Another mechanism of COp concentration is associated with C. 

 and Crassulacean-acid metabolism (CAM) plants. These pathways 

 reduce water loss via transpiration per amount of C0« fixed. In 

 plants of both groups, C0 2 is intermittently fixed by 

 phosphoenolpyruvate carboxylase (PEP) into malate. In C, plants, 

 malate is transported to a specialized tissue, decarboxylated, and 

 the C0 2 is assimilated by Rubisco. In CAM plants, initial fixation 

 occurs at night, when there is little evaporative water loss 

 through open stomatosis flow; final assimilation occurs in the same 

 cells during the day, when energy is not limited, with stomates 

 closed. 



The manifestation of both pathways is developmentally 

 programmed and, often, environmentally enhanced. Developmentally 

 and environmentally programmed sensors, transducers, signalling 

 molecules, and their target sites in membranes and in gene control 

 elements are poorly understood. Several gene probes are available, 

 and several mechanistic approaches have been initiated. Induced 

 genes appear to be characterized by cis-acting elements in their 

 promoters, which are recognized by several transacting factors. 

 The elements may confer stress-inducibility, and the factors may be 

 ubiquitous, so it is necessary to assume (an) other control 

 mechanism (s) that act on the protein factor bound to a particular 

 cis-element. 



One of the most significant model systems for these studies is 

 the induction of water-efficient CAM metabolism in the C-, succulent 

 Mesemsynanthemum crystallinum by salt and water stress (Winter, 

 1985) . The model proving most amenable to molecular evaluation of 

 the primary stress-protein response and the subsequent induction of 

 the whole suite of additional metabolic reactions used in the 

 energetically destructive CAM pathway (Bohnert et al., 1988), 

 including the primary CO^-fixing enzyme PEP carboxylase. 



Nitrogen Limitation 



The low concentration of dissolved inorganic nitrogen observed 

 in the euphotic zone of large portions of the oceans has prompted 

 biological oceanographers to hypothesize that phytoplankton growth 

 could often be limited by the availability of nitrogen. While the 

 physiological response to nitrogen limitation is well characterized 

 in marine phytoplankton, the molecular basis of nitrogen limitation 



1-3 



