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

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 CO(2) uptake and by (14)CO(2) 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 CO(2) 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.