Characterisation Of The Photosynthetic Responses Of Spring And Winter Wheat To Growth At Cold-hardening Temperatures

In vivo room temperature chlorophyll a fluorescence coupled with CO{dollar}\sb2{dollar} and O{dollar}\sb2{dollar} exchange are measured to determine the effect of cold-hardening on the photosynthetic capacity and the susceptibility to photoinhibition of six cultivars of wheat (Triticum aestivum L.), covering a range of freezing tolerance. The role of the photosystem II (PSII) repair cycle in resistance to, and recovery from, photoinhibition is assessed at both low temperatures (5{dollar}\sp\circ{dollar}C & {dollar}-3\sp\circ{dollar}C) and in the presence of the 70S protein synthesis inhibitor, chloramphenicol. The effect of long-term, repeated photoinhibitory events and sustained reduction in PSII efficiency on net carbon gained is also assessed.;Winter wheat cultivars grown at 5{dollar}\sp\circ{dollar}C show light-saturated rates of CO{dollar}\sb2{dollar} exchange and apparent photon yields for CO{dollar}\sb2{dollar} exchange and O{dollar}\sb2{dollar} evolution that are equal to or greater than those of winter cultivars grown at 20{dollar}\sp\circ{dollar}C. In contrast, spring wheat cultivars grown at 5{dollar}\sp\circ{dollar}C show 35% lower apparent photon yields for CO{dollar}\sb2{dollar} exchange and 25% lower light-saturated rates of CO{dollar}\sb2{dollar} exchange compared to 20{dollar}\sp\circ{dollar}C grown controls. The lower CO{dollar}\sb2{dollar} exchange capacity is not associated with a lower efficiency of PSII activity measured as either the apparent photon yield for O{dollar}\sb2{dollar} evolution or the variable to maximal fluorescence ratio, and is most likely associated with carbon metabolism. In addition, cold-hardened spring wheat cultivars are more sensitive to short-term photoinhibition at low temperatures than cold-hardened winter cultivars. This greater resistance of the winter cultivars is associated with the winter phenotype rather than freezing tolerance per se and is due to an increase in the capacity of cold-hardened winter wheat to keep the primary electron acceptor for PSII, Q{dollar}\sb{lcub}\rm A{rcub},{dollar} oxidised at high irradiance (Oquist et al, 1991a).;Repeated photoinhibition (daily reductions in {dollar}F\sb{lcub}\rm V{rcub}/F\sb{lcub}\rm M{rcub}{dollar} of up to 40%) and sustained depression of PSII efficiency are shown to have no negative effects of net carbon accumulation in both cold-hardened winter and spring wheat. This is related to the finding that up to 50% of the loss in photon yield during photoinhibition is not due to irreversible damage to PSII. It is due instead to a rapidly reversible form on photoinhibition that recovers at temperatures as low as {dollar}-3\sp\circ{dollar}C and without de novo protein synthesis. It is suggested that this rapidly reversible form of photoinhibition is evidence of PSII quenching centres resulting from light induced, reversible modifications to the PSII reaction centre. It is proposed that these rapidly reversible quenching centres act as a regulatory mechanism, adjusting the yield of electron transport to match utilisation by photosynthetic carbon reduction. Thus, in situations where photosynthesis may be limited by factors other than light availability. PSII efficiency can be regulated by these wheat cultivars such that the photosynthetic apparatus is protected from over excitation