The Role of Farnesol as a Regulator of Stomatal Opening inSorghum

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Abstract
Fine, very dilute aqueous emulsions of all-trans farnesol applied to intact leaves of Sorghum bicolor caused appreciable inhibition of stomatal opening which persisted for 2 d, after which time the stomata regained their capacity to open. The inhibitory effect of farnesol was not overcome by flushing the leaves with CO2-free air, indicating that it was not the result of an accumulation of CO2. This conclusion was supported by measurements of CO2 compensation, which increased only slightly after farnesol treatment. All-trans farnesol has previously been reported to be formed in water-stressed plants of Sorghum. The data presented here suggest that it could be acting as an endogenous antitranspirant, in a comparable role to that already established for abscisic acid in several species. It would appear, however, to have a less prolonged inhibitory effect than abscisic acid, and it could be responsible for the rapid responses of Sorghum stomata to water stress and their quick recovery after the plant has regained turgor, a characteristic which distinguishes Sorghum from many other genera so far investigated.