FLIM (fluorescence lifetime imaging microscopy) of avocado leaves during slow fluorescence transient (the P to S decline and the S to M rise)

Abstract

Fluorescence lifetime imaging measurements were made on intact avocado leaves (Persea americana Mill.) during the slow part of chlorophyll (Chl) a fluorescence transient, the P to S and the S to M phase. Contributions of lutein-epoxide and violaxanthin cycles operating in parallel on the ΔpH-dependent (transthylakoid H+ concentration gradient) thermal energy dissipation (qE) and slowly reversible ApH-independent fluorescence quenching (qI) were studied. A polar plot analysis of the lifetime data revealed three major chlorophyll a fluorescence lifetime pools for photosystem II. The longest lifetime pool (centered at 2 ns) was observed when linear electron transport and the resulting ΔpH build-up were inhibited in leaves. The other two lifetime pools (1.5 and 0.5 ns) were observed during ΔpH build-up under illumination. Interconversion between these two lifetime pools took place during the slow part of the chlorophyll a fluorescence transient. Formation of the 0.5 ns pool upon illumination was correlated with dark-retention of antheraxanthin and photo-converted lutein in leaves. In the absence of ΔpH, neither the intensity nor the lifetimes of fluorescence were affected by the presence of antheraxanthin and photo-converted lutein. We conclude that both antheraxanthin and photo-converted lutein are able to enhance ΔpH-dependent qE processes associated with the 0.5 ns lifetime pool.