Effects of arbuscular mycorrhizal fungi on plant-plant interactions under water and salt stress

Background/Question/Methods Plant-plant interactions could regulate the dynamics of plant populations and the structure of plant communities. Direction (positive or negative) and strength of plant interactions are affected by environmental conditions. It was been well documented about abiotic factors effects, however, how biotic factors such as arbuscular mycorrhizal fungi (AMF) affect plant interactions remains further explanation. Here we address how AMF mediate plant-plant interactions along water and salt stress gradients. Using Medicago sativa L. as model species, we conducted two experiments to test the changes of plant-plant interaction along water and salt stress gradient and the role AMF. Experiment 1, we investigated AMF mediation on plant neighbor effects along a simulated aridity gradient (simulated mean annual precipitation: 1455, 920, 632, and 312 mm). Isotope tracers were used to test whether AMF hyphae mediated competition for nitrogen (N) between target and neighbor plants. Experiment 2 studied the response of target plants to neighbors and the biomass-density relationship under three salt levels (0.05%, 0.2% and 0.5%) with high or low AMF status. A relative interaction index (RII) was calculated to indicate whether effects of neighbor plant on target plant were positive or negative (facilitative or competitive).

Results/Conclusions The results of experiment 1 showed that the effects of AMF on the interactions between neighboring M. sativa plants depended on water availability. RII shifted from negative to positive as water availability increased, indicating that AMF likely ameliorated competition intensity between neighbors under drought stress conditions. AMF hyphae helped neighbor plants compete for ¹⁵N when water was available but not when water was limiting. Experiment 2, AMF increased competitive plant neighbour effects when there is no salinity stress, but this enhancement was not significant with increasing salinity levels. The slope of self-thinning population was significantly steeper for the high AMF treatment than for the low AMF treatment when there was no salinity. Under the two higher salinity levels, the effect of AMF on the self-thinning exponent was not significant. Our results support the hypothesis that abiotic stress and AMF interact to influence plant neighbor effects. Under no stress conditions, AMF increased competition, but these effects of AMF were decreased with water or salinity stress increasing.