C4 photosynthesis and water stress

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Abstract
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. 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. 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.