Introduction
Arbuscular mycorrhizal (AM) associations are ubiquitous in desert ecosystems and may play an important role in plant establishment and growth by bridging between plant and soil (
Allen, 1983;
Skujins and Allen, 1986;
Dhillion and Zak, 1993). Mycorrhizal plants have a greater ability to absorb nutrients and soil water and increase plant fitness, both of which may lead to better survival under stressed environmental conditions (
Auge and Stodola, 1990;
Sylvia and Williams, 1992). AMF, especially, can form enormous hypha network systems in the rhizosphere, which can enhance the stability of soil aggregates, fix dune, and improve the physical and chemical conditions of the soil (
Bearden and Petersen, 1999). Therefore, AMF can play an important role in ecological system protection, restoration, and reconstruction. In addition, root endophytes such as dark septate endophytes (DSE) are abundant in many plant genera and many habitats worldwide (
Jumpponen and Trappe, 1998), may be as ubiquitous as or even more than AMF, especially under harsh climatic conditions (
Barrow, 2003;
Olsson and Tyler, 2004;
Addy et al., 2005), and have been reported to confer a positive effect on plant growth (
Read and Haselwandter, 1981). The DSE are less well known, but are potentially beneficial to their host plants (
Postma et al., 2007). As far as we know, little work has been done concerning the relationships between desert plants and DSE in arid and semi-arid area of northwestern China. Therefore, we examined the diversity of AMF structures and also observed the colonization of the DSE and AMF under the canopy of desert plants.
Mu Us Sandy Land of China is located between 107°20′E to 111°30′E and 37°27′N to 39°22′N, with an area of about 40 thousand km
2, including portions of the Inner Mongolia Autonomous Region, Shaanxi Province, and the Ningxia Hui Autonomous Region (
Department of Geography of Peking University et al., 1983). This area has a typical continental semi-arid climate. Annual precipitation ranges from 440 mm in the southeast to 250 mm in the northwest, of which 60%-80% is concentrated in the period from June to August. The annual mean temperature is about 6.0-8.5ºC, with monthly mean temperatures of 22ºC in July and -11ºC in January. This area is the biggest mobile sand land in the dry and nutrient-poor grasslands of China. With shortage of water, wind exposure, sand texture, and fluctuations in soil, the moisture and temperature make the ecological condition very fragile and sensitive.
The target plants belong to eight families of Salicaceae (Salix psammophila), Tamaricaceae (Tamarix chinensis), Leguminosae (Caragana intermedia, Caragana microphylla, Caragana korshinskii, Ammopiptanthus mongolicus, Oxytropis aciphylla, Hedysarum scoparium, Hedysarum fruticosum, Astragalus adsurgens, and Glycyrriza uralensis), Zygophyllaceae (Sarcozygium xanthoxylum), Asclepiadaceae (Periploca sepium), Compositae (Artemisia ordosica, Artemisia sphaerocephala, Artemisia scoparia), Gramineae (Stipa capillata, Psamrrmchloa villosa, Phragmites australis), and Liliaceae (Allium mongolicus). Among these species, H. fruticosum, C. korshinskii, A. sphaerocephala, and A. ordosica are four dominant psammophyte species in Mu Us Sandy Land. Due to its rapid growth and vegetative propagation characteristics, H. fruticosum has been widely used in the desertified land. C. korshinskii belongs to Leguminosae (Caragana Fabr.), uptaking fixed nitrogen in the air and improving soil fertility in the biological sand fixation. A. sphaerocephala was the advantages of the Ordos Plateau species, A. ordosica, as a key, for mobile dune to semifixed and fixed dunes succession process. A. mongolicus is the oldest sand Tertiary relict species, unique in Alashan Desert plant construction group, and others are common in Mu Us Sandy Land.
Materials and methods
Sites description and sampling
Twenty species of desert plants were collected from Mu Us Sandy Land mainly from the Ordos Sandy Land Ecological Station of the Institute of Botany, the Chinese Academy of Sciences (OSES), Shaanxi Yulin Rare Sandy-plants Conversation Field (RSCF), Ningxia Shapotou (SPT), Yanchi (YC), and Alaskan desert (ALS) in October, 2007. For widely distributed species, samples were collected from more than one habitat. Detailed information about the sampling species and sites is described in Table 1.
Soil samples and root samples were collected in four replicates from the rhizosphere of each plant. The rhizosphere soil at 0-30 cm layer from each replicate was placed in an individual plastic bag and transported to the laboratory. Air-dried soil samples were sieved (2 mm mesh size) for each sample. The subsamples of soil from each replicate were used to determine spore density.
Assessment of AM and DSE colonization
Fresh roots were cut into 0.5 to 1.0 cm long segments and processed by washing them free of soil and clearing in 10% (w/v) KOH at 90°C in a water bath for 15-30 min, depending on the degree of lignification of the roots. The root subsamples were cooled, washed, and stained with 0.5% (w/v) acid fuchsin. Thirty root fragments were examined at 100-400 times magnification using a Nikon YS100 microscope with an automatic photomicrographic system for the presence of AMF structures. Total AM, hyphal, vesicular, and arbuscular colonization were expressed as the percentage of root segments colonized for each root sample.
Mycorrhizal types were designated according to Smith and Smith (
1997), and DSEF tissues were named according to Jumpponen and Trappe (
1998). The hyphae and microsclerotium (MS) of DSEF colonization was calculated as the ratio of the number of infected sections to the total number of sections examined.
Extraction of AMF spores
Spores or sporocarps were extracted from 20 g air-dried soil of each soil sample by wet sieving followed by flotation centrifugation in 50% sucrose (
Dalpe, 1993). The spores were collected on a filter paper, washed several times with distilled water, and counted using a dissecting microscope at 75 times magnification. A sporocarp was counted as one unit.
