Seasonal Photoinhibition of Photosystem II in a Cold-Climate Canadian Cactus (Opuntia cespitosa)

Abstract: Few members of the Cactaceae family occur in cold climates resulting in our limited knowledge about their response to such conditions. We used chlorophyll fluorescence to assess the impact of seasonal changes in the extremes of a continental climate on the photosynthetic performance of Opuntia cespitosa in Canada. Quantum yield (QY, FV/FM) was sampled on opposing faces of four cladodes (pads) from April to December, and data for ambient air temperature, morning sky conditions and soil moisture were collected. Soil moisture was negatively related to FV/FM, but only on sun-facing pad faces. We found that FV/FM was variable at the start and ending periods while the summer months had consistently high values. FV/FM decline began more than a month earlier on sun-facing pad faces than shaded faces, with some shaded faces with FV/FM > 0.8 even in December. Indeed, when sun- and shade-facing faces were inverted on one pad in late November, FV/FM increased on the newly shaded face, suggesting remarkable resilience to recover from photoinhibition. Overall, we suggest photoinhibition of photosystem II (PSII) is related to low temperature and secondarily to light exposure. Although Opuntia only suffers from photoinhibition during the coldest months, photoinhibition appears to be dynamic and reversible. The seasonal response of Opuntia cespitosa appears to mimic that observed for the cold tolerant evergreen conifers.