Photosynthesis of salt marsh species

Abstract

The salt marsh environment is severe. Stressful edaphic factors include high and variable salinity, a strongly reducing environment, anoxia, and toxic compounds such as hydrogen sulfide. Mechanisms which adapt endemic species to these conditions include osmoregulation, anaerobic metabolism, aerenchyma, xeromorphism, tolerance of salt and water stress, and high water and nitrogen use efficiency. Species having the C 3 and C 4 photosynthetic pathway are common in salt marshes. Succulents are also present although none are known to operate the crassulacean acid metabolism. Photosynthetic rates measured in situ are relatively low, but high rates have been measured in laboratory-grown plants and this suggests that some or all of the above mentioned factors control photosynthesis in the field. Nutrient status may regulate the photosynthetic apparatus through effects on the metabolic components of carbon reduction and on the capacity of the plant to control water balance. Field and laboratory studies show that light and CO 2 saturated photosynthetic capacity decreases and, in C 4 species, the CO 2 compensation point increases under nitrogen limiting conditions. Water stress regulates growth and photosynthesis. Photosynthetic responses to increasing salinity are highly species specific, but within species the responses depend on light, temperature and nutrient status during growth. Both the stomatal conductance of CO 2 and the metabolic determinants of photosynthesis are affected by salinity but the available data suggest that the primary effect of salinity is on the metabolic components.