Saxifraga cernua, a perennial herb distributed throughout the arctic and subarctic regions, shows high levels of dark respiration. The amount of respiration exhibited by leaves and whole plants at any temperature is influenced by the pretreatment temperature. Plants grown at 10°C typically show higher dark respiration rates than plants grown at 20°C. The levels of alternative‐pathway respiration (or cyanide‐insensitive respiration) in leaves of S. cernua grown at high and low temperatures were assessed by treating leaf discs with 0.25 mol m−3 salicylhydroxamic acid during measurements of oxygen consumption. Alternative pathway respiration accounted for up to 75% of the total respiration. Tissues from 20°C‐grown plants yielded a Q10 of 3.37 for normal respiration, and of 0.97 for alternative‐pathway respiration. Tissues from 10°C‐grown plants yielded a Q10 of 2.55 for normal respiration, and of 0.79 for alternative‐pathway respiration. The alternative pathway does not appear to be as temperature sensitive as the normal cytochrome pathway. A simple energy model was used to predict the temperature gain expected from these high rates of alternative‐pathway respiration. The model shows that less than 0.02°C can be gained by leaves experiencing these high respiration rates.

Closure of stomata caused by low (10(-7)M) concentrations of abscisic acid (ABA) is specific for cis-trans ABA, and is initiated within 5 minutes. Upon withdrawal of the hormone supply, reopening starts within 5 minutes. Gas analysis of leaves treated with ABA or DCMU allows one to distinguish effects on the stomatal apparatus from inhibition of photosynthesis and to conclude that ABA acts on stomata directly.

In excised coleoptiles, it has been repeatedly shown that growth rate is promoted by the presence of auxin and by increase in turgor (achieved by reducing the amount of osmoticum in the medium). Any formal general statement about the mechanism of growth has to account for both kinds of promotion of rate.Several possibilities for the joint action of pressure and auxin on growth can be seen in Figure 1 and the simple formulaHere r is rate, P is turgor pressure (bar), Y is a yield threshold, a pressure (bar) below which no extension occurs and m is an "extensibility," the slope relating rate to (P -Y. Figure 1, the mode of action should be easily ascertained. If the true relation were concave upward, the mode of action of auxin action (curve shifting versus curve steepening) would be less obvious (12). Difficulty in answering this question centers on the determination of the rate to be considered typical for a given turgor value.Treatments in Parallel. If sections are cut and floated for several hours on a concentration series of solutions, rate can be determined as the length increase observed, divided by time. Unfortunately the quotient is a function of (a) the course of osmo-elastic equilibration of the tissue's turgor pressure to the new value, (b) any transient adjustment of the irreversible growth rate to the turgor shift (as found in the present study), (c) time course of loss of endogenous IAA in the -IAA experiment, (d) effects of aging, and (e) possible slow osmotic adjustment of the tissue cells. Ordin et al (21) give the time course of the approach to a steady elongation rate, seen in a range of mannitol concentrations. Rate at a given apparent turgor pressure changes with time, and this drift is not parallel for the various solutions. The same is seen in the data of Bennett-Clark (1). When rate is taken as mean rate over the entire treatment in such studies the results indicate that steady rate is increased by auxin, in terms of Figure 1, through an increase in m, (5,17). That is, IAA increases the relative ability of turgor pressure to increase rate.Treatments in Series. With continuous measurement of coleoptile length (singly or in the aggregate) one can attempt to measure the rate versus turgor relation by imposing a sequence of turgor shifts on the same tissue. This method could reveal details lost by averaging in the above method. A problem lies in choosing the time, after turgor shift, for taking steady rate. There is the possibility that an observed steady rate is really a composite phenomenon where, e.g., a decaying elastic shrinkage is balanced by a decaying acceleration, this latter being part of the irreversible growth response to the shift (6). Data taken in this way have suggested, in contrast to measurements in parallel, that auxin increases steady rate primarily by the lowering of Y, in equation 1 (8)

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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