We discuss the physical and enzymatic bases of carbone isotope discrimination during photosynthesis, noting how knowledge of discrimination can be used to provide additional insight into photosynthetic metabolism and the environmental influences on that process

Carbon Isotope Discrimination and Photosynthesis

Carbon isotope fractionation in plants

he efficiency of photosynthesis continues to interest biochemists, biologists, and plant physiologists. Scientists interested in CO2 uptake are concerned about the extent to which the uptake rate is limited by such factors as stomatal diffusion and the chemistry of the CO2 absorption process. The fractionation of carbon isotopes that occurs during photosynthesis is one of the most useful techniques for investigating the efficiency of CO2 uptake. Atmospheric carbon dioxide contains approximately 1.1% of the nonradioactive isotope carbon-13 and 98.9% of carbon-12. During photosynthesis, plants discriminate against C because of small differences in chemical and physical properties imparted by the difference in mass. This discrimination can be used to assign plants to various photosynthetic groups. The isotope fractionation also reflects limitations on photosynthetic efficiency imposed by the various diffusional and chemical components of CO2 uptake. When analyzed in detail, this fractionation provides information .about water use efficiency and indicates that different strategies are needed for improving wateruse efficiency in different kinds of plants. Isotope fractionation in simple physical and chemical processes is well understood and is commonly Current studies include

Carbon Isotopes in PhotosynthesisFractionation techniques may reveal new aspects of carbon dynamics in plants

The stable isotopic composition of modern soil carbonate and its relationship to climate

In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a “pulse” of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.

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Precipitation pulses and carbon fluxes in semiarid and arid ecosystems

Fertilization of cotton ovules was prevented by removal of styles and stamens on the morning of anthesis. Forty-eight hr later ovaries were harvested and ovules were aseptically transferred to liquid culture medium supplemented with various plant growth substances. In the absence of phytohormones, ovules browned and failed to increase in size or produce fibers. Indoleacetic acid and gibberellic acid provided for ovule growth and fiber development. Kinetin provided for ovule growth only. The ovule's capacity for indoleacetic acidor gibberellic acid-stimulation of fiber development was reduced by high concentrations of kinetin or abscisic acid. Low concentrations of kinetin partially reversed the inhibitory effect of

Effects of plant growth substances on in vitro fiber development from unfertilized cotton ovules

A detailed comparison of green leaf phosphoenolpyruvate carboxylases from the C4-species Atriplex spongiosa and the C3-species Atriplex hastata revealed significant physical and kinetic differences. The two alloenzymes can be separated by anion exchange chromatography but have comparable molecular weights (350,000). Maximal velocity estimates were 38.0 and 1.48 micromoles per minute per milligram of chlorophyll for the carboxylases of A. spongiosa and A. hastata, respectively. Km phosphoenolpyruvate estimates were 0.49 and 0.08 mm for the C4A. spongiosa and C3A. hastata and the Km Mg estimates were 0.33 mm for the C4 species and 0.017 mm for the C3 species. The activity of the phosphoenolpyruvate carboxylase of A. spongiosa is more sensitive to chloride and phosphate than the phosphoenolpyruvate carboxylase of A. hastata, but both are equally sensitive to Mg chelating substances such as ATP, ADP, and citrate if assayed at their respective Km Mg values. A survey of the phosphoenolpyruvate carboxylases from 18 C4 and C3 species resulted in mean maximal velocity estimates of 29.0 ± 13.2 and 1.50 ± 0.57 micromoles per minute per milligram of chlorophyll for the C4 species and C3 species, respectively. Km phosphoenolpyruvate estimates were 0.59 ± 0.35 mm and 0.14 ± 0.07 mm for the C4 and C3, and Km Mg estimates were 0.50 ± 0.30 and 0.097 ± 0.057 mm for C4 and C3. All differences between means were significant at the 0.01 confidence level, supporting our hypothesis that the phosphoenolpyruvate carboxylase alloenzymes of C4 and C3 plants are functionally different and are associated with different photosynthetic roles. Both function in the photosynthetic production of C4 acids, the phosphoenolpyruvate carboxylase of C4 species largely producing malate or aspartate (or both) as a photosynthetic intermediate and the phosphoenolpyruvate carboxylase of C3 species producing malate or aspartate (or both) as a photosynthetic product.

http://www.plantphysiol.org/content/plantphysiol/51/3/439.full.pdf

Photosynthetic phosphoenolpyruvate carboxylases: characteristics of alloenzymes from leaves of c(3) and c(1) plants.

