the followings: enhanced photosynthesis rate as well as the adjustment of photosynthates allocation to the roots and shoots (Suresh and Bagyarja, 2002). The significance of interaction between AM fungi and soil microbes is because such effects can impact the sustainability of agricultural systems through influencing soil fertility and hence plant growth and production (Johansson et al., 2004; Miransari, 2011). The mechanisms, related to the interactions between AM and soil microbes are not well understood yet, however with the help of novel approaches including PCR and molecular methods it has been likely to indicate the identity and location of soil microbes in the mycorrhizosphere (de Boer et al., 2005; Miransari, 2011). Through the following ways AM influence the communities of soil microbes in the rhizosphere: the direct effects of AM hypha and the exudated products and the indirect effects of AM fungi on soil microbes (Marschner

3 Soil Compaction and Mycorrhizal Fungi 1855 and Baumann, 2003; Miransari, 2011) by influencing rhizodeposition (Linderman, 1988). In addition, AM can affect the bacterial communities in the soil by producing high energy products via their hypha (Andrade et al., 1997), affecting soil structure (Rillig and Mummey, 2006), competition for nutrient uptake (Ranskov et al., 1999) and influencing root exudates (Soderberg et al., 2002). Soil bacteria may influence AM establishment and development through affecting spore germination and growth (Toljander et al., 2006; Miransari, 2011). Researchers have also indicated that AM fungi and soil microbes are very much interactive (Soderberg et al., 2002) probably through exchanging signals and metabolites (Toljander et al., 2006; Miransari, 2011). It has been indicated that AM fungi can alleviate the adverse effects of different stresses on plant growth (Miransari, 2010). After the exchange of the signals between the two partners (Vierheilig and Piché, 2002), which is a necessary step for the start of the symbiosis, the AM hypha develop into the plant roots producing some organelles including the branched like structure or arbuscules (the place for the exchange of nutrients between the fungi and the host plant) and vesicles (as the storage organelles) (Smith and Read, 2008). The fungi have the ability to develop an extensive network of hypha with very fine diameter, which can significantly increase plant uptake of water and nutrients (Miransari et al., 2009a, 2009b). In addition, the fungi produce nutrients mobilizing enzymes such as phosphatases and a glycolprotein called glomalin, which can improve soil structure through binding soil particles (Rillig and Mummey 2006). These abilities make such symbionts ideal partners for the host plant, especially under stress conditions. With respect to the significance of agricultural and ecological efficiency, which are affected by soil stresses such as compaction and also with respect to the abilities of AM fungi, especially under different stresses including compaction, we tested the following objectives in a two year greenhouse experiment: 1) the effects of soil compaction on corn growth, and 2) alleviation of soil compaction on corn growth using different species of AM under non-sterilized and sterilized conditions. MATERIALS AND METHODS Experimental Procedure The top soil (Xeric Haplocalcids, Banaei 2000) of the Agricultural Research Field of Shahed University, Tehran, Iran, was used for the experiment. After sieving the soil half of it was autoclaved at 121 Cand1.5Pafor1hr (Toshihiro et al., 2004) and was then used in cm pots weighing 6.5 kg. Using the standard laboratory methods the soil properties were analyzed (Miransari et al., 2007, 2008). The soil is not saline or calcareous, however its ph is more than 7 and the amount of its nutrients are much

