The effect of ectomycorrhizal symbiosis on drought tolerance in Populus caspica

Increasing need for wood and on the other hand declining wood resources have caused desire for afforestation with fast–growing species such as poplars, but poplar species usually are susceptible to drought. Ectomycorrhizal fungi can improve the water status of host plants and increase plant survival and growth under drought conditions. The precise mechanism of this function is still not clear, but amino acid metabolism has been reported to play an important role in plant tolerance to drought stress. In this study seedlings of Populus caspica were propagated through tissue culture and their symbiosis with Pisolithus arhizus was carried out to evaluate the plantlets resistance to drought in acclimatization process. It was found that establishment of this symbiosis enhanced seedling height, biomass of root, shoot and leaves, photosynthesis, transpiration, stomatal conductance and leaf chlorophyll. Determination of amino acids in ectomycorrhizal and non–ectomycorrhizal roots of P. caspica plantlets (in both drought and irrigation conditions) showed that generally, the concentration of most amino acids increased in ectomycorrhizal root systems, which proved amino acids are the most important assimilation and nitrogen transfer forms in ectomycorrhizal tissue. Significant accumulation of many amino acids such as asparagine, glutamine, glutamic acid, histidine, tyrosine, threonine, alanine and methionine in ectomycorrhizal roots under stress conditions (compared to the ectomycorrhizal roots under irrigation and the control roots) was found, thus it can conclude that tolerance of ectomycorrhizal roots to drought stress is closely associated with their accumulation of free amino acids. Arginine, citrulline and aspartic acid accumulated in non–inoculated P. caspica root systems under irrigation conditions (compared to the mycorrhizal root systems), which confirmed the use of these amino acids in some metabolic pathways in the mycorrhizal root systems. Implications of these changes in the functioning of the ectomycorrhizal symbiosis are discussed.