Abstract
Abundant secondary metabolites, such as condensed tannins, and their interpopulation genotypic variation can remain through plant leaf senescence and affect litter decomposition. Whether the intrapopulation genotypic variation of a more diverse assortment of secondary metabolites equally persists through leaf senescence and litter decomposition is not well understood. We analyzed concentrations of intracellular phenolics, epicuticular flavonoid aglycones, epicuticular triterpenoids, condensed tannins, and lignin in green leaves, senescent leaves and partly decomposed litter of silver birch, Betula pendula. Broad-sense heritability (H2) and coefficient of genotypic variation (CVG) were estimated for metabolites in senescent leaves and litter using 19 genotypes selected from a B. pendula population in southern Finland. We found that most of the secondary metabolites remained through senescence and decomposition and that their persistence was related to their chemical properties. Intrapopulation H2 and CVG for intracellular phenolics, epicuticular flavonoid aglycones and condensed tannins were high and remarkably, increased from senescent leaves to decomposed litter. The rank of genotypes in metabolite concentrations was persistent through litter decomposition. Lignin was an exception, however, with a diminishing genotypic variation during decomposition, and the concentrations of lignin and condensed tannins had a negative genotypic correlation in the senescent leaves. Our results show that secondary metabolites and their intrapopulation genotypic variation can for the most part remain through leaf senescence and early decomposition, which is a prerequisite for initial litter quality to predict variation in litter decomposition rates. Persistent genotypic variation also opens an avenue for selection to impact litter decomposition in B. pendula populations through acting on their green foliage secondary chemistry. The negative genotypic correlations and diminishing heritability of lignin concentrations may, however, counteract this process.
Highlights
Plants produce an abundance of diverse secondary metabolites such as phenolics and terpenoids
We focus on the fate of foliar secondary metabolites and the persistence of their genotypic variation through leaf senescence and litter decomposition in a Betula pendula Roth population
Changes in Metabolite Concentrations during Leaf Senescence and Litter Decomposition. All those secondary metabolites that were found in green summer leaves were detected in senescent leaves, except for caffeoylquinic acids and their derivatives (CQAs) (Figure 1A)
Summary
Plants produce an abundance of diverse secondary metabolites such as phenolics and terpenoids These compounds were thought to be waste products until Fraenkel (1959) recognized that they have an important role in herbivore defense. Secondary metabolites can remain in leaf litter, and by affecting soil microbial activity, litter decomposition, and nutrient cycling have effects on ecosystem functioning (Northup et al, 1998; Hättenschwiler and Vitousek, 2000; Schweitzer et al, 2004; Kotilainen et al, 2009) While these ‘afterlife’ effects are widely recognized, understanding of metabolite dynamics in plant litter is often based on one dominant group of compounds such as condensed tannins (e.g., Schweitzer et al, 2004, 2008b) and studies on the persistence of a wider range of individual compounds (e.g., Gallet and Lebreton, 1995) have remained scarce. Focusing more on the diversity of less abundant metabolites is encouraged by a recent hypothesis that production of low-abundance, low-molecular weight secondary compounds may serve as a mechanism for trees in tropical forests to enforce energy starvation of soil decomposers and conserve the nutrients in litter, where they are accessible by plant-associated mycorrhizal fungi (Hättenschwiler et al, 2011)
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