Transpiration, measured by the cut-leaf method, was greatly retarded during the cold season in Rhododendron catawbiense Michx., Kalmia latifolia L., and Pinus nigra Arn. growing out-of-doors in New Haven, Conn. Stomatal opening, measured by observing leaf replicas under the microscope, was at least roughly related to the seasonal changes in transpiration. The winter decline in transpiration was clearly not related to changes in leaf water content, but the spring increase in percent of stomata open, and in transpiration, was related to a decline in leaf moisture in spite of continued ample soil moisture. Cold hardiness, related only in a general way to stomatal opening and transpiration, was more closely related to photosynthesis, the latter being measured under constant temperature and illumination in leaves cut from the plants. Despite an apparent relationship between stomatal opening and photosynthesis, it is suggested that photosynthesis is curtailed by a nonstomatal factor, probably biochemical, such as a sugar increase. Hardiness, more closely related to starch conversion than to total sugar levels, may be related to the seasonal changes in photosynthesis through this biochemical change.
Document Type: Journal Article
Assistant Professor, Yale School of Forestry, New Haven, Conn.