Alleviation of salt stress by arbuscular‐mycorrhizal Glomus species in Lactuca sativa plants

Abstract

Improved salt tolerance of mycorrhizal plants is commonly attributed to their better mineral nutrition, particularly phosphorus. However, the effect of arbuscular‐mycorrhizal (AM) fungi on salt tolerance may not be limited to this mechanism. We investigated the possibility that non‐nutritional effects of AM fungi, based on proline accumulation or increased photosynthesis and related parameters, can influence the tolerance of lettuce (Lactuca sativa L.) to salinity. Three levels of salt (3, 4 and 5 g NaCl kg‐1 dry soil) were applied and plants were maintained under these conditions for 7 weeks. The salt‐treated AM plants produced greater root and shoot dry weights than unfertilized or P‐fertilized non‐AM controls. With increasing salinity, both shoot and root dry weights were reduced, but this decrease was greater in uninoculated plants. In particular, shoot dry weight was not reduced in G. fasciculatum‐colonized plants as a consequence of salt, whereas in uninoculated plants it was reduced by about 35% at the highest salt level. Proline accumulation was considerably lower for P‐amended non‐AM and for AM plants except for G. mosseae‐colonized plants than was the case for unamended plants. Transpiration, carbon dioxide exchange rate (CER), stomatal conductance and water use efficiency (WUE) were higher in mycorrhizal plants. At 5 g NaCl kg‐1, both photosynthesis and WUE increased by more than 100% in mycorrhizal treatment relative to uninoculated plants. The contents of phosphorus of P‐fertilized non‐AM plants was similar to or higher than those of G. mosseae‐ and G. fasciculatum‐colonized plants. Plants colonized by G. deserticola had the highest P‐content regardless of salt level. Hence, the effect of G. mosseae and G. fasciculatum on salt tolerance in this experiment could not be attributed to a difference in the P content. The mechanisms by which these two fungi alleviated salt stress appeared to be based on physiological processes (increased CER, transpiration, stomatal conductance and WUE) rather than on nutrient uptake (N or P).

Co-inoculations of arbuscular mycorrhizal fungi and rhizobia under salinity in alfalfa

“…(Aziz and Khan 2001) have been shown to produce higher yields in comparison to control samples under salinity conditions. In accordance with our findings, the increase in yield of the soybean specimen inoculated with Rhizobium japonicum and Glomus mosseae was also reported by Ruiz-Lozano et al (1996). Increase in forage yield of subterranean clover inoculated with mycorrhizae under both salinity and non-salinity conditions was also reported by Asghari et al (2005), and it was concluded that inoculation increases mycorrhizae effectiveness and enhances root colonization under salt and non-salt stresses.…”

“…This fact can be justified as follows: such symbioses may cause hydrogen (H) pumps that generate the driving force for increasing the value of the K/Na ratio which enhance the plant salinity tolerance (Rabie and Almadini 2005). In addition, AMF may also increase plant salinity tolerance by increasing mineral nutrition acquisition (Cordovilla et al 1995), improving rhizospheric conditions (Linderman 1994), enhancing water potential (Hildebrandt et al 2001;Marulanda et al 2003), altering physiological and biochemical properties of the host plants (Smith and Read 1997), and enhancing host physiological processes such as increasing the carbon dioxide exchange rate, transpiration, stomatal conductance, root hydraulic conductivity and water use efficiency (Ruiz-Lozano et al 1996;Smith and Read 1997;Al-Karaki et al 2001).…”

“…They promote salinity tolerance by employing various mechanisms, such as enhancing nutrient uptake, especially phosphorus (P) nutrition (Giri et al 2002), producing plant growth hormones, improving rhizospheric condition of the soil (Linderman 1994), altering the physiological and biochemical properties of the host plants (Grant et al 2001) and improving host physiological processes (Pfetffer and Bloss 1988). In addition, AMF colonization can reverse the effect of salinity on potassium (K) and Na + nutrition by enhancing K + absorption under saline conditions (Ruiz-Lozano et al 1996;Giri et al 2002;Sharifi et al 2007) while preventing Na translocation to shoot tissues. These benefits of AMF have prompted it to be a suitable candidate for bio-amelioration of saline soils.…”

Interactions between arbuscular mycorrhizal fungi and phosphate-solubilizing fungus (Mortierella sp.) and their effects on Kostelelzkya virginica growth and enzyme activities of rhizosphere and bulk soils at different salinities

“…Approximately 7% of the global land surface is covered with saline soil (Ruiz-Lozano et al 1996); the soil salinity is becoming one of the major limiting factors for crop production and an appropriate solution to this situation is to use PSF as bioinoculants. Rietz and Haynes (2003) and Tripathi et al (2006) had reported that soil microbial communities and their activities are greatly influenced by salinity.…”

Managing arbuscular mycorrhizal fungi in cropping systems

“…This mechanism is particularly important for the many soil types that have low levels of bioavailable phosphorus (Hayman and Mosse 1971. Arbuscular mycorrhizae can also provide many non-nutritional benefits to crops, including: alleviation of water (Safir et al 1971;Allen and Boosalis 1983) and salt (Ruizlozano et al 1996) stress, protection against some soil-borne pathogens (Baltruschat and Schönbeck 1972;Roncadori and Hussey 1977) and improvements to soil structure (Hamel et al 1997;Schreiner and Bethlenfalvay 1995). Since the pioneering work of Baylis (1959), Daft and Nicolson (1966), Gerdemann (1968), and Hayman and Mosse (1971), the scientific community has advocated greater use of AM fungi in agricultural practices (Gianinazzi and Schüepp 1994;Harrier and Watson 2003), particularly to reduce the use of P fertilizers, but so far this has not happened.…”