Parenchyma Abundance in Wood of Evergreen Trees Varies Independently of Nutrients.

Martyna M Kotowska,Ian J Wright,Mark Westoby

doi:10.3389/fpls.2020.00086

Martyna M Kotowska, Ian J Wright + Show 1 more

Open Access

https://doi.org/10.3389/fpls.2020.00086

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Abstract

The abundance of living cells in wood—mainly as interconnected axial and ray parenchyma networks—varies widely between species. However, the functional significance of this variation and its role in plant ecological strategies is poorly understood, as is the extent to which different parenchyma fractions are favored in relation to soil nutrients and hydraulic functions. We analyzed wood tissue fractions of 16 Australian angiosperm species sampled from two nearby areas with similar climate but very different soil nutrient profiles and investigated structure-function links with soil and tissue nutrient concentrations and other plant traits. We expected the variation in parenchyma fractions to influence nutrient concentrations in wood xylem, and to find species with lower parenchyma fractions and accordingly lower nutrient requirements on lower-nutrient soils. Surprisingly, both axial and ray parenchyma fractions were mostly unrelated to tissue and soil nutrient concentrations, except for nitrogen concentration in stem sapwood. Species from low nutrient soils showed higher fractional P translocation from both leaves and sapwood, but little patterning with respect to tissue nitrogen. While species from high and low nutrient soils clearly clustered along the soil-fertility axis, their tissue composition varied independently from plant functional traits related to construction costs and hydraulic anatomy. Our findings imply that there is considerable variation among species in the nutrient concentrations within different parenchyma tissues. The anatomical composition of wood tissue seems unrelated to plant nutrient requirements. Even though xylem parenchyma is involved in metabolic functions such as nutrient translocation and storage, parenchyma abundance on its own does not directly explain variation in these functions, even in co-occurring species. While parenchyma is highly abundant in wood of angiosperm trees, we are still lacking a convincing ecological interpretation of its variability and role in whole-tree nutrient budgets.

Highlights

Wood is often considered a dead tissue serving the tree mainly for structural stability to lift the photosynthetically active leaves above neighbors competing for light
For nitrogen (N) there was a positive relationship with total parenchyma fraction in stem wood, but not in twig wood
There was no significant correlation with soil nutrients either as total N content or available P

Summary

Introduction

Wood is often considered a dead tissue serving the tree mainly for structural stability to lift the photosynthetically active leaves above neighbors competing for light. Functionality within parenchyma is likely to be influenced by the orientation and connectivity of parenchyma cells—radially as rays, or axially—and by the arrangement type and proximity to tracheary elements—paratracheal or apotracheal— which are commonly used as anatomical classification criteria (Carlquist, 2001). While these distribution patterns have been confirmed for their taxonomic value (Baas, 1982; Wheeler and Baas, 1998), inferences on their functionality often remain indirect or based on morphological observation (Secchi et al, 2017). A link between total parenchyma volume in wood and its nutrient concentrations has been suggested in the past (Merrill and Cowling, 1966; De Vries, 1975) and a general pattern of lower nutrient concentrations in gymnosperm wood having low presence of axial parenchyma as compared to angiosperm wood has been described (Meerts, 2002)

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