During fall leaf senescence in deciduousspecies, photosynthesis nears completion dueto chlorophyll breakdown and re-assimilation.However, several other processes such as leafnutrient uptake, re-translocation, and storage,or tissue dehydration to avoid frost damage,may be important and dependent upon stom-atal opening. We report here on measuredchanges in photosynthesis (A), leaf conduc-tance to water vapor (g), and WUE (estimatedby A/g) in three deciduous tree species (Acersaccharum, Cornus florida, and Ginkgo biloba)during the weeks of leaf senescence precedingabscission. Substantial decreases in Aof 60 upto 80% were not matched quantitatively bysimilar declines in g (40 to 70%), resulting incorresponding decreases in WUE (estimatedby A/g) from near 50% to over 300% among thethree species. This shift to a lower WUE mayreflect adaptive value in maintaining a highergrelative to Aduring the fall leaf senescenceperiod.IntroductionIn
