biostudies-literatureUnknown36(3)Frechette ENSERCCanada Foundation for InnovationUniversity of Toronto and Ontario Graduate Scholarships.University of Toronto CenterThe photochemical reflectance index (PRI) is a proxy for the activity of the photoprotective xanthophyll cycle and photosynthetic light use efficiency (LUE) in plants. Evergreen conifers downregulate photosynthesis in autumn in response to low temperature and shorter photoperiod, and the dynamic xanthophyll cycle-mediated non-photochemical quenching (NPQ) is replaced by sustained NPQ. We hypothesized that this shift in xanthophyll cycle-dependent energy partitioning during the autumn is the cause for variations in the PRI-LUE relationship. In order to test our hypothesis, we characterized energy partitioning and pigment composition during a simulated summer-autumn transition in a conifer and assessed the effects of temperature and photoperiod on the PRI-LUE relationship. We measured gas exTree physiology311-24https://www.ebi.ac.uk/biostudies/studies/S-EPMC4885948biostudies-literaturePhotoperiod and temperature constraints on the relationship between the photochemical reflectance index and the light use efficiency of photosynthesis in Pinus strobus.PMC4885948Frechette EEnsminger IChang CYfalsePhotoperiod and temperature constraints on the relationship between the photochemical reflectance index and the light use efficiency of photosynthesis in Pinus strobus.The photochemical reflectance index (PRI) is a proxy for the activity of the photoprotective xanthophyll cycle and photosynthetic light use efficiency (LUE) in plants. Evergreen conifers downregulate photosynthesis in autumn in response to low temperature and shorter photoperiod, and the dynamic xanthophyll cycle-mediated non-photochemical quenching (NPQ) is replaced by sustained NPQ. We hypothesized that this shift in xanthophyll cycle-dependent energy partitioning during the autumn is the cause for variations in the PRI-LUE relationship. In order to test our hypothesis, we characterized energy partitioning and pigment composition during a simulated summer-autumn transition in a conifer and assessed the effects of temperature and photoperiod on the PRI-LUE relationship. We measured gas ex2016-01-01T00:00:00Z2016 Mar2026-07-15T18:20:48.797Z2019-03-27T02:14:49ZS-EPMC48859482684698010.1093/treephys/tpv143