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

Winter wheat (Triticum aestivum L cv. Monopol) plants grown under either co ntrol (20 degrees C, 250 PFD), low temperature (5 degrees C, 250 PFD) or hi gh light conditions (20 degrees C, 800 PFD) were compared in order to asses s the roles of the Calvin cycle and the Mehler reaction in the differential sensitivity to chronic photoinhibition. Despite similar photosynthetic res ponses to irradiance, the partial pressure of CO2 [p(CO2)] and photoinhibit ion, photosynthetic acclimation to cold temperature appears to be quite dis tinct from acclimation to high light. First, the lower ratio of Rubisco oxy genation/Rubisco carboxylation and the reduced effects of p(CO2) on number of electrons per mole of CO2 fixed in cold-acclimated compared to high ligh t-grown wheat indicate that photorespiration is differentially suppressed i n cold-acclimated Monopol. Second, inhibition of the Calvin cycle by glycer aldehyde during photoinhibition indicated that the sensitivity of high ligh t-acclimated Monopol to photoinhibition was more dependent on Rubisco activ ity than the sensitivity of cold-acclimated plants to photoinhibition. Thir d, cold-acclimated Monopol exhibited higher electron transport rates in the presence of either ambient CO2, 2 kPa O-2 or N-2, 2 kPa O-2 (either 77% or 68%, respectively) relative to controls compared to high light-acclimated plants exposed to similar gaseous environments (either 57% or 38%, respecti vely). Last, the activation state of NADP-malate dehydrogenase indicated th at the stroma is highly reduced during cold acclimation relative to either controls or high light-grown Monopol. Thus, in cold-acclimated wheat, the M ehler reaction appears to play an important role while photorespiration pla ys a minimal role in mitigating the sensitivity to photoinhibition. In cont rast, both photorespiration and the Mehler reaction appear to mitigate the sensitivity to photoinhibition in high light-grown Monopol. This is consist ent with the differential sensitivity to methylviologen and the differentia l SOD activity observed between cold-acclimated and high light-grown Monopo l.