4 1856 M. Miransari less than the sufficiency level for plant growth and production. Soil texture is a silty loam (Gee and Bauder, 1986). On 20 April 2007 and 2009 soils in the pots were compacted at four different levels including control (C1 = 1.15), 4-(C2 = 1.24), 12-(C3 = 1.28) and 20-time (C4 = 1.34 g/cm 3 )compaction, using 2-kg weights, released from a 20-cm height (Miransari et al., 2007, 2008). Based on the number of the released weight the pots were compacted. The 20-time release of the weight was the highest likely level of compaction, to be created in the pots. Although in some experiments it has been stated that soil must be compacted in different layers (Place et al., 2008), however to make the conditions similar to the field, pots soil were compacted in one layer. In addition, dry soil was compacted in the pots because compacting wet soil creates a very hard and unfavorable compacted soil for plant growth. Compaction levels were selected based on our previously conducted experiments (Miransari et al., 2007, 2008). Soil bulk density for dry samples (using oven at 104 Cfor24hr)was measured using a 100-cm 3 cylinder for 16 randomly selected pots for different replicates and treatments. In addition, soil resistance to penetrometer for all pots with the exception of a few missing measurements for the sterile pots was determined using a handy penetrometer (Model SL135, Impact Co., Scotland, UK) on a kg cm 2 basis at certain moisture. At the time of soil resistance measurements, soil samples were collected to determine soil moisture using oven. Hence, for the precise measurement of soil moisture the collected samples were subsampled making the total of 18 soil subsamples. Plantation and Inoculation Two kinds of soil treatment, four compaction levels and four (first experiment) and three (second experiment) arbuscular mycorrhizal species were tested in the experiment using a (first experiment) and (second experiment) factorial on the basis of a completely randomized design. The experiment was performed in four replicates making a total of 32 (first experiment) and 24 (second experiment) treatments and 128 (first experiment) and 96 (second experiment) pots. Almost one week after compacting the pots and at seeding four corn seeds (cultivar 704) were planted in each pot and were inoculated with 40 and 10 g of different AM inoculum species including Glomus etunicatum (M2), G. mosseae (M3) and G. intraradices (M4) in non-(s1) and sterilized (S2) soils, respectively in the first experiment. In the second experiment all pots were inoculated with 10 g of G. mosseae and G. etunicatum. A control treatment was also used in the experiments. AM inoculum was produced on sorghum roots in a four months period using sterilized sand (Miransari et al., 2007, 2008). After seed germination, corn plants were thinned to one plant in each pot.

5 Soil Compaction and Mycorrhizal Fungi 1857 In the non-sterilized pots weeds were removed manually and in the sterilized pots no weeds grew. Two months after seeding and according to soil testing pots were fertilized with nitrogen (N), phosphorus (P), and potassium (K) using urea, triple superphosphate and potassium sulfate fertilizer at 1.48, 0.46 and 1.32 g per pot, respectively (Miransari et al., 2007, 2008). Pots were irrigated to their maximum capacity allowing the excess water drained out. Mean daily and nightly temperatures in the greenhouse were around 25 and 20 C, respectively and plants received natural sunlight during the day. At harvest, which was on the 20 February 2008, corn plants including roots and leaves were harvested. If necessary, tap water was used to remove soil particles from the corn roots. Plant samples were placed in plastic bags and their dry weights were immediately determined (using paper bags) in the laboratory using oven at 104 Cfor24hr. Statistical Analysis Data were subjected to analysis of variance using SAS (SAS Institute, Cary, NC, USA). Means were compared using Least Significant Difference (LSD) test. In addition, contrast comparisons between different AM species and for different plant measurements were also determined (Steel and Torrie, 1980). RESULTS Soil Resistance Data related to soil resistance at the soil moisture of 15.4 ± 1.6% are presented in Tables 1, 2, and 3. According to the analysis of variance compacting soil significantly increased soil resistance in non-sterilized pots. The highest level of soil resistance (1.96 kg cm 2 ) was related to the highest level of compaction (C4), which was significantly different from the first (C1) and second (C2) level of compaction. In the sterilized soil the conditions were also similar, although the differences were not significant. Corn Growth as Affected by Different Species of AM Fungi in a Compacted Soil Although analysis of variance indicated that the trend of AM species effects on root growth was almost significant (P = 0.07) in the compacted soil, however according to the contrast comparisons, species M4 significantly enhanced root growth in the non-sterilized soil, relative to the control treatment. The effects of the experimental model, soil compaction, and the interaction effects of soil compaction and AM species on root growth were significant in the non-sterilized soil. AM species did not affect shoot growth in the non-and sterilized soil, however the interaction effect of soil compaction

6 1858 M. Miransari TABLE 1 Pot soils resistances (kg cm 2 ) and their related standard error values at different levels of soil compaction in the non-sterilized soil in the first experiment Rep. Compaction level SR1 SR2 SR3 Non-sterilized soil R1 C ± ± ± 0.28 R1 C R1 C R1 C ± ± ± 0.14 R2 C ± ± ± 0.82 R2 C ± ± ± 0.35 R2 C ± ± ± 2.07 R2 C ± ± ± 1.98 R3 C R3 C ± ± ± 0.20 R3 C ± ± ± 0.28 R3 C ± ± ± 1.01 R4 C ± ± ± 0.25 R4 C ± ± ± 0.25 R4 C ± ± ± 0.18 R4 C ± ± ± 0.36 Sterilized soil R1 C ± ± ± 0.35 R2 C ± ± ± 0.01 R2 C ± ± ± 0.29 R2 C R3 C R3 C ± ± ± 2.12 R3 C ± ± 0.35 R4 C R4 C ± ± ± 0.47 Rep.: replicate. SR1, SR2 and SR3: soil resistance in the first, second and third measurements, respectively. C1, C2, C3 and C4, first, second, third and fourth level of compaction, respectively. and AM species on shoot growth in the sterilized soil was significant. The effects of AM species on plant growth were completely different in non- and sterilized-soil. Some of the beneficiary effects of increased soil compaction up to C3 level like enhanced shoot growth was also observed, however the high levels of soil compaction decreased root growth. In the sterilized soil species M2 and in the non-sterilized soil species M4 performed the best to alleviate the stress of soil compaction on corn growth (Tables 4, 5, and 6). DISCUSSION Soil Resistance According to the results, compacting soil increased soil resistance and soil bulk density, which is in agreement with the results of Miransari et al. (2007, 2008). In addition, increased soil moisture decreased soil resistance

