Ferns preferentially use the daily carbon assimilated to the synthesis of secondary rather than primary metabolites
In order to better understand the metabolic differences between ferns and angiosperms, we performed an unprecedented LC-MS analysis of ferns leaves, which, combined with our previous published GC-MS-based metabolite profiling analysis (Lima et al. 2019), provided a clear depiction both regarding the metabolic differences among ferns and angiosperms and metabolic aspects underpinning the regulation of stomatal speediness. Multivariate analyses indicate that ferns and angiosperms differ substantially at metabolic level, as evidenced by their separation following HCA, orthoPLSDA and PCA. The LC-MS-based metabolic fingerprinting analysis further showed that several features are solely found in ferns (Figure 4), indicating a higher chemical diversity, compared to the angiosperms species studied here. Furthermore, several features found in both plant groups have higher level in ferns than angiosperms throughout the diel course (Figure 6), suggesting that ferns have a higher accumulation of secondary metabolites than angiosperms. This idea is further supported by the fact that amino acids precursors of secondary metabolism such as phenylalanine and tryptophan are present at higher levels in ferns than angiosperms throughout the diel course, while the majority of sugars and other amino acids have higher levels, throughout the diel course, in angiosperms, when compared to ferns (Lima et al. 2019). These results strongly indicate that ferns prioritize carbon allocation towards secondary rather than primary metabolism. Given the role of secondary metabolites for plant defense against (a)biotic stresses (Martins et al. 2014; Tohge et al. 2016; Austen et al. 2019; Li et al. 2021) and the evidence indicating that ferns have greater tolerance to different stress conditions, when compared to angiosperms (Proctor & Tuba 2002; C.H et al. 2017; Salachna & Piechocki 2020), it seems likely that the higher allocation of carbons toward the secondary metabolism could be a mechanism to improve stress tolerance in ferns, at expenses of reduced growth (Figure 9). These results contribute to answer the elusive question as to why the growth of ferns is slow and sheds light on the challenge that plant breeding programs face to produce stress tolerant genotypes in the absence of a major yield penalty.