Evidence for involvement of photosynthetic processes in the stomatal response to CO2

Stomatal conductance (g(s)) typically declines in response to increasing intercellular CO2 concentration (c(i)). However, the mechanisms underlying this response are not fully understood. Recent work suggests that stomatal responses to c(i) and red light (RL) are linked to photosynthetic electron transport. We investigated the role of photosynthetic electron transport in the stomatal response to c(i) in intact leaves of cocklebur (Xanthium strumarium) plants by examining the responses of g(s) and net CO2 assimilation rate to c(i) in light and darkness, in the presence and absence of the photosystem II inhibitor 3-(3,4-dichlorophenyl)1,1-dimethylurea (DCMU), and at 2% and 21% ambient oxygen. Our results indicate that (1) g(s) and assimilation rate decline concurrently and with similar spatial patterns in response to DCMU; (2) the response of gs to c(i) changes slope in concert with the transition from Rubisco- to electron transport-limited photosynthesis at various irradiances and oxygen concentrations; (3) the response of g(s) to c(i) is similar in darkness and in DCMU-treated leaves, whereas the response in light in non-DCMU-treated leaves is much larger and has a different shape; (4) the response of gs to c(i) is insensitive to oxygen in DCMU-treated leaves or in darkness; and (5) stomata respond normally to RL when c(i) is held constant, indicating the RL response does not require a reduction in c(i) by mesophyll photosynthesis. Together, these results suggest that part of the stomatal response to c(i) involves the balance between photosynthetic electron transport and carbon reduction either in the mesophyll or in guard cell chloroplasts.