Stomatal movement is an energetic oxygen-requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red-light-dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10|xmoles O2 mg ' chl h~', while guard cell protoplasts respired at rates of 200-300 (xmoles O2 mg chl"' h"', depending on the species. Lowering the oxygen concentration below 50-60 mmol m~^ resulted in a decrease in guard cell respiration rates, while rates hy mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis hy guard cell protoplasts isolated from V. faba and B. napus decreased eoncomitantly with respiration. Oligomycin, an inhihitor of oxidative phosphorylation, reduced photosynthesis in mesophyll cell protoplasts hy 27-46% and in guard cell protoplasts hy 51-58%. The reduction in hoth guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metaholic coupling hetween the two activities, possihiy mediated hy the availahility of suhstrate for respiration associated with photosynthetic electron transport activity and suhsequent export of redox equivalents.

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Thermal acclimation by Saxffraga cernua to low temperatures results in a change in the optimum temperature for gross photosynthetic activity and may directly involve the photosynthetic apparatus. In order to test this hypothesis photosynthetic electron transport activity of S. cernua thylakoids acclimated to growth temperatures of 200C and 10°C was measured in vitro. Both populations exhibited optimum temperatures for whole chain and PSII electron transport activity at temperatures close to those at which the plants were grown. Chlorophyll a fluorescence transients from 10°C-acclimated leaves showed higher rates in the rise and subsequent quenching of variable fluorescence at low measuring temperatures; 20°C-acclimated leaves showed higher rates of fluorescence rise at higher measuring temperatures. At these higher temperatures, fluorescence quenching rates were similar in both populations. The kinetics of State 1-State 2 transitions in 20°C-and 10°C-acclimated leaf discs were measured as changes in the magnitude of the fluorescence emission maxima measured at 77K. Leaves acclimated at 10°C showed a larger F730/F695 ratio at low temperatures, while at higher temperatures, 20°C-acclimated leaves showed a higher F730/F695 ratio after the establishment of State 2. High incubation temperatures also resulted in a decrease in the F695/F685 ratio for 10°C-acclimated leaves, suggesting a reduction in the excitation transfer from the light-harvesting complex of photosystem 11 to photosystem 11 reaction centers. The relative amounts of chlorophyllprotein complexes and thylakoid polypeptides separated electrophoretically were similar for both 20°C-and 10°C-acclimated leaves. Thus, photosynthetic acclimation to low temperatures by S. cernua is correlated with an increase in photosynthetic electron transport activity but does not appear to be accompanied by major structural changes or different relative amounts in thylakoid protein composition.Thermal acclimation of photosynthetic activity, usually measured as changes in the optimum temperature for photosynthesis resulting from a change in growth temperature, has been demonstrated in a number of species from diverse environments (for reviews, see Refs. 4 and 25). From an ecological viewpoint, the potential, or capacity for thermal acclimation is considered to be an important strategy for maximizing carbon fixation and seasonal net productivity, ' Supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada to WRC. 2Present address:

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Mesophyll cells were isolated rapidly from the cladophylls of Asparagus sprengeri Regel, by gentle grinding of the tissue by hand. About 60 to 65% of the leaf cells on a chlorophyll basis were released, of which 85 to 90% were cells with an intact plasmalemma. The isolated cells had photosynthetic rates of 50 to 80 μmol O2 per milligram chlorophyll per hour at pH 7.2 and 25 °C although rates of 90 to 105 μmol O2 per milligram chlorophyll per hour were recorded. The apparent Km of photosynthetic bicarbonate uptake at pH 7.2 was 176 μM in air and 104 μM under nitrogen, which corresponds to apparent Km's of CO2 fixation of 23 μM and 13.5 μM respectively. The isolated cells maintained high photosynthetic rates for 50 h when stored in the light at 4 °C and for over 160 h after storage in the dark at 4 °C.