7 Soil Compaction and Mycorrhizal Fungi 1859 TABLE 2 Pot soils resistances (kg cm 2 ) and their related standard error values at different levels of soil compaction in the non-sterilized soil in the second experiment Rep. Compaction level SR1 SR2 SR3 Non-sterilized soil R1 C ± ± ± 1.48 R1 C ± ± ± 0.21 R1 C ± ± ± 2.47 R1 C ± ± R2 C ± ± ±.44 R2 C ± ± 0.17 R2 C R2 C ± ± ± 1.06 R3 C ± ± ± 1.41 R3 C R3 C ± ± ± 0.39 R3 C ± ± ± 1.06 R4 C ± ± ± 0.49 R4 C R4 C ± ± ± 0.28 R4 C ± ± ± 1.62 Sterilized soil R1 C ± ± 0.99 ND R1 C R1 C ± ± ± 2.31 R1 C ± ± ± 0.89 R2 C ± ± ± 0.78 R2 C R2 C ± R2 C R3 C ± ± ± 0.29 R3 C ± ± ± 1.41 R3 C R4 C R4 C ± ± 0.11 R4 C R4 C ± ± ± 0.35 Rep.: replicate. SR1, SR2 and SR3: soil resistance in the first, second and third measurements, respectively. C1, C2, C3 and C4, first, second, third and fourth level of compaction, respectively. to penetrometer, which is similar to the results of Whalley et al. (1995) and Passioura (2002). Effects of Soil Compaction on Corn Growth Soil compaction significantly affected corn growth. Increased soil compaction increases root length and volume, because under stress more carbon is allocated to the roots. It should also be mentioned that according to Jones et al. (1991) and Amato and Ritchie (2002) increased soil bulk density up to some level does not adversely affect root growth. However, higher levels of soil compaction can have unfavorable effects on root growth and hence

8 1860 M. Miransari TABLE 3 Mean comparison of soil resistances (kg cm 2 ) at different levels of soil compaction for sterilized soil, using Least Significant Difference (LSD) test at P = 0.05 Soil resistance (kg/cm 2 ) Soil resistance (kg/cm 2 ) Non-sterilized soil Sterilized soil Soil resistance Compaction level Experiment 1 Experiment 2 Experiment 1 Experiment 2 SR1 C1 0.68c SR1 C2 0.73bc SR1 C3 1.72ab SR1 C4 1.96a SR2 C1 0.76b SR2 C2 0.65b SR2 C3 1.80a SR2 C4 1.75a SR3 C1 0.70c SR3 C2 0.77bc SR3 C3 1.82a SR3 C4 1.71ab SR1, SR2 and SR3: soil resistance in the first, second and third measurements, respectively. C1, C2, C3 and C4, first, second, third and fourth level of compaction, respectively. Values, within each column, followed by different letters, are significantly different at P = plant growth (Li et al., 1997), which is in agreement with the results of this research work. Effects of AM fungi on root growth are significant, which is similar to the results of Harrison (1997), Al-Karaki and Clark (1998) and Miransari et al. (2007, 2008). Also compared with the control treatment, AM species significantly increased root growth in the compacted soils in this experiment and hence increased plant growth through enhancing water and nutrient uptake (Miransari et al., 2009a, 2009b). In mycorrhizal plants the fungal hypha increase root surface absorbing area and hence it explores a higher volume of soil resulting in enhanced water and nutrient uptake (Camel et al., 1991; Clark and Zeto, 2002). It should also be mentioned that adverse soil conditions such as compaction can affect the morphology of AM hypha; however, AM can adjust its hyphal growth in compacted soil (Evans and Miller, 1990; Drew et al., 2003). Under compaction stress enhanced P uptake by mycorrhizal corn can increase root growth (Miransari et al., 2007, 2009a). Interestingly, very recently Pupin et al. (2009) found that under compaction stress while the production of urease decreases the amount of phosphatase and dehydrogenase enzymes increases. They also found that the fungal population is stimulated under compaction stress, however the bacterial activities and population including the nitrifying bacteria decreases. Also in a field experiment (Miransari, 2005; Miransari et al., 2006) using different tractor passes the field soil was compacted at different levels and corn seeds were inoculated with different species of AM. Signs of compaction stress were apparent on the corn plants

