Dear Colleagues,

Quantifying the components of the photosynthetic CO₂ uptake pathway from air to the chloroplast is fundamental to understanding limitations to plant carbon assimilation and growth. These limitations to CO₂ gas exchange can occur at the canopy, crown, leaf, cell, subcellular, and molecular levels, and involve a host of plant structural and environmental variables. Physiologically, the light reactions of photosynthesis also influence the CO₂ uptake pathway by providing the energy driving carbon assimilation. Quantifying the resistance (1/conductance) of each component of the pathway from the ambient air to the chloroplast continues to be a challenge today, potentially providing specific targets for identifying limitations expressed at the molecular level. Moreover, these pathway resistors are often driven by environmental variables such as sunlight incidence, temperature, humidity, soil water and nutrient availability. The specific resistors most limiting to CO₂ uptake (e.g., stomatal versus mesophyll cell) are continuing to be evaluated and linked to molecular control mechanisms in both agricultural and native species. Additionally, the concomitant loss of water vapor accompanying CO₂ uptake may be controlled by stomata to optimize water-use efficiency. The recognition that too much sunlight can lead to severe photoinhibition of photosynthesis is now also well established. In addition, the possible use of internally generated CO₂ (e.g., from mitochondrial and photorespiration) is a continuing area of research that is being investigated with the use of natural isotopes. Similarly, the influence of stored carbon and carbon processing (respiration) on CO₂ uptake is only recently being evaluated. The effects of elevated atmospheric CO₂ on photosynthetic gas exchange is currently being contemplated both physiologically and ecologically at multiple organizational and spatial scales. The primary purpose of this Special Issue is to point out what topics in this broad area are currently unknown, but important to our understanding of plant photosynthetic gas exchange in both natural and agricultural systems.

Prof. Dr. William K. Smith
Dr. Keith Reinhardt
Guest Editors

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• Photosynthetic gas exchange
• CO₂ resistance/conductance pathway
• Carbon processing and respiration
• Internal CO₂ recycling
• Photoinhibition