Light and nutrient treatments had strong effects on plant growth and phenotype
The largest source of variation in our experiment was environmental, namely light and nutrient availability, which impacted growth rates, frond area, root length and colony size in a strong and consistent way for all genotypes, with or without the presence of the microbiome. Both light and nutrients were limiting at low levels (Fig. 1).
In low nutrient conditions the main phenotypic response was an increase in root length. This response to low light was combined with a decrease in frond area, as plants invested a larger portion of their biomass to root tissue (Fig. 3c). Although in L. minor , nutrient uptake takes place via both the roots and fronds, longer roots increase rates of Nitrogen uptake due to the increased surface area, and an increase in root length in low nutrient conditions is consistent with other studies (Cedergreen and Madsen 2002, 2004). This plastic response in root length was strengthened in high light conditions, perhaps since the increase in plant growth resulted in more severe nutrient limitation.
Fronds became smaller in low nutrient conditions (Fig. 3b), which is a common response to stress in L. minor (Mohan and Hosetti 1999, Naumann et al. 2007, O’Brien et al. 2020a). In low light conditions, fronds were also visibly darker green in colour (Fig. 3a), another standard plant response to light limitation due to an increase in leaf chlorophyll content (Björkman 1981, Minotta and Pinzauti 1996).
Colony size is controlled by the abscission of the stipe, vascular tissue connecting the mother and daughter fronds (Landolt 1986). Here we find that higher resource levels resulting in increased growth led to increased abscission and therefore smaller colony size. One possible interpretation is that in low resource environments, daughter fronds act as a sink by continuing to receive fixed carbon from the rest of the colony through prolonged attachment.