All available evidence suggests that C4 plants have evolved from ancestors possessing the C3 pathway of photosynthesis and this has occurred independently many times in taxonomically diverse groups (3,21). At present, the precise evolutionary transition, at the anatomical, physiological, and biochemical levels, from a C3 to a C4 plant is not clear. It is generally believed that studies of C3-C4 intermediate species might provide insight into the evolution of C4 photosynthesis. In addition, since most of the world's important crops are C3 plants, there has been considerable interest in improving their productivity by screening for mutants with reduced rates of photorespiration or by incorporating C4 characteristics into C3 plants (3,19,20). Thus, the search for naturally '
Several aspects of photosynthetic adaptation to temperature were examined in four graminoid species from the Colorado shortgrass steppe. The experimental species were chosen to provide examples of a variety of in situ seasonal phenology patterns. The cool season grass, Agropyron smithii (C), exhibited higher photosynthesis rates when grown in a cool temperature regime (20/15°C), and compared to warm grown plants (35/15°C). The warm season species, Bouteloua gracilis (C) and Buchloe dactyloides (C), exhibited higher photosynthetic capacities when grown in the warmer temperature regime. The sedge, Carex eleocharis (C), which exhibits seasonal growth potential during the cool and warm portions of the growing season, exhibited a marked capacity for photosynthetic temperature acclimation. Differential effects of growth temperature on the intracellular conductance to CO appeared to have a greater regulatory role in these responses for the two C species, relative to stomatal conductance or photorespiration (O inhibition of photosynthesis). In the two C species decreases in the intracellular conductance in cool grown plants were correlated with the decreased photosynthetic capacity in normal air for B. gracilis, but not for B. dactyloides. Analysis of the Arrhenius relationship for CO saturated net photosynthesis at low leaf temperatures (4.5-17°C) indicated sharp breaks in the apparent energy of activation at 5.8-9.0°C in the warm season species B. gracilis and B. dactyloides. Leaves of A. smithii and C. eleocharis exhibited no significant low temperature limitations according to this analysis. The low temperature limitations in the warm season species were partially reflected in an inhibition of the quantum yield for CO uptake after 2 h at 5-6°C in the presence of high photon flux densities. Temperature dependent increases in the chlorophyll fluorescence yield at high temperatures revealed the lowest breakpoint values for A. smithii, and the highest values for B. gracilis. The differential patterns of temperature adaptation among the species further extend the proposal of Kemp and Williams (1980; Ecology 61:846-858) that seasonal temperature gradients in the shortgrass steppe have a regulatory role in maintaining offset patterns of resource utilization and decreasing interspecific competition.
The quantum yield for CO2 uptake was measured in C3 and C4 monocot species from several different grassland habitats. When the quantum yield was measured in the presence of 21% O2 and 340 cm(3) m(-3) CO2, values were very similar in C3 monocots, C3 dicots, and C4 monocots (0.045-0.056 mole CO2 · mole(-1) quanta absorbed). In the presence of 2% O2 and 800 cm(3) m(-3) CO2, enhancements of the quantum yield values occurred for the C3 plants (both monocots and dicots), but not for C4 monocots. A dependence of the quantum yield on leaf temperature was observed in the C3 grass, Agropyron smithii, but not in the C4 grass, Bouteloua gracilis, in 21% O2 and 340 cm(3) m(-3) CO2. At leaf temperatures between 22-25°C the quantum yield values were approximately equal in the two species.
A northern latitude population of Opuntia erinacea in eastern Washington State, U.S.A. was investigated with regard to daily and seasonal activity of Crassulacean acid metabolism and water relations. Fresh samples were collected throughout the light and dark periods on 21 dates between October 1979 and December 1980. Daily activity of CAM varied seasonally with daily maximum titratable acidity values ranging from 190 μEq·g(-1) fresh weight in October 1979 to 84 μEq·g(-1) in January 1980, to 230 μEq· g(-1) fresh weight in June 1980. Daily patterns of acid fluctuation were generally typical of CAM plants. Afternoon increases in acid concentration were noticed occasionally, and morning peaks in acidity were frequently observed. Plant water potentials were generally more closely correlated with median cladophyll temperature than with precipitation patterns, exhibiting sharply increased negativity around 6C. The data suggest the importance of available moisture and temperature in regulation of CAM, the existence of a cryoprotection mechanism involving reduction of plant water content, and the necessity of spring recovery for post-winter resumption of CAM activity.
Plants exhibiting Crassulacean acid metabolism characteristically display a 'morning burst' in the rate of CO2 uptake during the early morning period (16). This MB3 generally follows a brief, rapid reduction of CO2 fixation which is triggered by the onset of daytime light and temperature conditions. The MB is usually of short duration (20-30 min, or longer depending on growth conditions) and marks the beginning of the decarboxylation phase and of decreased net CO2 uptake (phase II,15). The metabolic activities of this transition phase are not well understood, but involve a complexity of carboxylating and decarboxylating events (7,14). A previous investigation of initial products of 14C02 fixation in Kalanchoedaigremontiana implicated PEPC I
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