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

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 Ktgrown plants were not higher than the 2 millimolar K -grown wheat, and rates were inhibited below 0.5 millimolar KV. Photosynthesis was measured by both attached whole leaf CO2 uptake and by '4C02 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 83% 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 KV. 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.Inhibition of photosynthesis in isolated chloroplasts, protoplasts, and leaf slices by dehydration in hypertonic solutions (4,16) diating water stress inhibition of the photosynthetic process.One line of evidence suggests that the stromal pH is too low to support optimal photosynthetic rates in dehydrated plastids, and that cellular processes which facilitate stromal alkalization may modulate stress effects on chloroplasts. Inhibition of photosynthesis of isolated chloroplasts in hypertonic medium was partially reversed by addition of K+ and NH4Cl (2, 3), which raise stromal pH (13,20

Maintenance of Photosynthesis at Low Leaf Water Potential in Wheat

“…Aliquots of protoplasts were then added to resuspension medium which was brought to -1.3, -1.8, -2.3, or -2.8 MPa with betaine. After 5 min incubation in media of varying osmotic strength, protoplast volume, protoplast K+, and chloroplast K+ were measured using silicone oil microcentrifugation techniques (17). For determination of chloroplast K+, protoplasts were passed through a 10 um nylon net attached to the end of a 1 mL syringe six times prior to microcentrifugation for 30 s in a Beckman B microcentrifuge.…”

“…For determination of chloroplast K+, protoplasts were passed through a 10 um nylon net attached to the end of a 1 mL syringe six times prior to microcentrifugation for 30 s in a Beckman B microcentrifuge. Control experiments were performed to ensure the following: (a) virtually all protoplasts were broken at all treatments and at all Is after six passes through a 10 Mm net; (b) replacement of the standard osmoticum (sorbitol) with betaine (this was done in order to allow the optimal silicone oil mixture [17,22] to support media with an osmotic strength up to -2.8 MPa) did not alter protoplast or chloroplast K+ at a given Is; and (c) after microcentrifugation of intact protoplasts, no cytoplasm (i.e. phosphoenol pyruvate carboxylase activity) was recovered in the supernatant above the silicone oil layer.…”

“…Leaf 4fI', and Is were measured using a pressure chamber, and thermocouple psychrometers, respectively, as described previously (17). RWC was determined by measuring the fresh, rehydrated (turgid), and dry weights of leaf discs (21).…”

“…Pots were watered as described previously (17) with complete fertilizer containing either 0.2, 2, or 6 mm K+ supplied as K2SO4. These three fertilizer regimes were the treatments used in all studies.…”

“…Previous work in this laboratory (6,17) (6,17), they found no effect of altered K+ status on C02-and light-saturated photosynthesis. However, Rao et al indicated that due to variability in the leaf K+ content in their experiments, they could not completely rule out a role of K+ in alleviating stress effects on chloroplasts.…”