Summary: Seedlings of Betula spp. were grown in conjunction with isolates of Amanita muscaria Hooker and Paxillus involutus Fr. under aseptic conditions. Mycorrhizas were established in sterilized peat and vermiculite to which nutrient media had been added. Zinc was supplied at various levels and growth of the host was assessed after eight weeks. The mycorrhizas increased the tolerance to Zn of both tolerant and non‐tolerant Betula. This was coupled with a reduction in the translocation of zinc to the shoots of Betula, but an accumulation of zinc in the mycorrhizas.

Both Ca²⁺ and reactive oxygen species (ROS) play critical signaling roles in plant responses to biotic and abiotic stress. However, the positioning of Ca²⁺ and ROS (in particular H₂O₂) after a stress stimulus and their subcellular interactions are poorly understood. Moreover, although information can be encoded in different patterns of cellular Ca²⁺ signals, little is known about the subcellular spatiotemporal patterns of ROS production or their significance for downstream responses. Here, we show that ROS production in response to hyperosmotic stress in embryonic cells of the alga Fucus serratus consists of two distinct components. The first ROS component coincides closely with the origin of a Ca²⁺ wave in the peripheral cytosol at the growing cell apex, has an extracellular origin, and is necessary for the Ca²⁺ wave. Patch-clamp experiments show that a nonselective cation channel is stimulated by H₂O₂ and may underlie the initial cytosolic Ca²⁺ increase. Thus, the spatiotemporal pattern of the Ca²⁺ wave is determined by peripheral ROS production. The second, later ROS component localizes to the mitochondria and is a direct consequence of the Ca²⁺ wave. The first component, but not the second, is required for short-term adaptation to hyperosmotic stress. Our results highlight the role of ROS in the patterning of a Ca²⁺ signal in addition to its function in regulating cell wall strength in the Fucus embryo.