MECHANISM OF NONPHOTOCHEMICAL CHLOROPHYLL FLUORESCENCE QUENCHING .1. THE ROLE OF DE-EPOXIDIZED XANTHOPHYLLS AND SEQUESTERED THYLAKOID MEMBRANE PROTONS AS PROBED BY DIBUCAINE
N. Mohanty et Hy. Yamamoto, MECHANISM OF NONPHOTOCHEMICAL CHLOROPHYLL FLUORESCENCE QUENCHING .1. THE ROLE OF DE-EPOXIDIZED XANTHOPHYLLS AND SEQUESTERED THYLAKOID MEMBRANE PROTONS AS PROBED BY DIBUCAINE, Australian journal of plant physiology, 22(2), 1995, pp. 231-238
Dibucaine reportedly inhibits the light-induced transthylakoid proton
gradient of chloroplasts without inhibiting energy-dependent non-photo
chemical chlorophyll fluorescence quenching (Laasch, H. and Weis, E. (
1989). Photosynthesis Research 22, 137-146). We show that dibucaine ca
n inhibit fluorescence quenching, depending on the de-epoxidation stat
e of the xanthophyll cycle. Whereas dibucaine (20-40 mu M) had little
effect on fluorescence quenching in pre-illuminated-type thylakoids (l
oaded with zeaxanthin and antheraxanthin), it strongly inhibited quenc
hing in dark-adapted-type thylakoids (no preinduction of de-epoxidatio
n). Dibucaine inhibited lumen acidification similarly in both types of
thylakoids and also the induction of violaxanthin deepoxidation in da
rk-adapted thylakoids. Thus dark-adapted and pre-illuminated thylakoid
s differed in de-epoxidation states and their suspectibility to dibuca
ine inhibition of fluorescence quenching corresponded to this differen
ce. The mechanism of inhibition of de-epoxidation by dibucaine is uncl
ear. It could be due to the inhibition of lumen acidification but an i
nhibition of the violaxanthin available for de-epoxidation is not excl
uded. High dibucaine concentrations inhibited de-epoxidase activity di
rectly. Dibucaine inhibition of fluorescence quenching, however, is no
t limited to the inhibition of de-epoxidation. Small but clear effects
on fluorescence quenching were present in thylakoids even with de-epo
xidation preinduced. Moreover, thylakoids with preinduced deepoxidatio
n were more resistant to dibucaine inhibition of fluorescene quenching
when poised by salt treatments for proton partitioning into membrane-
sequestered domains than when poised for proton partitioning into delo
calised domains. We conclude that non-photochemical quenching of chlor
ophyll fluorescence depends on both de-epoxidised xanthophylls and seq
uestered proton domains in the thylakoid membranes.