The redox state of the apoplast influences the acclimation of photosynthesis and leaf metabolism to changing irradiance

The redox state of the apoplast is largely determined by ascorbate oxidase (AO) activity. The influence of AO activity on leaf acclimation to changing irradiance was explored in wild type (WT) and transgenic tobacco (Nicotiana tobaccum) lines containing either high (PAO) or low (TAO) AO activity at low (LL; 250 µmol m(-2) s(-1) ) and high (HL; 1600 µmol m(-2) s(-1) ) irradiance and following the transition from HL to LL. AO activities changed over the photoperiod, particularly in the PAO plants. AO activity had little effect on leaf ascorbate, which was significantly higher under HL than LL. Apoplastic ascorbate/ dehydroascorbate (DHA) ratios and threonate levels were modified by AO activity. Despite decreased levels of transcripts encoding ascorbate synthesis enzymes, leaf ascorbate increased over the first photoperiod following the transition from HL to LL, to much higher levels than LL-grown plants. Photosynthesis rates were significantly higher in the TAO leaves than WT or PAO plants grown under HL but not LL. Sub-sets of amino acids and fatty acids were lower in TAO and WT leaves than the PAO plants under HL, and following the transition to LL. Light acclimation processes are therefore influenced by the apoplastic as well as chloroplastic redox state.