Regulation of energy partitioning and alternative electron transport pathways during cold acclimation of lodgepole Pine is oxygen dependent
Plant and Cell Physiology: 2010 51:1555-1570
Second year needles of Lodgepole pine (Pinus contorta L.) were exposed for 6 weeks to either simulated control summer [‘summer’; 25°C/250 photon flux denisty (PFD)], autumn (‘autumn’; 15°C/250 PFD) or winter conditions (‘winter’; 5°C/250 PFD). We report that the proportion of linear electron transport utilized in carbon assimilation (ETRCO2) was 40% lower in both ‘autumn’ and ‘winter’ pine when compared with the ‘summer’ pine. In contrast, the proportion of excess photosynthetic linear electron transport (ETRexcess) not used for carbon assimilation within the total ETRJf increased by 30% in both ‘autumn’ and ‘winter’ pine. In ‘autumn’ pine acclimated to 15°C, the increased amounts of ‘excess’ electrons were directed equally to 21 kPa O2-dependent and 2 kPa O2-dependent alternative electron transport pathways and the fractions of excitation light energy utilized by PSII photochemistry (ΦPSII), thermally dissipated through ΦNPQ and dissipated by additional quenching mechanism(s) (Φf,D) were similar to those in ‘summer’ pine. In contrast, in ‘winter’ needles acclimated to 5°C, 60% of photosynthetically generated ‘excess’ electrons were utilized through the 2 kPa O2-dependent electron sink and only 15% by the photorespiratory (21 kPa O2) electron pathway. Needles exposed to ‘winter’ conditions led to a 3-fold lower ΦPSII, only a marginal increase in ΦNPQ and a 2-fold higher Φf,D, which was O2 dependent compared with the ‘summer’ and ‘autumn’ pine. Our results demonstrate that the employment of a variety of alternative pathways for utilization of photosynthetically generated electrons by Lodgepole pine depends on the acclimation temperature. Furthermore, dissipation of excess light energy through constitutive non-photochemical quenching mechanisms is O2 dependent.