Three causes of variation in the photochemical reflectance index (PRI) in evergreen conifers

Christopher Y.S. Wong, John A. Gamon

Research output: Contribution to journalArticlepeer-review

161 Scopus citations


The photochemical reflectance index (PRI) reflects diurnal xanthophyll cycle activity and is also influenced by seasonally changing carotenoid : Chl pigment ratios. Both changing pigment pools and xanthophyll cycle activity contribute to photoprotection in evergreen conifers exposed to boreal winters, but they operate over different timescales, and their relative contribution to the PRI signal has often been unclear. To clarify these responses and their contribution to the PRI signal, leaf PRI, pigment composition, temperature and irradiance were monitored over 2 yr for two evergreen conifers (Pinus contorta and Pinus ponderosa) in a boreal climate. PRI was affected by three distinct processes operating over different timescales and exhibiting contrasting spectral responses. Over the 2 yr study period, the greatest change in PRI resulted from seasonally changing carotenoid : Chl pigment ratios, followed by a previously unreported shifting leaf albedo during periods of deep cold. Remarkably, the smallest change was attributable to the xanthophyll cycle. To properly distinguish these three effects, interpretation of PRI must consider temporal context, physiological responses to evolving environmental conditions, and spectral response. Consideration of the separate mechanisms affecting PRI over different timescales could greatly improve efforts to monitor changing photosynthetic activity using optical remote sensing.

Original languageEnglish (US)
Pages (from-to)187-195
Number of pages9
JournalNew Phytologist
Issue number1
StatePublished - Apr 1 2015
Externally publishedYes


  • Acclimation
  • Carotenoid : Chl ratios
  • Cold stress
  • Conifers
  • Leaf pigments
  • Photochemical reflectance index (PRI)
  • Photosynthetic down-regulation
  • Xanthophyll cycle

ASJC Scopus subject areas

  • Physiology
  • Plant Science


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