Thigmomorphogenetic responses of an aquatic macrophyte to hydrodynamic stress.

Jonas Schoelynck,Eric Struyf,Sara Puijalon,Patrick Meire

doi:10.3389/fpls.2015.00043

Abstract

The response of aquatic plants to abiotic factors is a crucial study topic, because the diversity of aquatic vegetation is strongly related to specific adaptations to a variety of environments. This biodiversity ensures resilience of aquatic communities to new and changing ecological conditions. In running water, hydrodynamic disturbance is one of the key factors in this context. While plant adaptations to resource stress (nutrients, light…) are well documented, adaptations to mechanical stress, particularly flow, are largely unknown. The submerged species Egeria densa was used in an experiment to detect whether the presence or absence of hydrodynamic stress causes plant thigmomorphogenetic responses (i) in terms of plant biogenic silica (BSi), cellulose and lignin concentrations, and (ii) in terms of plant strength. Plant silica concentrations, as well as lignin concentrations were significantly higher in presence of hydrodynamic stress. These physiological changes are accompanied by some significant changes in stem biomechanical traits: stem resistance to tensile forces (breaking force and breaking strength) and stiffness were higher for plants exposed to hydrodynamic stress. We conclude that the response of this aquatic plant species to mechanical stress is likely the explaining factor for a higher capacity to tolerate stress through the production of mechanically hardened shoots.

Highlights

Aquatic plants can be exposed to important external mechanical forces resulting from pressure exerted by water movement, in flowing ecosystems such as rivers and streams (Puijalon et al, 2011; Puijalon and Bornette, 2013)
The submerged species Egeria densa was used in an experiment to detect whether the presence or absence of hydrodynamic stress causes plant thigmomorphogenetic responses (i) in terms of plant biogenic silica (BSi), cellulose and lignin concentrations, and (ii) in terms of plant strength
These physiological changes are accompanied by some significant changes in stem biomechanical traits: stem resistance to tensile forces and stiffness were higher for plants exposed to hydrodynamic stress

Summary

Introduction

Aquatic plants can be exposed to important external mechanical forces resulting from pressure exerted by water movement, in flowing ecosystems such as rivers and streams (Puijalon et al, 2011; Puijalon and Bornette, 2013). When growing under permanent flow conditions, aquatic plants may present important thigmomorphogenetic responses (i.e., developmental responses to external mechanical stimulation; Braam, 2005; Telewski, 2006), which can increase the plant capacity to tolerate hydrodynamic forces. These thigmomorphogenetic responses involve many morphological traits (e.g., reduced plant mass and height, reduced leaf sizes, higher biomass allocation to below-ground organs; Doyle, 2001; Strand and Weisner, 2001), sometimes resulting in extremely modified morphologies such as dwarfed individuals (Puijalon and Bornette, 2013). Both stem cross-sectional area and proportion of strengthening tissues may be affected by thigmomorphogenetic responses (Bociag et al, 2009), but the actual consequences for aquatic plant resistance to breakage still needs to be investigated

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