9 Soil Compaction and Mycorrhizal Fungi 1861 TABLE 4 Effects of different species of arbuscular mycorrhiza on corn (Zea mays L.) shoot and root dry weight and their related standard errors at different levels of soil compaction for non- and sterilized soils in the first experiment SDW (g), RDW, (g), SDW, (g), RDW, (g), AM species Compaction level S1 S1 S2 S2 M1 C ± ± ± ± 0.45 M2 C ± ± ND M3 C ND ND M4 C ± ± ± M1 C M2 C ± ± ± ± 0.95 M3 C ± ± M4 C ± ± ± ± 0.50 M1 C ± ± ± ± 0.86 M2 C ± ± 0.11 M3 C ± ± ± ± 1.19 M4 C ± ± ± ± 0.50 M1 C ± ± ± ± 0.98 M2 C ± ± ± 0.60 M3 C ND M4 C ± ± Model n.s. n.s. n.s. Rep. n.s. n.s. Compaction n.s. n.s. n.s. Rep compaction n.s. n.s. n.s. AM n.s. P = 0.07 P = 0.14 n.s. AM compaction n.s. n.s. M1 vs. M2 n.s. n.s. M1 vs. M3 P = 0.17 M1 vs. M4 P = 0.14 P = 0.22 n.s. Arbuscular mycorrhiza (AM) species including control (M1), Glomus etunicatum (M2), G. mosseae (M3) and G. intraradices (M4). SDW and RDW: shoot and root dry weight (g), respectively. S1 and S2: non- and sterilized soil. ND: not determined. including reduced corn height, pale leaves, and pan cake like and cluster growth of corn roots. AM species alleviated the stress of compaction on corn growth. The physiological adjustments in plant tissues by AM fungi result in structural and biochemical alterations in roots cells and membrane permeability and hence affecting the quantity and quality of roots exudates (Linderman, 1992; Harrison, 1999; Ramos et al., 2009; Zhu et al., 2010). AM fungi are also able to utilize mechanisms for the use of the host plant C (Blee and Anderson, 1998). Although non-mycorrhizal plants can alleviate soil stresses by using different mechanisms, however the mycorrhizal plants rely very much on their symbiosis with the AM fungi. Hence, under stress AM fungi adjust plant physiology in a way, which enable the host plant to more efficiently handle the stress (Miransari et al., 2008). In a very interesting experiment Taylor et al. (2008) compared the abilities of mycorrhizal tomato (Lycopersicon esculentum L.) and mycorrhizal corn

