Active stomatal control of Marsilea crenata, an amphibious fern, in response to CO_2 and exogenous application of ABA

Angiosperms have active stomatal control in response to rising CO_2 and plant regulator abscisic acid (ABA). Whether ferns have similar response is controversial. To evaluate its stomatal response, we measured leaf photosynthetic gas exchange of Marsilea crenata (an amphibious fern), grown under full light and shaded condition, in response to variations in CO_2 concentration ([CO_2]) and exogenous application of ABA. The results showed that stomatal conductance (g_s) of M. crenata significantly decreased while photosaturated photosynthetic rate (A_(max)) increased as [CO_2] increased from 0 to 600 ppm, resulting in increments in water use efficiency (WUE). The reduction in g_s when [CO_2] was elevated from 400 to 800 ppm was more in leaves of full light-grown than those of shade-grown plants, however, the increment in A_(max) was similar. Leaves of M. crenata gradually closed stomata after 30 minutes of application of exogenous ABA, resulting in a 52.1 % reduction in g_s and a 40 % in Amax, hence a 25 % increase of WUE. A more than two-fold increment of ABA contents was also measured in the leaves after the ABA application. This study showed that stomata of M. crenata do respond to the increase of ambient [CO_2] from 0 to 600 ppm and to the ABA application, and the response to the elevated ambient [CO_2] is affected by growth conditions.