Evidence for Involvement of Photosynthetic Processes in the Stomatal Response to CO

Stomatal conductance (g[subscript s]) typically declines in response to increasing intercellular CO₂ concentration (c[subscript i]). However, the mechanisms underlying this response are not fully understood. Recent work suggests that stomatal responses to c[subscript 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[subscript i] in intact leaves of cocklebur (Xanthium strumarium) plants by examining the responses of g[subscript s] and net CO₂ assimilation rate to c[subscript 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[subscript s] and assimilation rate decline concurrently and with similar spatial patterns in response to DCMU; (2) the response of g[subscript s] to c[subscript 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[subscript s] to c[subscript 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 g[subscript s] to c[subscript i] is insensitive to oxygen in DCMU-treated leaves or in darkness; and (5) stomata respond normally to RL when c[subscript i] is held constant, indicating the RL response does not require a reduction in c[subscript i] by mesophyll photosynthesis. Together, these results suggest that part of the stomatal response to c[subscript i] involves the balance between photosynthetic electron transport and carbon reduction either in the mesophyll or in guard cell chloroplasts.