Estimating the internal conductance to CO₂ movement

Area of Science:

• Plant Physiology
• Photosynthesis Research
• Biophysical Chemistry

Background:

• Chloroplast CO₂ concentration is lower than atmospheric due to gas and liquid phase resistances.
• Historically, chloroplast CO₂ was assumed equal to intercellular CO₂, but evidence shows otherwise.
• Internal conductance (g_i) quantifies CO₂ decrease from intercellular spaces to chloroplasts.

Purpose of the Study:

• To describe the theoretical basis and protocols for estimating internal conductance (g_i).
• To evaluate common and alternative methods for measuring g_i.
• To highlight limitations and suggest improvements for accurate g_i estimation.

Main Methods:

• Simultaneous measurements of gas exchange and chlorophyll a fluorescence.
• Instantaneous discrimination against ¹³CO₂.
• Alternative methods: CO₂ photocompensation points, A/C_i curve curvature, and initial A/C_i slope vs. A/C_c slope.

Main Results:

• Common g_i estimation methods exhibit poor precision (RSD > 10%).
• Precision of alternative methods is less understood.
• Accuracy of all methods is largely unknown due to shared assumptions.

Conclusions:

• Internal conductance (g_i) is a significant limitation to photosynthesis.
• Current methods for estimating g_i require further validation and improvement.
• Using multiple methods with different assumptions and ensuring accurate A and C_i measurements can improve confidence in g_i estimates.