Salicylic acid (SA) results in an increase in oxygen consumption and heat production by excised wheat roots. Activation of respiration and heat production can be caused by protonophoric abilities of SA, which can acidify cell cytoplasm. This is accompanied by increased H+-ATPase activity of the plasma membrane. Oxygen consumption and heat production by root cells in the presence of SA are sensitive to KCN, an inhibitor of cytochrome oxidase. As cytoplasm acidification leads to electron transfer via the outer mitochondrial membrane, it is believed that these electrons are transferred to cytochrome oxidase. Malate is a unique substrate that may be oxidized by the outer mitochondrial membrane. In the presence of SA, malate causes an increase in respiration, which is sensitive to KCN. Ascorbate, as the electron donor for cytochrome c and cytochrome oxidase, stimulates respiration of roots in the presence of SA as well, and this stimulation is also sensitive to KCN. Unike SA, malate and ascorbate do not combine with protons in the membrane and thus do not disrupt the energy accumulation due to ion translocations, that do not lead to the extra heat production by roots. However, as for SA, respiration of roots treated with malate and ascorbate is stimulated because their ions remain dissociated in the aqueous phase and can be transported through the outer mitochondrial membrane via cytochrome/redox reactions. (C) 2004 Elsevier B.V. All rights reserved.