CHILLING SENSITIVITY OF VIOLAXANTHIN DEEPOXIDATION INHIBITS THE DEVELOPMENT OF ENERGY-DEPENDENT CHLOROPHYLL FLUORESCENCE QUENCHING IN-VIVO

The temperature dependences of in vivo chlorophyll (chi) fluorescence quenching and zeaxanthin formation, and of in vitro electron transport and transthylakoid proton gradient formation were determined in chill ing tolerant spinach (Spinacia oleracea) and compared to chi fluoresce nce, proton gradient and xanthophyll cycle data from chilling-sensitiv e pumpkin (Cucurbita maxima). In spinach, photochemical quenching (q(P )) decreased with decreasing temperature, indicating an increase of th e reduction state of Q(A) through impaired NADPH utilization. Below 8- 10 degrees C, the fast-relaxing component (q(F)) of nonphotochemical q uenching (q(N)) decreased slightly in moderate and q(N) decreased stro ngly in low light. In dithiothreitol (DTT) treated leaf discs, the low remaining q(N) under low light was less temperature dependent. The q( F) (q(N)) decrease was not the result of temperature effects on couple d photosynthetic electron and proton transport or the permeability of the thylakoid membrane for protons, as judged from Arrhenius plots of whole-chain electron transport and the temperature dependence of light -induced Delta pH in isolated chloroplasts. However, zeaxanthin format ion decreased strongly below 10 degrees C in vivo. In pumpkin, q(F) br oke down completely between 10 and 7 degrees C under moderate light an d this correlated with a complete loss of violaxanthin deepoxidation b elow 7 degrees C, while the in vitro formation of Delta pH by linear e lectron transport was unaffected. It is concluded that leaf zeaxanthin content modulates the q(E) vs. T curve at moderate light in spinach a nd that it is responsible for the breakdown of q(E) below 10 degrees C in pumpkin and, under low light, also in spinach.