C₄ photosynthesis and water stress

• Published in: Annals of botany Vol. 103; no. 4; pp. 635 - 644
• Main Author: Ghannoum, Oula
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01.02.2009
• Subjects: C3 and C4 photosynthesis; C4 photosynthesis; C4 plants; Carbon - metabolism; Carbon Dioxide - metabolism; Corn; Dehydration; Drought; Droughts; Enzymes; high CO2; Leaves; Photorespiration; Photosynthesis; Plant Stomata - metabolism; Plants; REVIEW; Reviews; stomatal and non-stomatal limitation; Water - physiology; water stress; and C; water stress; high CO; C; photosynthesis; stomatal and non-stomatal limitation
• ISSN: 0305-7364; 1095-8290
• DOI: 10.1093/aob/mcn093

BACKGROUND: In contrast to C₃ photosynthesis, the response of C₄ photosynthesis to water stress has been less-well studied in spite of the significant contribution of C₄ plants to the global carbon budget and food security. The key feature of C₄ photosynthesis is the operation of a CO₂-concentrating mechanism in the leaves, which serves to saturate photosynthesis and suppress photorespiration in normal air. This article reviews the current state of understanding about the response of C₄ photosynthesis to water stress, including the interaction with elevated CO₂ concentration. Major gaps in our knowledge in this area are identified and further required research is suggested. SCOPE: Evidence indicates that C₄ photosynthesis is highly sensitive to water stress. With declining leaf water status, CO₂ assimilation rate and stomatal conductance decrease rapidly and photosynthesis goes through three successive phases. The initial, mainly stomatal phase, may or may not be detected as a decline in assimilation rates depending on environmental conditions. This is because the CO₂-concentrating mechanism is capable of saturating C₄ photosynthesis under relatively low intercellular CO₂ concentrations. In addition, photorespired CO₂ is likely to be refixed before escaping the bundle sheath. This is followed by a mixed stomatal and non-stomatal phase and, finally, a mainly non-stomatal phase. The main non-stomatal factors include reduced activity of photosynthetic enzymes; inhibition of nitrate assimilation, induction of early senescence, and changes to the leaf anatomy and ultrastructure. Results from the literature about CO₂ enrichment indicate that when C₄ plants experience drought in their natural environment, elevated CO₂ concentration alleviates the effect of water stress on plant productivity indirectly via improved soil moisture and plant water status as a result of decreased stomatal conductance and reduced leaf transpiration. CONCLUSIONS: It is suggested that there is a limited capacity for photorespiration or the Mehler reaction to act as significant alternative electron sinks under water stress in C₄ photosynthesis. This may explain why C₄ photosynthesis is equally or even more sensitive to water stress than its C₃ counterpart in spite of the greater capacity and water use efficiency of the C₄ photosynthetic pathway.