Despite the extensive use of glyphosate, how it alters the physiology and metabolism of plants is still
unclear. Photosynthesis is not regarded to be a primary inhibitory target of glyphosate, but it has been
reported to be affected by this herbicide. The aim of the current research was to determine the effects of
glyphosate on the light and dark reactions of photosynthesis by comparing glyphosate-susceptible and
glyphosate-resistant Lolium perenne biotypes. After glyphosate treatment, accumulation of reduced
carbohydrates occurred before a decrease in gas exchange. Stomatal conductance and CO2 assimilation
were reduced earlier than chlorophyll fluorescence and the amount of chlorophyll in susceptible plants.
In the glyphosate-resistant biotype, stomatal conductance was the only parameter slightly affected only
5 days post-application. In susceptible plants, the initial glyphosate effects on gas exchange could be
a response to a feedback regulation of photosynthesis. Since the herbicide affects actively growing tissues
regardless of the inhibition of photosynthesis, the demand of assimilates decreased and consequently
induced an accumulation of carbohydrates in leaves. We concluded that stomatal conductance could be
a very sensitive parameter to assess both the susceptibility/resistance to glyphosate before the phytotoxic
symptoms become evident.