Maintenance of Photosynthesis at Low Leaf Water Potential in Wheat

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Abstract
The interaction of low water potential effects on photosynthesis, and leaf K+ levels in wheat (Triticum aestivum L.) plants was studied. Plants were grown at three K+ fertilization levels; 0.2, 2, and 6 millimolar. With well watered plants, 2 millimolar K+ supported maximal photosynthetic rates; 0.2 millimolar K+ was inhibitory, and 6 millimolar K+ was superoptimal (i.e. rates were no greater than at 2 millimolar K+). Photosynthesis was monitored at high (930 parts per million) and low (330 parts per million) external CO2 throughout a series of water stress cycles. Plants subjected to one stress cycle were considered nonacclimated; plants subjected to two successive cycles were considered acclimated during the second cycle. Sensitivity of photosynthesis to declining leaf water potential was affected by K+ status; 6 millimolar K+ plants were less sensitive, and 0.2 millimolar K+ plants were more sensitive than 2 millimolar K+ plants to declining water potential. This occurred with nonacclimated and acclimated plants at both high and low assay CO2. It was concluded that the K+ effect on photosynthesis under stress was not mediated by treatment effects on stomatal resistance. Differences between the K+ treatments were much less pronounced, however, when photosynthesis of nonacclimated and acclimated plants was plotted at a function of declining relative water content during the stress cycles. These results suggest that K+ effects on the relationship between relative water content and water potential in stressed plants was primarily responsible for the bulk of the K+-protective effect on photosynthesis in stressed plants. In vitro experiments with chloroplasts and protoplasts isolated from 2 millimolar K+ and 6 millimolar K+ plants indicated that upon dehydration, K+ efflux from the chloroplast stroma into the cytoplasm is less pronounced in 6 millimolar K+ protoplasts.