Correlation between changes in photosynthetic activity and changes in total protoplast volume in leaf tissue from hygro-, meso- and xerophytes under osmotic stress

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Abstract
Rates of photosynthesis of leaf slices from various hygro-, meso- and xerophytes were measured in the absence of stomatal control in various stages of osmotic dehydration. The external osmotic potential π° for a 50% inhibition of photosynthesis varied between 20 bar in some hygrophytes up to 50 bar in xerophytes. The response of photosynthetic enzymes to increased salt concentrations in the reaction medium was similar in leaf extracts from hygro-, meso- and xerophytes. The total protoplast volume in vacuum-infiltrated leaf discs from various plants was measured as the difference between 3H2O-labeled space and [14C]sorbitol-labeled space. In all plants, the protoplast volume could be reduced to about 55% of the maximum volume of tissue in equilibrium with water, without decreasing photosynthesis. Reduction of the maximal protoplast volume below 55% decreased photosynthesis in all tissues to the same decreased photosynthesis in all tissues to the same degree. At 20% maximal volume, photosynthesis of all plants was completely inhibited. The differential decrease of protoplast volumes of various leaf tissues in response to changes in π° was mainly due to the different osmotic potential of the cell sap (πcs). The relative contribution of sugars to the overall osmolarity of the cell sap was up to nineteen times higher in xerophytes than in hygrophytes. Short-term recovery of photosynthesis after hypertonic stress was good in xerophytes, incomplete in mesophytes and absent in hygrophytes. There was also a large discrepancy between the partial recovery of protoplast volumes and the complete absence of a recovery of photosynthesis in hygrophytes.