Environmental conditions frequently prevent photosynthetic carbon fixation by plants, leading to the absorption of excess light that can potentially damage photosynthetic machinery if not dissipated. To do so, plants utilize a variety of photoprotective mechanisms, some optically detectable with remote sensing using Photochemical Reflectance Index (PRI). The two components of PRI have been associated with the facultative engagement of the xanthophyll cycle (?PRI) and constitutive changes in chlorophyll and xanthophyll pigment pool sizes (PRI0), representing distinct mechanisms regulating shorter and longer-term photoprotective responses, respectively. This study aimed to assess the variation in the expression of facultative and constitutive photoprotective mechanisms within and between canopies of six deciduous and evergreen tree species across the growing season, and how this variation is structured by trade-offs in leaf function related to leaf lifespan and insolation. We quantified species pigment responses by exposing dark-adapted leaves to high irradiance (“dark-to-light” method) while sampling reflectance to measure constitutive (PRI0) and facultative (?PRI) responses. Reflectance measurements were also collected in situ on leaves from canopies under low (“shade”) and high (“sun”) irradiance conditions to derive both constitutive and facultative PRI-light responses. Deciduous and evergreen species had contrasting PRI0 and ?PRI values at mid-growing season, reflecting differences in both constitutive investments in photoprotective pigments and facultative xanthophyll cycle engagement. Towards the end of the growing season, PRI0 and ?PRI shifted more dramatically for deciduous species undergoing leaf senescence than evergreens that retained their leaves. PRI-light responses derived from canopy positions in contrasting irradiance conditions resembled differences detected among functional groups using the dark-to-light method, demonstrating the utility of canopy light gradients for evaluating the constitutive and facultative PRI components at the leaf-level. Our results indicate that deciduous and evergreen species have distinct, seasonally dynamic photoprotective strategies, likely related to different functional constraints tied to leaf lifespan. The identification of coordinated strategies in photoprotection and photosynthesis may improve the ability to distinguish photoprotective mechanisms operating dynamically under different environmental conditions in functionally diverse communities.

Bio Sketch:
Alec Piper is a master’s student at the University of Nebraska-Lincoln, working in the Center for Advanced Land Management Information Technologies (CALMIT) under Dr. John A. Gamon and Dr. Sabrina E. Russo. He earned his bachelor’s degree in biological sciences from Doane University in December 2020. Alec’s master’s research focuses on the variation in photoprotective pigment responses among trees using proximal remote sensing. Studies investigating variation in species’ photoprotective strategies, aimed at better understanding the mechanisms regulating photosynthesis and photoprotection, can enhance the ability to monitor plant stress in forest communities with remote sensing technology.