Plant-Plant Interactions And The Effects Of Root Systems And Arbuscular Mycorrhizae Under Salt Stress

Plant-plant interactions (competition or facilitation) play important roles in shaping the structures of plant populations or communities. The nature and intensity of plant-plant interactions are affected by both abiotic and biotic factors. Facilitation often occurs under stress environments. The mechanisms explaining facilitation between plant neighbors have been focused on aboveground interactions and abiotic stress, while belowground interactions and biotic interactions are less well known. The overall goal of this study was to examine the plant-plant interactions under salt stress and the mechanisms underlying these interactions. Specific objectives of this study were (1) to test whether species differing in biomass-density relationship resulted from the nature of plant-plant interaction of a specific species; (2) to quantify responses of plant-plant interaction to environmental stress gradient; (3) to examine the contributions of plant canopy or belowground to plant-plant interactions under stressed conditions; (4) to investigate whether arbuscular mycorrhizal fungi (AMF) played a role in the response of plant-plant interaction to stress environment. To address these objectives, experiments were conducted along an increasing gradient of salt concentration in fields and in controlled environmental conditions in the greenhouse. The halophytic shrub Tamarix chinensis L. was employed as model plant system. Literature analysis and meta-analysis were also integrated in the study. The results were as follows:1 Species differing in the nature of plant-plant interactions resulted in the differences of biomass-density (B-D) relationships among species in a salt marshBoth results of literature review and field experiment showed that slope (a) of B-D relationship line varied among plant species under higher stress. The field study showed that species with different salinity-tolerance differed in the direction and strength of neighbor effects (Rll varied from-0.42 to 0.19). There was a positive relationship between Rll value and a-value among the tested species:negative neighbor effects increased as the slope of the biomass-density relationship became more negative.2 The outcome of neighbor effects changed from net competition to facilitation with increased salt stress gradientGreenhouse experiments showed that plant-plant interaction of T. chinensis shifted from competition to facilitation as soil salinity increased from 0.05% to 0.8%: Rll value changed from -0.21 to -0.02 with seedling neighbor, and changed from-0.34 to 0.25 with adult neighbor. Strength of facilitation did not decrease under the highest soil salinity.3 Neighbor plants facilitated target plants mainly through belowground under salt stressNeighbor plant increased shoot biomass and AMF colonization of target seedlings via whole plant effects (above-plus belowground) under salt stress (soil salinity content 0.62%). Canopy removal experiment showed that T. chinensis neighbors significantly promoted the growth of target plants mainly through belowground, while canopy effect was not significant (P> 0.05). Compared to no neighbor, adult neighbor reduced soil salinity content (by 72.83%), increased soil organic matter content (by 75.33%) and AMF spore density (by 90.97%); while seedling neighbor reduced soil salinity content by 23.90%, and increased soil organic matter content and AMF spore density by 21.16 and 16.27%, respectively. These results indicated that neighbor plants may promote the growth of target seedlings via ameliorating soil conditions.4 AMF played a role in facilitation between plants through mediating nutrient uptake under salt stressResults of meta-analysis indicated that positive effect of AMF occurred under stressed conditions when neighbors existed, the Effect Size was significantly greater than zero (P<0.001). When there was no environment stress, however, the effect of AMF was not significant. Field experiment showed that AMF relaxed competition intensity or even enhanced plant facilitation under stress conditions (soil salinity content 0.62%), especially when adult neighbor was presented; AMF increased plant competition under no salinity treatment. In the greenhouse experiment, neighbors with AMF hyphal connection increased shoot biomass, AMF colonization, and 15N content of target seedlings under the high salt level (soil salinity content 0.8%), suggesting that neighbors can facilitate surrounding seedlings’growth under salt stress via AMF symbiosis.5 Adult plants promoted neighboring seedling growth through AMFBoth field experiment and greenhouse experiment showed that when salt stress existed (soil salinity content 0.62% and 0.8%), adult neighbor promoted, but seedling neighbor did not promote the growth of target seedlings (Rll value did not differ with zero). Greenhouse experiment further showed that seedlings obtained more 15N via AMF network when with adult neighbor. Meta-analysis also indicated that when there was an adult neighbor, the growth of seedling target positively responded to AMF network (Effect Size=1.28), but not when grown with a seedling neighbor (Effect Size=0.08).