Two specific areas where an understanding of rate-limiting 

 processes is crucial to the global carbon cycle are terrestrial and 

 oceanic photosynthesis. In terrestrial ecosystems, the effects of 

 both changes in temperature and of the availability of water and 

 C0 2 on higher-plant production are largely unknown. On a global 

 scale, water is the major determinant of the distribution and 

 production of terrestrial plants. Climate models suggest that a 

 major effect of long-term alteration of the radiation balance in 

 the atmosphere will be on the hydrological cycle. The ability to 

 withstand drought is highly variable among higher plants. Drought 

 stress induces changes in growth rate, morphology (root/shoot 

 ratios) , physiology (leaf conductance) , and gene expression. The 

 types of changes depend on the stage of the life cycle of the 

 plant, and the severity and duration of water deficit. Genes 

 encoding channel-forming proteins, a thiol protease, and enzymes 

 involved in osmotic adjustment are induced by a mild water deficit. 

 Protection from more severe dehydration is correlated with the 

 accumulation of a specific set of hydrophilic proteins, the 

 dehydrins, whose exact function is unknown. The genes which encode 

 these proteins are induced by the hormone abissic acid (ABA) , which 

 increases in tissues that are partially dehydrated. The DNA 

 elements which mediate responsiveness to ABA and the proteins which 

 bind to the ABA, cis-elements, have been identified. In situ 

 measurements of proteins, and their mRNAs, will be diagnostic 

 markers of water stress and will provide an understanding of how 

 these genes are regulated in nature. 



The effects of increased CO- levels on plant growth may also 

 be beneficial; CO,, itself may limit photosynthesis in terrestrial 

 plants. In natural ecosystems, secondary limiting factors, such as 

 potassium, sulfur, or nitrogen, may retard or eliminate any 

 increase in production resulting from C0 2 fertilization. These 

 factors have specific molecular diagnostic markers, which may 

 reveal the limitation of carbon fixation by plants in nature. 

 Little is understood about the overall short- and' long-term 

 coordinate adaption of plants to various changes in nutrients and 

 other environmental conditions. 



About 50% of global photosynthesis occurs in the oceans. 

 Identifying the factors which limit oceanic photosynthesis has 

 occupied the attention of biological oceanographers for three 

 decades. Traditional methods for determining the limiting factor 



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