Abstract

Leaf physiology of eastern redbud (Cercis canadensis L.) was assessed under natural photoperiod when grown in 100% sun or under polyethylene shade with a light transmittance of 69%, 47%, or 29% sun. Net CO₂ assimilation rate (A) was similar under 100%, 69%, and 47% sun; A was reduced under 29% sun. Adaptations to shade included a near perpendicular leaf orientation to the sun, reduction in specific leaf weight (SLW), and a decreased chlorophyll a: chlorophyll b ratio. Conversely, eastern redbud adapted to 100% sun by manifesting an increased SLW and a vertical orientation of leaves that curled inward toward the midrib. Light response curves were similar for sun- and shade-acclimatized plants. When all data were analyzed collectively, A was most closely related to photosynthetic photon flux (PPF) (R² = 0.52), whereas stomatal conductance to water vapor (gs) was primarily influenced by vapor pressure deficit (VPD) (R² = 0.75). Hence, A and gs were not well correlated (R² = 0.41). The lack of strong coupling between A and gs allowed the stomates to remain open under low PPF, resulting in an elevated intercellular CO² concentration. Thus, A was stimulated above what might have normally occurred under low PPF.