Abstract
Increased frequency of droughts and degraded edaphic conditions decreases the success of many reforestation efforts in the Pacific Northwest. Microbial endophyte consortia have been demonstrated to contribute to plant growth promotion and protection from abiotic and biotic stresses – specifically drought conditions – across a number of food crops but for limited tree species. Our research aimed to investigate the potential to improve establishment of economically and ecologically important conifers through a series of in situ field trials and ex situ simulations. Microbial endophyte consortia from Salicaceae, previously shown to confer drought tolerance, and conifer endophyte strains with potentially symbiotic traits were selected for trials with Douglas-fir (Pseudotsuga menziesii) and western redcedar (Thuja plicata). Reductive experimentation was used to subject seedlings to a spectrum of simulated drought levels or presence/absence of fertilizer, testing hypotheses that endophyte consortia impart improved drought resistance and growth promotion, respectively. Inoculation from Salicaceae consortia significantly (p ≤ 0.05) improved survival among seedlings of both species subject to increasing drought stress, with T. plicata seedlings surviving at twofold higher rates in extreme drought conditions. Both species demonstrated improved growth 540 days after inoculation of seed with conifer derived consortia. In the carefully controlled greenhouse experiments with both species, seedling Fv/Fm and SPAD values remained significantly (p ≤ 0.05) more stable in inoculated treatment groups as stress increased. Our findings confirm that multi-strain consortia may be applied as seed or field amendment to conifers, and the approach is efficient in garnering a positive growth response and can mitigate abiotic stressors.
Highlights
Endophytes, microorganisms living within plants, are diverse, widely distributed, and essential in a range of ecological functions
Following a second growing season (540 days after inoculation (DAI)), seedlings treated with the Consortium 3 had a higher survival rate than those treated with the WP1 or Consortium 1+2 and 4; the survival rate of T. plicata seedlings receiving the endophyte treatments was similar to the control group
root-collar diameter (RCD) growth of T. plicata treated with the Consortium 4 was significantly greater than the control treatment and Consortium 3 at 1,600+ DAI
Summary
Endophytes, microorganisms living within plants, are diverse, widely distributed, and essential in a range of ecological functions. Endophytes may influence shifts in biogeography and plant communities (Rodriguez et al, 2009; Bulgarelli et al, 2013; Sikes et al, 2016), contribute to nutrient cycling (Moyes et al, 2016), and enhance crop production (Le Cocq et al, 2017). Fungal and bacterial endophytes isolated as individual strains and consortia (i.e., communities of strains) have been subjected to reductive analyses to determine their role in enhancing plant growth in some agricultural species (Chanway, 1996; Mitter et al, 2013; Hardoim et al, 2015). Endophyte applications to food crops can improve survival, growth, yields, and pathogen resistance (Senthilkumar et al, 2011; Le Cocq et al, 2017; Rho et al, 2018a,b). Endophyte inoculated trees have shown: (1) positive growth trends (Chanway and Holl, 1991; Chanway et al, 1994; Shishido et al, 1999; Khan et al, 2015; Castro et al, 2017), (2) adaptive phenotypic changes (Knoth et al, 2014; Vivas et al, 2015), (3) stress mitigation (Khan et al, 2016; Doty, 2017; Doty et al, 2017; Rho et al, 2018a,b), (4) nitrogen fixation (Anand and Chanway, 2013; Anand et al, 2013; Knoth et al, 2014), and (5) reduced damage from insect herbivores and fungal pathogens (Brooks et al, 1994; Miller, 2011; Pirttilä and Frank, 2011)
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