10 1862 M. Miransari TABLE 5 Effects of different species of arbuscular mycorrhiza on corn (Zea mays L.) shoot and root dry weight and their related standard errors at different levels of soil compaction for non- and sterilized soils in the second experiment SDW (g), RDW, (g), SDW, (g), RDW, (g), AM species Compaction level S1 S1 S2 S2 M1 C ± ± ± ± 0.98 M2 C ± ± ± M3 C ± ± ± ± 6.09 M1 C ± ± ± M2 C ± ± ± ± 1.21 M3 C ND M1 C M2 C ± ± ± ± 2.01 M3 C ± ± ± ± 8.75 M1 C ± ± ± M2 C ± ± ± M3 C ± ± ± ± 1.39 Model P = 0.16 n.s. n.s. n.s. Rep. n.s. n.s. n.s. n.s. Compaction P = 0.08 n.s. n.s. n.s. Rep compaction n.s. n.s. n.s. n.s. AM P = 0.15 n.s. n.s. n.s. AM compaction P = 0.19 n.s. n.s. n.s. M1 vs. M2 n.s. n.s. n.s. n.s. M1 vs. M3 n.s. P = 0.19 n.s. Arbuscular mycorrhiza (AM) species including control (M1), Glomus etunicatum (M2), G. mosseae (M3) and G. intraradices (M4). SDW and RDW: shoot and root dry weight (g), respectively. S1 and S2: non- and sterilized soil. ND: not determined. (Zea mays L.) to find the likely reasons for the enhanced root absorption area. They found that AM fungi are able to increase the absorption area in corn roots through increasing the number of membrane cells, but not in tomato. The regulation of H + across the corn symbiotic cellular membrane (Ramos et al., 2009) can affect root growth and physiology through affecting the passage of phosphate (Poulsen et al., 2005), sugars (Schüβler et al., 2006) and amino acids (Cappellazzo et al., 2008). Such activities are regulated by H + -ATPases and H + -pyrophosphatases located in the plasmalemma and vacuole membrane (Gaxiola et al., 2007). Additionally, under stress the production of oxygen radicals can adversely affect plant growth; however, AM fungi can alleviate the stress through the production of osmolytes and antioxidant enzymes resulting in the reduction of membrane lipid peroxidation (Sajedi et al., 2010; Zhu et al., 2010). The significant interaction effects between different levels of soil compaction and the arbuscular mycorrhiza are in agreement with the results of Miransari et al. (2007, 2008) indicating that AM fungi may behave differently in different levels of soil compaction. In addition, significant interaction effects indicate that the alleviating effects of AM fungi on the stress of soil compaction becomes more evident with increasing soil compaction levels

11 TABLE 6 Mean comparison of shoot and root dry weight (g) at different levels of soil compaction for non-sterilized and sterilized soil using Least Significant Difference (LSD) test at P = 0.05 Compaction level SDW (g) RDW (g) AM species SDW (g) RDW (g) Compaction level SDW (g) RDW (g) AM species SDW (g) RDW (g) Experiment 1 Non-sterilized soil Experiment 2 C a M b C1 2.94b 0.52 M C b M b C a 1.20 M C b M b C3 6.43ab 0.67 M C b M a C4 8.42ab 0.94 Sterilized soil C M C M C M C M C M C M C M C SDW and RDW: shoot and root dry weight (g), respectively. C1, C2, C3 and C4: compaction levels including first, second, third and fourth level, respectively. Arbuscular mycorrhiza (AM) species including control (M1), Glomus etunicatum (M2), G. mosseae (M3) and G. intraradices (M4). Values,within each column, followed by different letters, are significantly different at P =

12 1864 M. Miransari (Miransari et al., 2007, 2008, 2009a, 2009b). Miransari et al. (2007) found that soil sterilization enhanced the alleviating abilities of AM fungi in a compacted soil, relative to non-sterilized soil. In this experiment the results are somehow different in this respect, which can be attributed to the different amounts of AM inoculum used in this experiment for the non- and sterilized soil. The other reasons indicating the superb abilities of AM fungi in compacted soils are like the followings: the fungal hypha are much thinner (average diameter 3 4 μm) than even the thinnest root hairs (average diameter more than 10 μm) enabling them to grow in the smallest soil micropores in a compacted soil, the extensive network of hypha and hence plant roots explore a higher volume of soil resulting in the enhanced uptake of water and nutrients by plant roots (Nadian et al., 1997, 1998). In addition, AM hypha are capable of absorbing higher rates of P from a compacted soil, compared with a non-compacted soil (Li et al., 1997; Miransari et al., 2009a, 2009b). The reason that why plant roots are more affected than plant shoots in a compacted soil is the reduced N uptake by plants resulting in less production of cytokinin by plant roots and their consequence movement to the roots. Decreased cytokinin production in the shoots reduces the rate of cell division, while in the roots it may inhibit the unfavorable effects of cytokinin on cell growth and development. The higher translocation of sucrose to the roots under such conditions also increases root growth (van der Werf and Nagel, 1996). The superior performance of Glomus etunicatum to alleviate the stress of soil compaction on corn growth is in agreement with Miransari et al. (2007, 2009). CONCLUSION The results of this experiment are very importantly complementary to the previously little results regarding the effects of AM species on corn growth under compaction. These results indicate that, among other reasons, one of the most important effects of AM fungi on enhancing corn growth under compaction is through increasing root growth. These results elucidate even more than before the necessity and important roles of AM species in agriculture and ecosystem, especially under stresses like compaction. AM species, performed differently under stress and G. etunicatum was the most efficient species to enhance corn growth. REFERENCES Al-Karaki, G, N., and R. B. Clark Growth, mineral acquisition, and water use by mycorrhizal wheat grown under water stress. Journal of Plant Nutrition 21:

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