Intercellular Diffusion Limits to CO(2) Uptake in Leaves : Studies in Air and Helox

Area of Science:

• Plant Physiology
• Photosynthesis Research
• Biophysical Chemistry

Background:

• Photosynthetic carbon assimilation is crucial for plant growth.
• Gaseous diffusion of carbon dioxide (CO2) within leaves can potentially limit this process.
• Leaf anatomy, specifically stomatal distribution (hypo- vs. amphistomatous), may influence diffusion limitations.

Purpose of the Study:

• To quantify the extent to which intercellular CO2 diffusion limits photosynthetic carbon assimilation in plants.
• To compare diffusion limitations between hypostomatous and amphistomatous leaves.
• To develop a more accurate gas-exchange equation for analyzing diffusion in different gas mixtures.

Main Methods:

• Studied eleven plant species: five hypostomatous and six amphistomatous.
• Measured photosynthetic assimilation rates in both air and helox (a helium-based gas mixture with higher CO2 diffusivity).
• Reformulated the standard gas-exchange equation to account for varying gas diffusivities.

Main Results:

• Assimilation rates increased by up to 27% in hypostomatous leaves and 7% in amphistomatous leaves when using helox compared to air.
• These findings indicate that intercellular CO2 diffusion is a significant limiting factor for photosynthesis.
• CO2 pressure is not uniform throughout the mesophyll in many leaf types.

Conclusions:

• Intercellular CO2 diffusion is a critical, often overlooked, limitation on photosynthetic rates, especially in hypostomatous leaves.
• The non-uniformity of CO2 pressure within the mesophyll must be considered in photosynthesis models.
• A new gas-exchange equation was developed for analyzing CO2 and water vapor diffusion in various gas mixtures.