light, and pressure. 



C0 2 Limit at ion/ Water Limitation 



Photosynthetic organisms play a major role in the global 

 carbon cycle by depleting the atmospheric pool of C0«. For 

 photosynthetic carbon fixation to reduce atmospheric C0 2 , the 

 external factor (s) which limit photosynthetic processes must be 

 alleviated. On geological time scales, however, global 

 photosynthetic processes reach a relative steady-state with 

 respiratory processes. An understanding of the factors limiting 

 primary production is essential to understanding the transient 

 response to anthropogenic C0 ? emissions. 



In the terrestrial environment, the major factors limiting 

 biomass production are C0 ? concentrations, water and nutrient 

 availability, fluctuating salinity levels, and temperature. The 

 global distribution of terrestrial plants is related to the 

 availability of water more than to any other single factor. In 

 fact, all three factors limit the availability of water for 

 physiological reactions. Operationally, CO- levels, drought, 

 salinity, and temperature are signals that elicit specific 

 responses; only the persistence of the signal causes stress. 



One aspect of study must be the pathways by which these 

 signals are sensed and transmitted, what the transmitters are, and 

 how they interact with receptors and target sites to elicit 

 specific physiological and molecular responses. In plants, such 

 stresses involve the hormone, ABA. In all plants studied, these 

 stresses induced the expression of "lea" proteins (late 

 embryogenesis abundant in seeds) in vegetative tissues. The 

 proteins (and their mRNAs) are markers of water stress. 



Ribulose bisphosphate carboxylase (Rubisco) is a relatively 

 inefficient enzyme for C0 ? fixation. For example, many 

 cyanobacteria have developed a CO- concentration mechanism, whose 

 elucidation was critically dependent on molecular technology (Price 

 and Badger, 1989) . Its presence can be inferred from a few 

 proteins for which there are antibody probes. The number of 

 different cyanobacteria species with functional carboxysomes should 

 be an indicator that CO- is a limiting factor in marine systems 

 which may seasonally dominate C-f luxes at some latitudes. 



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