Responses of Alnus glutinosa to Anaerobic Conditions - Mechanisms and Rate of Oxygen Flux into the Roots

Abstract

Upon exposure to waterlogged growing conditions two-year-old alder trees reduced total root mass. Roots were concentrated in the uppermost soil horizon, and only few coarse roots penetrated into deeper soil layers. Root porosity was only slightly affected and did not exceed 8 % in fine roots. Porosity of coarse roots was higher (27 %) but unaffected by growing conditions. The stem base area covered by lenticels increased strongly and so did the cross section diameter of the stem base. The latter showed a highly significant correlation with O₂ transport into the roots, measured by a Clark type oxygen electrode. Exposure of the lower 5 cm of the stem base, where lenticels were concentrated, to pure N₂ led to a cessation of O₂ transport, confirming that lenticels were the major site of air entry into the stem. In alder plants grown under waterlogged conditions, temperature had a pronounced effect on O₂ gas exchange of the root system. The temperature compensation point, i.e., the temperature where O₂ transport equals O₂ consumption by respiration, was 10.5 °C for the entire root system, when measured in a range of 0.15 - 0.20 mmol dissolved O₂ L^-1, which is typical for an open water surface equilibrated with air. O₂ net flow was inversely related to O₂ concentration in the rooting media, indicating that higher root and microbial respiration induced higher net fluxes of O₂ into the root system. With 0.04 mmol dissolved O₂ L^-1 nutrient solution, the temperature compensation point increased to 20 °C. Measurement of O₂ gradients in the rhizosphere of agar-embedded roots using O₂ microelectrodes showed a preference for O₂ release in the tip region of coarse roots. Increasing stem temperature over air temperature by 5 °C stimulated O₂ flux into the roots as suggested by the model of thermo-osmotic gas transport. However determination of stem and air temperature in a natural alder swamp in northern Germany revealed that within the experimental period of almost one year, temperature gradients required for thermo-osmotic gas transport were very seldom. From this it is concluded that under natural conditions in northern Germany, oxygen diffusion along the stem into the root system is driven by O₂ concentration gradients rather than by thermo-osmosis.

Root aeration - waterlogging - diffusive gas flux - radial oxygen loss - thermo-osmosis

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1 Much to our regret, Prof. B. Sattelmacher deceased in November 2005.

Editor: M. Riederer