Chlorophyll a fluorescence transient during freezing and recovery in winter wheat

Chlorophyll (Chl) a fluorescence measurements as evaluators of plant freezing tolerance are frequently insufficiently sensitive to detect the early metabolic changes that are initiated following exposure to freezing temperatures. Using cold-acclimated winter wheat, I analysed the polyphasic transience (from 50 mu s to 1 s) of Chl a fluorescence. This enabled detailed studies of the progressive energy flows and efficiencies within the photosystem 2 (PS2) complex that ensue following initial exposure to freezing temperatures right through to the plant recovery stage. The initial consequences of mild frosts that may cause primary damage involve a disturbance to the energy transfer subsequent to Q(A) (the primary quinone electron acceptor of PS2). Lower freezing temperatures, on the other hand, may deter energy flow between the PS2 reaction centre (RC), Chl, and Q(A). All primary damage could only be repaired partially. Further freezing-triggered dysfunction of the electron transfer between the PS2 RCs and Q(A) was connected with secondary damage that could lead to PS2 deactivation. Both primary and secondary freezing damages were reflected in decreased PI(ABS), the Performance Index based on equal absorption that characterizes all energy biftircations in PS2. PI(ABs) also differentiated cultivars with contrasting freezing-tolerance either subsequent to the onset of freezing or during the recovery stage. In contrast, the potential quantum yield of PS2 (F(v)/F(m),), which characterizes efficiency of energy trapping in the PS2 RCs, was only different in cultivars with contrasting freezing-tolerance during the recovery stage.