Exclusive and hypersensitive stomatal control by blue-light in Equisetum

Most ferns are adapted to lower-light environments and have relatively low rates of leaf gas exchange compared with seed plants. Recent studies suggest that certain fern groups adapted to full sun, yet ever-wet, environments have evolved novel stomatal regulatory mechanisms, particularly in response to light, enabling higher rates of leaf gas exchange. Among these lineages, the genus Equisetum, a morphologically and ecologically distinctive group of ferns, remains poorly understood in terms of stomatal physiology. Here, we investigated stomatal control by light in Equisetum by combining observations of stomatal conductance in the field with measurements of canopy conductance in a controlled environment and stem-level gas exchange responses to varying wavelengths and intensities of light. We found that Equisetum stomatal closure in the dark occurs over 5 min, which is extremely fast compared with other ferns. Additionally, Equisetum has lost a stomatal response to red-light, with stomata exclusively regulated by blue-light. This novel regulation of stomata by blue-light and rapid stomatal closure in the dark results in strong co-ordination between canopy conductance and light intensity at the end of the day. Our results have implications for understanding the regulation of stomata by light and suggest that there is considerable ecologically relevant diversity in stomatal regulation within ferns.