Increasing water use efficiency along the C3 to C4 evolutionary pathway: a stomatal optimization perspective.

Area of Science:

• Plant Physiology
• Evolutionary Biology
• Biochemistry

Background:

• C4 photosynthesis evolved multiple times, likely driven by declining atmospheric CO2, high temperatures, and aridity.
• Understanding the evolutionary transition from C3 to C4 photosynthesis is crucial for plant adaptation to changing environments.

Purpose of the Study:

• To examine environmental factors controlling stomatal behavior and leaf-level carbon/water exchange across the C3-C4 continuum.
• To develop a unified stomatal optimization model for C3 and C4 species.

Main Methods:

• Developed and parameterized a stomatal optimization model using data from C3, C3-C4 intermediates, and C4 Flaveria species.
• Assessed traits like marginal water use efficiency and C4 pump strength across the C3-C4 evolutionary stages.
• Investigated carbon and water exchange at current and low atmospheric CO2 concentrations.

Main Results:

• Marginal water use efficiency and C4 pump strength increased significantly from early C3-C4 intermediates to C4-like intermediates with an operational C4 cycle.
• At low CO2, net photosynthetic rates increased across the C3-C4 continuum.
• Only C4-like intermediates and C4 species showed higher water use efficiencies than C3 Flaveria at low CO2.

Conclusions:

• Both marginal water use efficiency and C4 pump strength increase in C4 Flaveria, enhancing photosynthesis and water use efficiency compared to C3 species.
• The primary advantage of early C3-C4 intermediate stages is carbon-based, rather than water-related.