Acclimation to low temperature or high light mitigates sensitivity to photoinhibition: roles of the Calvin cycle and the Mehler reaction

Abstract

Winter wheat (Triticum aestivum L cv. Monopol) plants grown under either control (20˚C, 250 PFD), low temperature (5˚C, 250 PFD) or high light conditions (20˚C, 800 PFD) were compared in order to assess the roles of the Calvin cycle and the Mehler reaction in the differential sensitivity to chronic photoinhibition. Despite similar photosynthetic responses to irradiance, the partial pressure of CO₂ [p(CO₂)] and photoinhibition, photosynthetic acclimation to cold temperature appears to be quite distinct from acclimation to high light. First, the lower ratio of Rubisco oxygenation/Rubisco carboxylation and the reduced effects of p(CO₂) on number of electrons per mole of CO₂ fixed in cold-acclimated compared to high light-grown wheat indicate that photorespiration is differentially suppressed in cold-acclimated Monopol. Second, inhibition of the Calvin cycle by glyceraldehyde during photo-inhibition indicated that the sensitivity of high light-acclimated Monopol to photoinhibition was more dependent on Rubisco activity than the sensitivity of cold-acclimated plants to photoinhibition. Third, cold-acclimated Monopol exhibited higher electron transport rates in the presence of either ambient CO₂, 2 kPa O₂ or N₂, 2 kPa O₂ (either 77% or 68%, respectively) relative to controls compared to high light-acclimated plants exposed to similar gaseous environments (either 57% or 38%, respectively). Last, the activation state of NADP–malate dehydrogenase indicated that the stroma is highly reduced during cold acclimation relative to either controls or high light-grown Monopol. Thus, in cold-acclimated wheat, the Mehler reaction appears to play an important role while photorespiration plays a minimal role in mitigating the sensitivity to photoinhibition. In contrast, both photorespiration and the Mehler reac-tion appear to mitigate the sensitivity to photoinhibition in high light-grown Monopol. This is consistent with the differential sensitivity to methylviologen and the differential SOD activity observed between cold-acclimated and high light-grown Monopol.