ABSTRACT
C3, C4, and crassulacean acid metabolism (CAM) are the three types of photosynthetic pathways in plants. These differ structurally, functionally, and also in terms of response to various environmental stresses. C4 plants are said to have emerged in somewhat arid regions, characterized by high temperatures in addition to water stress. C4 plants have two types of cells arranged in a specialized Kranz-type anatomy wherein mesophyll cell chloroplasts perform the light-dependent reactions, and the bundle sheath cells perform the light independent reaction. C4 plants show high water-use efficiency (WUE), as they can perform photosynthesis even at low intercellular CO2 concentrations. Besides, the efficient use of RuBisCO suppresses photorespiration, thereby improving the nitrogen-use efficiency (NUE). In addition to C4 metabolism, efficient antioxidant defense system also contributes to stress tolerance. CAM plants have adapted to severe water deficit in semiarid and arid regions of the world, including deserts. This involves a special type of photosynthetic adaptation that allows CO2 uptake at night when the 164stomata are open and its fixation during the day when the stomata are closed. These characteristics allow CAM plants to use 80% less water than C3 plants in order to produce similar amount of biomass. High temperature and salinity tolerance is beneficial in fulfilling the agricultural demands during global climate change scenarios. Many stress-adaptive responses involve abscisic acid (ABA)-dependent and ABA-independent pathways which are involved in the stress signaling of CAM plants. Stress adaptation in C4 and CAM plants also includes accumulation of osmoprotectants, enhanced antioxidant system, and development of specialized structures. This chapter explains the numerous adaptive mechanisms of C4 and CAM plants toward various environmental stresses.
