CO<sub>2</sub> Demand-Supply Coordination in Photosynthesis Reflecting the Plant-Environment Interaction: Extension and Parameterization of Demand Function and Supply Function

Abstract

Photosynthesis consists of a biochemical process named demand and a CO2 diffusion process named supply function. The intersection (Ci, An) at equal to the demand function and the supply function reflects a steady state of the plant subjected to the environment. The intersections of these demand-supply functions under different photosynthetically active radiation (PAR) can be fitted to a regression line (names DSF) in which slope (ΔAn/ΔCi) can be defined as dsf. We found that DSF information was embedded in both Laisk method (CO2 response curve (A/Ci) measured at three sub-saturated PARs, and their intersections were used to estimate daytime respiration (Rd), and CO2 compensation point (Ci*) and light response curve measurements, which could be used to estimate dsf values. This study investigated the relationship between dsf and the parameters related to the biochemical process and the CO2 diffusion process of photosynthesis. The results showed that dsf was negatively correlated with gs, apparent carboxylation efficiency, and apparent quantum yield. This suggests that DSF may coordinate the influence of environmental conditions (light, CO2 and water) on photosynthesis in the biochemical and CO2 diffusion process. Moreover, dsf was independent of gas exchange measurement conditions and showed species specificity. In conclusion, we speculated that dsf seems to be a comprehensive parameter that might be related to the intrinsic adaptation mechanism of plants to environmental conditions. We proposed an auxiliary line perpendicular to the DSF and used it to improve the stability of Ci* and Rd estimated from the Laisk dataset.