Effects of salt stress on photosynthesis under ambient and elevated atmospheric CO2 concentration

Abstract

Soil salinization is one of the most important factors which limit plant productivity. About 3.6 billion of the world's 5.2 billion hectares of dryland used for agriculture have already suffered erosion, soil degradation, and salinization. Global climate change caused by rising atmospheric trace gases such as CO2 and forced migration add to the urgency of this global problem. Therefore, solutions are desperately needed, such as the improvement of drought and salinity resistance of crops or the use of (xero-) halophytes instead of glycophytic crops. As photosynthesis is a prerequisite for biomass production, this chapter focuses on information related to this essential sequence of reactions, thereby discussing the different levels of photosynthesis. At first, there are primary reactions of photosynthesis, namely absorption of light energy and (1) its conversion to redox energy, conserved in the coenzyme NADPH, and (2) energy of chemical bounds, conserved in the coenzyme ATP. On the second level, we find reactions of the Calvin cycle, nitrate and sulfate reduction as well as sugar, lipid, and amino acid metabolism. Typical reactions on the third level are transmembrane and inter tissues transport of metabolites. The fourth level of photosynthesis relates to physiological aspects of gas exchange and water relations. Apart from these general effects of salinity on photosynthesis, we will review the probable photosynthetic performance of salt stressed plants under future atmospheric conditions, namely under elevated CO2 concentration. Special emphasis will be put on gas exchange and photosynthesis of C3 and C4 plants because these two photosynthesis types show different responses to elevated CO2, leading to different interactions with salinity.