Respiration is typically measured by monitoring either carbon dioxide release or oxygen consumption. An alternative approach, used here, is to monitor the product of oxygen consumption, namely water. Although the metabolic water produced will, like carbon dioxide, ultimately diffuse away from mitochondria or be carried off by other processes, it will do so at a much slower rate and may therefore be captured at or near the site of respiration. In this way, the amount and within limits the

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... n of the label provided can be known. Plant species and tissues under investigation were vacuum infiltrated with 18 C>2 (99.97%) and kept devoid of light for a 30 minute period to prevent photosynthesis and allow sufficient time for the label to be incorporated. Labelled metabolic water was easily detected and recovered against a large background of normal tissue water, in Medicago sativa L. sprouts. We investigated four other species, representing different functional groups: a C3 plant {Helianthus giganteus L.), a CAM plant (Crassula ovata (Miller) Druce), a desert plant {Disocactus flagelliformis (L.) Barthlott), and a tree (Populus trichocarpa Torr & Gray). The <5 18 0 values of tissue water ranged from +7.93%o to +216%o. Higher values were associated with tissues with high respiration rates per unit water content. In some experiments, labelling was followed by immersing the tissues in unlabelled water to monitor exchange (i.e. leakage) over periods of 5 to 60 minutes. Exchange of labelled water was more rapid during the first 5 minutes (X = 63.4% complete over all species, tissues and treatments) than over the 5-10 min interval, suggesting the existence of two pools of water available for exchange; namely apoplastic and symplastic water. Attempts to modify aquaporin activity failed to influence exchange. A model system was constructed to test whether high rates of exchange could also be expected in a soil environment, using Helianthus. In conclusion, the movement of metabolic water across cellular membranes was very rapid, severely limiting the utility of 1 O labelling for pinpointing sites of respiratory activity.