Contrasting metabolic regimes operate in Opuntia basilaris Engelm. and Bigelov, before and after precipitation. During periods of drought, atmospheric CO2 exchange and transpiration are greatly reduced throughout the day/night cycle by stomatal closure and a highly impervious cuticle. The hypothesis is that endogenously produced CO2 is retained and recycled through dark CO2 fixation, organic acid transformations, photosynthesis, and respiration. Immediately following precipitation, nighttime stomatal opening is initiated, permitting increased atmospheric CO2 assimilation and organic acid synthesis.

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Drought Adaptation in Opuntia basilaris: Significance of Recycling Carbon through Crassulacean Acid Metabolism

Measurements of internal gas phase CO(2) concentration, stomatal resistance, and acid content were made in Crassulacean acid metabolism plants growing under natural conditions. High CO(2) concentrations, sometimes in excess of 2%, were observed during the day in a range of taxonomically widely separated plants (Opuntia ficus-indica L., Opuntia basilaris Engelm. and Bigel., Agave desertii Engelm., Yucca schidigera Roezl. ex Ortiges, Ananas comosus [L.] Merr., Aloe vera L., Cattleya sp. and Phalanopsis sp.) and below ambient air concentrations were observed at night.Stomatal resistance was always high when CO(2) concentration was high and experiments in which attempts were made to manipulate internal CO(2) concentrations gave data consistent with stomatal behavior in Crassulacean acid metabolism being controlled by internal CO(2) concentration. Exogenous CO(2) applied in darkness at a concentration similar to those observed in the light caused stomatal resistance to increase.In pads of Opuntia basilaris Engelm. and Bigel. subjected to severe water stress internal gas phase CO(2) concentrations exhibited fluctuations opposite in phase to fluctuations in acid content. Stomatal resistance remained high and the opening response to low CO(2) concentration was almost entirely eliminated.

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Relationships between Stomatal Behavior and Internal Carbon Dioxide Concentration in Crassulacean Acid Metabolism Plants

The physical and kinetic properties of multiple forms of phosphoenolpyruvate carboxylase were studied in leaves of C4 and C3 species, their F1 and F3 hybrids, in greening maize leaves, in Crassulacean acid metabolism plants, and in nongreen root tissues Four different forms are suggested: a C4 photosynthetic phosphoenolpyruvate carboxylase with high Km for phosphoenolpyruvate (∼059 mm), Km Mg (∼05 mm), and Vmax (∼29 micromoles per minute per milligram of chlorophyll); a C3 photosynthetic phosphoenolpyruvate carboxylase with low Km for phosphoenolpyruvate (∼014 mm), Km for Mg (∼0097 mm), and Vmax (15); a Crassulacean acid metabolism type with low Km for phosphoenolpyruvate (014 mm), and high Vmax (14 micromoles per minute per milligram of chlorophyll); and a nongreen or nonautotrophic type with low Km for phosphoenolpyruvate, Km for Mg, and low Vmax In closely related species or within species, the types can be differentiated by anion exchange column chromatography Each of the four forms is associated with a different metabolic pathway: the phosphoenolpyruvate carboxylase of C4 species for malate generation as a photosynthetic intermediate, the phosphoenolpyruvate carboxylase of C3 species in malate generation as a photosynthetic product, the phosphoenolpyruvate carboxylase of Crassulacean acid metabolism species in malate generation as a CO2 donor for photosynthesis during the subsequent light period, and a nongreen or root type producing malate for ionic balance and reduced nicotinamide adenine dinucleotide phosphate generation The data in this paper in conjunction with published information support the notion of different molecular forms of a protein functioning in different metabolic pathways which have common enzymic steps

Irwin P. Ting

Papers

Effects of plant growth substances on in vitro fiber development from unfertilized cotton ovules

Photosynthetic phosphoenolpyruvate carboxylases: characteristics of alloenzymes from leaves of c(3) and c(1) plants.

Drought Adaptation in Opuntia basilaris: Significance of Recycling Carbon through Crassulacean Acid Metabolism

Relationships between Stomatal Behavior and Internal Carbon Dioxide Concentration in Crassulacean Acid Metabolism Plants

Multiple Forms of Plant Phosphoenolpyruvate Carboxylase Associated with Different Metabolic Pathways