Modulation of Water Stress Effects on Photosynthesis by Altered Leaf K+

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Abstract
Wheat irrigated with nutrient solutions containing 0, 0.2, 0.5, 1, 2, or 6 millimolar K+ had maximum photosynthetic rates at 1 to 2 millimolar K+ concentrations. Rates in the 6 millimolar K+-grown plants were not higher than the 2 millimolar K+-grown wheat, and rates were inhibited below 0.5 millimolar K+. Photosynthesis was measured by both attached whole leaf CO2 uptake and by 14CO2 fixation of leaf slices in solution. Exposure of leaf slices from 0.2, 2, and 6 millimolar K+-grown wheat to various assay media water potentials showed that photosynthesis of the 0.2 millimolar K+-grown wheat decreased from control (high water potential) rates by 35%, that of the 2 millimolar K+-grown wheat by 20.4%, and that of the 6 millimolar K+-grown wheat by only 8.3% at −3.11 megapascals. Also, photosynthesis of the 6 millimolar K+-grown wheat was enhanced by 28% over that of the 2 millimolar K+ wheat at the most severe water stress (−3.11 megapascals), indicating that the excess leaf K+ in the 6 millimolar K+-grown wheat partially reversed dehydration effects on photosynthesis. Oligomycin eliminated the protective effects of high K+ on photosynthesis in dehydrated leaf slices. These results suggest that the protective effect of high K+ under water stress may involve the exchange of K+ in the cytoplasm for stroma H+, thus altering stromal pH and restoring photosynthesis. The protective effect of high K+ was also observed in attached whole leaf photosynthesis of in situ water-stressed wheat grown on 0.2, 2, and 6 millimolar K+. Under water stress, rates of the 6 millimolar K+-grown wheat were enhanced by 66.2% and 113.9% over that of 2 millimolar K+-grown wheat in two separate experiments. Internal CO2 concentration of the 6 millimolar K+-grown wheat was lower than that of the 0.2 and 2 millimolar K+-grown wheat. These results suggest that the high K+ effects on chloroplast photosynthesis seen in leaf slices also occur at the whole plant level.