Line Lapointe

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Photoinhibition of photosynthesis has been shown to be more severe when combined with other stresses. This suggests a possible correlation between the capacity to grow at low temperature and an increased resistance to photoinhibition at low temperature. Previous work has shown with chlorophyll a fluorescence that rye plants (Secale cereale L. cv Musketeer) acclimated to low temperature were more resistant to photoinhibition than non-hardened rye plants. The goal of the present study was to assess whether the morphological changes observed at the leaf level and the structural changes observed at the thylakoid level during the cold hardening process were responsible for the increased resistance of cold hardened rye to photoinhibition.;Photoinhibition was monitored in hardened and non-hardened winter rye at the leaf level, with isolated mesophyll cells, and with isolated thylakoids. They were exposed to high photon fluxes at 20 and at 5{dollar}\sp\circ{dollar}C and recovery from photoinhibition was assessed upon return to lower photon fluxes. Room temperature chlorophyll a fluorescence, light limited CO{dollar}\sb2{dollar} fixation rates, electron transport rates, photoacoustic spectroscopy and atrazine binding were measured during and following photoinhibition.;Cold-hardened isolated rye cells were more resistant to photoinhibition when monitored with chlorophyll a fluorescence. However, CO{dollar}\sb2{dollar} fixation rates showed similar extent of photoinhibition in cold hardened and non-hardened isolated rye cells. Photoinhibition was similar at 5 and at 20{dollar}\sp\circ{dollar}C but the recovery from photoinhibition was slower when occurring at 5{dollar}\sp\circ{dollar}C. Both groups of cells showed similar rates of recovery. In contrast, cold hardened isolated rye thylakoids were more susceptible to photoinhibition which mainly affected Photosystem II activity.;Leaf morphological differences and structural changes occurring at the thylakoid level during cold hardening of winter rye are not responsible for its increased resistance to photoinhibition. Photoinhibition as monitored with chlorophyll a fluorescence showed different trends and different kinetics compared to CO{dollar}\sb2{dollar} fixation. CO{dollar}\sb2{dollar} fixation data showed that regulatory mechanisms also play a role during photoinhibition and subsequent recovery. Therefore, CO{dollar}\sb2{dollar} fixation should be monitored in intact leaves before the increased resistance of cold hardened winter rye is conclusively proven.



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