Photosynthesis and Photorespiration- II. C4 Plants: Advantages and Paradoxes

Abstract

After a brief description of the main characteristics of C3 plants, a review on the main features of C4 plants with benefits such as improved photosynthetic performance under high temperature and strong light, better water and nitrogen use efficiency was consid­ered essential to discuss. The advantages of C4 plants are mainly the result of a system combining a CO2 pump and an anatomical specialization of differentiated cells in the leaves. These theoretical benefits were compared with results of agronomy and eco-physiological analysis to reveal the paradoxical correlation between water use efficiency (WUE) and a lower resistance to severe water limitations and a low dominance in hot and arid sunny regions for non-grass plant species. The main differences between the C3 and C4 photosynthetic systems are illustrated by unidirectional exchanges of oxygen. In contrast to C3 plants, the low level of photorespiration results in a low O2 uptake and a lack of complementary variation in reactions of carboxylation and oxygenation. This ‘mirror effect’ illustrates the role of a ‘sink’ for photochemical energy that might indicate a recognized positive role of photorespiration during water limitation, and probably during variations of bright light. This absence may cause the paradox stated above. An evolutionary analysis of the kinetics of Rubisco, the enzyme that fix CO2, shows that Rubisco of C4 plants can be considered relatively poorly advanced with a low ratio (Vo:Vc) of the maximum rate of oxygenation (Vo) to carboxylation (Vc). This ratio is considered to be a factor that characterizes, in higher plants, the protective role of photorespiration.