Results
Among the 20 plants in this experiment, we found different plant species formed different mycorrhizal morphology, consisting of mainly three AM types of Arum type, Paris type, and intermediate type. The Arum type formed extensive intercellular hyphae. The Paris type was characterized by the absence of intercellular hyphae and replaced by extensive intracellular hyphal coils (Fig. 1 (a)). Moreover, if two morphological structures were represented in the same root system, then it was termed as intermediate type (Fig. 1 (b)). The mycorrhizal types of the target plants are described in Table 2.
All the species were colonized by AMF, although A. mongolicus and A. mongolicum were not observed to have hyphae and vesicles, respectively, and the colonization of S. psammophila was low. The morphology of AMF hyphae (Figs. 1 (a), (b) and Fig. 2 (b)) and vesicles (Figs. 2 (d), (e), (g)) was various. Most of the studied species had no arbuscular colonization, with only low arbuscular colonization of A. ordosica, C. intermedia, C. microphylla, and C. korshinskii (Fig. 2 (f)). The spore density (expressed as per 1 g dry soil) under the canopy differed greatly among plants, ranging from 2.45 to 83.60 (Table 2).
In this investigation, plant species belonging to a same or different families formed different AM morphology, suggesting that the morphology of AM was depended on the genotype of host plant (
Ahulu et al., 2007).
Coexistence of DSEF and AMF was detected in all investigated plants (Fig. 1 (c) and Fig. 2 (a)), although some species had lower colonization, such as DSE in P. sepium and AM in S. psammophila. All 20 species had DSE and formed microsclerotium (Fig. 2 (h)); the colonization of microsclerotium in C. intermedia was especially high, reaching about 50%. The results indicated the dominant and constructive plants could commonly form AM and DSEF tissues.
Correlation analyses (Table 3) demonstrated that the total percentage of highly significant colonization was positively correlated with hyphal (P < 0.01) and vesicular (P < 0.01) colonization, whereas the hyphal colonization had a significantly negative correlation (P < 0.05) with DSE tissues. Moreover, the microsclerotium colonization was positively correlated with vesicular and arbuscular colonization.
Discussion
AMF plays an important role in plant survival and community stability of vegetation in natural ecosystems (
Gange et al., 1993;
Francis and Read, 1994;
Hartnett and Wilson, 2002;
Moraes et al., 2004). It is well known that AMF can improve plant nutrient uptake (
Tarafdar and Praveen-Kumar, 1996), water use efficiency (
Auge and Stodola, 1990), and resistance to abiotic stress under certain conditions (
Brundrett, 1991). The present study indicated that AMF was formed in the roots of various plants in Mu Us Sand Land, which demonstrated that these associations might be one of the valid countermeasures of acclimatizing plants to the arid and infertile environments. Plant community structure and AMF distribution and colonization status might be used to monitor desertification and soil degradation. Several studies on AMF have been conducted in recent years in desert ecosystems (
Collier et al., 2003;
Ferrol et al., 2004;
Alarcón and Cuenca, 2005).
The variation in spore density and colonization of AMF associated with different host plant species may be generated by a variety of potential mechanisms, including biological characteristics of rhizosphere under host species, variation in host species, mycorrhizal dependency, host plant-mediated alteration of the soil microenvironment, or other unknown host plant traits, as described by Lorgio et al. (
1999) and Eom et al. (
2000). In our present study, no significant correlation was observed between mycelial colonization and spore density. However, some researchers found a positive relationship between AM colonization and spore density (
Ebbers et al., 1987;
Sigüenza et al., 1996), whereas others found a negative relationship (
Fontenla et al., 1998).
We found that DSE infected all tested plants and cooccurred with AMF in the plant roots, although the colonization of some species was lower, including
P. sepium and
S. psammophila. The results indicated that the common and beneficial species of desert plants could easily form AM and DSEF tissues. Mandyam and Jumpponen (
2005) speculated that DSE fungi would be prevalent in various habitats and colonize a substantial proportion of the species present in mixed plant communities. This group of fungi cannot be overlooked while assessing the fungal communities of any ecosystem, as their abundance may equal or even exceed that of the AM fungi. Although different opinions exist regarding whether DSEF should be treated as mycorrhizal fungi or not, there seems to be an agreement that some members play an important role in soil ecosystems in some cases, with DSEF functioning much like mycorrhizal fungi (
Jumpponen and Trappe, 1998;
Jumpponen, 2001).
In the present study, correlation analysis demonstrated that total colonization was significantly and positively correlated with hyphal and vesicular colonization. This result is due to the mycelium and vesicles, which are the primary structures existing for months or years. Generally, arbuscules begin to form approximately two days after root penetration and begin to collapse after a few days (
Smith, 1995), and the sampling in October lead us to observe only low colonization of arbuscular in
A. ordosica,
C. intermedia,
C. microphylla, and
C. korshinskii.
The hyphal colonization negatively correlated with the DSE tissue may be AMF competing with DSEF for the econiche. The microsclerotium was positively correlated with vesicular and arbuscular colonization. The microsclerotia of some species of DSE had some special storage substances and may function as propagules (
Yu et al., 2001). Therefore, we suggest that the function and growth characteristics of microsclerotium may be similar to the vesicular and arbuscular. In general, the DSEF function and the interaction on AMF are less well known. Our study indicates that the plants in Mu Us Sandy Land may establish a good symbiosis with DSEF, and the symbionts may depend on each other for survival in these extreme environments. Moreover, further studies will be required to elucidate the interactional mechanisms with AMF and the operating mechanisms in desert ecosystems.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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