Assessing Spatial and Temporal Variation in Photoprotective Responses of Deciduous and Evergreen Tree Canopies with Leaf Spectroscopy

Environmental conditions frequently prevent carbon fixation by plants, leading to the absorption of excess light that can damage photosynthetic machinery if not dissipated. To do so, plants utilize several photoprotective mechanisms, some detectable remotely using Photochemical Reflectance Index (PRI). Two components of PRI correspond to the facultative engagement of the xanthophyll cycle (ΔPRI) and constitutive changes in xanthophyll pigment pool sizes (PRI0), representing distinct mechanisms regulating shorter and longer-term photoprotection, respectively. Our understanding of interspecific and intraspecific variation in these mechanisms is limited, primarily because PRI components are often not clearly distinguished. This study aimed to assess the variation in facultative and constitutive photoprotection within and between canopies of deciduous and evergreen species across the growing season, and how this variation is structured by functional trade-offs related to leaf lifespan and insolation. We addressed the following questions: How do dynamic photoprotective pigment responses vary (1) among species of different functional groups varying in leaf lifespan, (2) seasonally among species and groups, (3) according to canopy light environment? We quantified pigment responses by exposing dark-adapted leaves to high irradiance (“dark-to-light” method) while sampling reflectance to measure constitutive (PRI0) and facultative (ΔPRI) responses. Leaf reflectance measurements were also collected in situ under low and high irradiance conditions within canopies. Deciduous and evergreen species had contrasting PRI0 and ΔPRI values at mid-season, reflecting differences in constitutive pigment investments and facultative xanthophyll cycle engagement. Towards the end of the growing season, PRI0 and ΔPRI shifted more dramatically for deciduous species undergoing leaf senescence than evergreens. In situ PRI values resembled PRI responses detected using the dark-to- light method, demonstrating the utility of canopy light gradients for evaluating PRI-light responses. Our results indicate that deciduous and evergreen species have distinct, seasonally dynamic photoprotective strategies, likely related to functional constraints tied to leaf lifespan. Identifying coordinated strategies in photoprotection and photosynthesis may improve the ability to distinguish photoprotective mechanisms operating dynamically under different environmental conditions. Advisors: John A. Gamon and Sabrina E. Russo