Growth rate (fitness)
The plants grew rapidly over the four-week growth assay with an average doubling time of 6 days across all treatments. Growth rate was affected by environmental conditions (F3,21=362.6, p<0.001), with plants growing 1.23x faster in high nutrient conditions than in low nutrient conditions, and more than 1.95x faster in high light conditions than in low light (Fig. 1). In the most favourable environmental conditions (high light - high nutrients), average doubling time was 4 days, whereas in the most stressful environmental conditions (low light – low nutrients), it was 10 days.
There was considerable variation in growth rate among genotypes, all of which responded to light and nutrients in the same direction (increased growth rate with higher resource levels). The extent of this increase varied among genotypes however, leading to a significant genotype x environment interaction (F21,128=2.92, p<0.001), indicating the presence of a genotype by environment (GxE) effect. Presence of the microbiome had a strong and consistent effect on growth rate (F1,7=32.3, p<0.001), reducing growth rate in all environmental conditions (Fig. 1) and for seven of the eight genotypes (Fig. 2a). Again, the magnitude of this negative effect varied among genotypes and environmental conditions leading to significant interactions between microbiome x genotype (F7,128=3.39, p=0.002), and microbiome x environmental condition (F3,128=5.63, p=0.001). Full Anova tables for all analyses can be found in the Supplementary materials (Table S3).