Fe Resupply to Fe-deficient Sugar Beet Plants Leads to Rapid Changes in the Violaxanthin Cycle and other Photosynthetic Characteristics without Significant de novo Chlorophyll Synthesis.

The effects of Fe resupply to Fe-deficient plants have been investigated in hydroponically-grown sugar beet. In the short-term (24 h) after Fe resupply, major changes were observed, although de novo chlorophyll (Chl) synthesis had not begun yet. Approximately 50% of the zeaxanthin was converted into violaxanthin, whereas the actual Photosystem II (PS II) efficiency increased by 69% and non-photochemical quenching (NPQ) and the amount of thermally dissipated energy decreased markedly (by 47% and 40%, respectively). At the same time, photosynthetic rate increased approximately by 50%. From one to two days after Fe resupply, there was a gradual increase in the leaf concentrations of Chl and other photosynthetic pigments, accompanied by a further conversion of zeaxanthin into violaxanthin, increases in actual PS II efficiency and photosynthetic rates and decreases in NPQ and the amount of thermally dissipated energy. At 72-96 h after Fe resupply, leaf pigment concentrations, photosynthetic rates and actual PS II efficiency had increased further, although both photosynthetic rate and leaf pigment concentrations were still lower than those found in Fe-sufficient leaves. Good correlations were observed between the amount of light thermally dissipated by the PS II antenna, NPQ and the antheraxanthin + zeaxanthin concentration after Fe resupply, confirming the photoprotective role of the xanthophyll cycle in Fe-deficient sugar beet leaves. Similar correlations were observed for lutein, suggesting a possible role of this pigment in photoprotection.