George J. Williams

Photosynthetic adaptation to temperature in four species from the Colorado shortgrass steppe: a physiological model for coexistence

SummarySeveral 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 (C3), 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 (C4) and Buchloe dactyloides (C4), exhibited higher photosynthetic capacities when grown in the warmer temperature regime. The sedge, Carex eleocharis (C3), 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 CO2 appeared to have a greater regulatory role in these responses for the two C3 species, relative to stomatal conductance or photorespiration (O2 inhibition of photosynthesis). In the two C4 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 CO2 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 CO2 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.

Field measurements of photosynthesis, water-use efficiency, and growth in Agropyron smithii (C3) and Bouteloua gracilis (C4) in the Colorado shortgrass steppe

SummaryField measurements of gas exchange and growth were conducted on a C3 grass,Agropyron smithii, and a C4 grass,Bouteloua gracilis, in order to further establish the adaptive significance of the C4 pathway under natural conditions. Maximum rates of leaf area expansion in tillers and maximum seasonal photosynthesis rates of both species occurred during the cool, early summer month of June. The occurrence of maximum growth and photosynthesis inB. gracilis during this cool period was apparently related to its occupation of warm microenvironments next to the ground surface. As temperatures increased during the midsummer, photosynthesis rates decreased to 47% and 55% of the seasonal maximum inB. gracilis andA. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher forB. gracilis during June, the period of maximum growth. By mid-July, however, leaves of the C3 grass,A. smithii, exhibited water-use efficiencies approximately half as high asB. gracilis. These differences in water-use efficiency were the result of differences in stomatal conductance, rather than differences in daily CO2 uptake rates which were similar in both species. The results demonstrate that in certain environments there are no offset periods of growth and maximum photosynthesis during the growing season in these C3 and C4 species. The greater amounts of daily water use inA. smithii during the midsummer might contribute to its much greater abundance in lowland sites in the shortgrass steppe. The C4 grass,B. gracilis, occurs in dry upland sites in addition to the more mesic lowland sites.

A correlation between photosynthetic temperature adaptation and seasonal phenology patterns in the shortgrass prairie

SummaryThe temperatures at which chlorophyll fluorescence yield is substantially increased and the temperatures at which the quantum yield for CO2 uptake is irreversibly inhibited were measured for three shortgrass prairie species. The experimental taxa include, a cool season species (Agropyron smithii), a warm season species (Bouteloua gracilis), and a species which grows throughout the cool and warm seasons (Carex stenophylla). Agropyron smithii exhibited lower high temperature damage thresholds (43°C in cool grown plants, 46°C in warm grown plants), relative to the other two species. Bouteloua gracilis exhibited the highest tolerance to high temperature, with threshold values being 44–49°C for cool grown plants and 53–55°C for warm grown plants. Carex stenophylla exhibited threshold values which were intermediate to the other two species (43–47°C for cool grown plants, and 51–53°C for warm grown plants). Seasonal patterns in the fluorescence rise temperatures of field grown plants indicated acclimation to increased temperatures in all three species. The results demonstrate a correlation between the high temperature thresholds for damage to the photosynthetic apparatus, and in situ seasonal phenology patterns for the three species.

The quantum yield for CO2 uptake in C3 and C4 grasses

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 cm3 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 cm3 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 cm3 m-3 CO2. At leaf temperatures between 22–25°C the quantum yield values were approximately equal in the two species.

Temperature and Salinity Regulation of Growth and Gas Exchange of Salicornia fruticosa (L.) L.

SummarySalicornia fruticosa was collected from a salt marsh on the Mediterranean sea coast in Libya. Growth and gas exchange of this C3 species were monitered in plants pretreated at various NaCl concentrations (0, 171, 342, 513 and 855 mM). Maximum growth was at 171 mM NaCl under cool growth conditions (20/10° C) and at 342 mM NaCl under warm growth conditions (30/15° C) with minimum growth at 0 mM NaCl (control). Net photosynthesis (Pn) was greatest in plants grown in 171 mM NaCl with plants grown at 513 and 855 mM having lowest rates. Maximum Pn was at 20–25° C shoot temperatures with statistically significant reductions at 30° C in control plants while salt treated plants showed such reductions at 35° C. Salt treatments increased dark respiration over the control at 171 and 342 mM but reduced it at higher concentrations. Photorespiration was reduced by salt treatment and increased by increasing shoot temperature. Greatest transpiration was in 171 mM NaCl treated plants and increasing shoot temperature increased transpiration in all treatments. Stomatal resistance to CO2 influx was influenced only moderately by temperature while increasing salinity resulted in increased stomatal resistance. In general both temperature and salinity increased the mesophyll resistance to CO2 influx. The species seems adapted to the warm saline habitat along the Mediterranean sea coast, at least partially, by its ability to maintain relatively high Pn at moderate NaCl concentrations over a broad range of shoot temperatures.

Simultaneous Measurement of Leaf and Root Gas Exchange of Shortgrass Prairie Species

Agropyron smithii, C3, and Bouteloua gracilis, C4, were compared using infrared gas analysis to measure gas exchange of leaves and roots. Photosynthetic response to quantum irradiance was measured at two analysis temperatures, 20 and 30 C, for two temperature pretreatments, 20/15 C and 35/15 C. Saturation for A. smithii occurred between 900 and 1,200 μEinsteins (μE)·m-2·s

Diurnal and seasonal variations in activity of crassulacean acid metabolism and plant water status in a northern latitude population of Opuntia erinacea

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Temperature response of CO2 fixation in isolated Opuntia cells

for all combinations of pretreatment and analysis temperatures; B. gracilis exhibited saturation at a similar quantum flux for all combinations except plants grown at 35/15 C and analyzed at 30 C, which required 2,000 μE·m-2·s

A morning peak in acidity during phase II crassulacean acid metabolism in Opuntia erinacea, echinocereus echinocereus and Mammillaria vivipara

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A Physiological Basis for Niche Separation Between Agropyron Smithii (C3) and Bouteloua Gracilis (C4)

. Comparison of the species grown at 20/15 C shows that rates of net photosynthesis, transpiration, photorespiration, and dark respiration were greatest in A. smithii. Resistance to CO2 flux by stomates is greater in B. gracilis than in A. smithii, while the inverse was found for mesophyll resistance. Root respiration of both species was measured simultaneously with net photosynthesis and dark respiration; neither species showed great changes in root respiration if leaves were in the dark or in the light. Changes of leaf temperature had little effect on root respiration. A slight trend for a decrease in root respiration with time in the dark period was noted in both species. Root respiration was slightly greater in the C3 plants. Root respiration was not particularly sensitive to CO2 concentration in the root media over the range of 300-3,000 ppm.