Leaf Biomechanics and Biomass Investment in Support in Relation to Long-Term Irradiance in Fagus

Ü Niinemets; S. Fleck

doi:10.1055/s-2002-34132

Plant Biology, Table of Contents

Plant Biol (Stuttg) 2002; 4(4): 523-534
DOI: 10.1055/s-2002-34132

Original Paper

Georg Thieme Verlag Stuttgart ·New York

Leaf Biomechanics and Biomass Investment in Support in Relation to Long-Term Irradiance in Fagus

Ü. Niinemets ¹ , S. Fleck ²

¹ Department of Plant Physiology, Institute of Molecular and Cell Biology, University of Tartu, Tartu, Estonia
² Department of Plant Ecology, University of Bayreuth, 95440 Bayreuth, Germany

Abstract

We investigated biomass investment in support and assimilative leaf biomass in Fagus orientalis Lipsky and F. sylvatica L., and foliar biomechanical characteristics in F. orientalis to gain mechanistic insight into the determinants of leaf inclination in Fagus along the canopy light gradient. Because the leaf laminas of Fagus are elliptical, with petioles comprising only ca. 8 % of total leaf length, a leaf was approximated as a continuous sine load. Lamina load increased with increasing seasonal integrated quantum flux density in the canopy (Q _int), but leaf length was independent of irradiance. Despite greater load, leaf deflection under leaf own weight was lower for leaves at higher Q _int, indicating that foliage flexural stiffness (EI), that is a variable characterizing the resistance of beam-like structures to bending, scaled positively with irradiance. The components of EI - the leaf apparent Young's modulus of elasticity (E), which is a measure of leaf material properties, and lamina second moment of area (I), which characterizes the distribution of mass around the axis of bending - were also related to irradiance, with E decreasing, but I increasing with Q _int. The positive scaling of I with Q _int was associated with increases in leaf thickness and, in particular, with increases in the degree of leaf rolling, allowing the distribution of leaf mass further away from the neutral axis. Decreases in E were correlated with decreased leaf biomass investments in the midrib at higher irradiance. Both lamina and midrib nitrogen concentrations decreased with increasing Q _int, suggesting that foliage dry mass based physiological activity was lower at higher irradiance, possibly because of an interaction of Q _int with water stress in the canopy. Given that the veins also provide a pathway for water and nutrient transport to the leaf cells, as well as for carbon translocation from the leaf, lower leaf physiological activity in high light may provide an explanation for the lower biomass investment in major veins in high light. We conclude that foliage biomechanical characteristics and leaf inclination in the canopy are significantly affected by irradiance, and that the light effects may be modified by the reverse correlation between light and water availabilities in the canopy.

Key words

Carbon content - elastic modulus - flexural stiffness - light interception - nitrogen content - support costs

Full Text

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Ü. Niinemets

Department of Plant Physiology
Institute of Molecular and Cell Biology
University of Tartu

Riia 23
EE 51010 Tartu
Estonia

Email: ylo@zbi.ee

Section Editor: L. A. C. J. Voesenek