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Environmental and Stress Physiology

Modulation of Water Stress Effects on Photosynthesis by Altered Leaf K⁺¹

Department of Horticulture and Forestry, Cook College, Rutgers University, New Brunswick, New Jersey 08903

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.