Abstract:

The characteristics of chlorophyll metabolism and change of chlorophyll fluorescence parameters in the silvered leaves and leafstalks of summer squash induced by silverleaf whitefly (SLW) were studied. The content of Chlorophyll a+b (Chl.a+b) 、Chlorophyll a (Chl.a) and Chlorophyll b (Chl.b) in the silvered leaves and leafstalks decreased. The Chl.a/b in the slight and complete silvered leaves was higher than that in the control, but the Chl.a/b in the moderate silvered leaves was not significantly different. The Chl a/b in the leafstalks decreased. Chl.b was affected more seriously in leaves while the Chl.a compacted in the leafstalks. Chlorophylase (chlase) activity significantly increased with the increasing disease grade, so the ratio of chlorophyll degradation increased. Compared with the control treatment, the amount of precursors including Protoporphyrin IX (Proto IX), Magnesium protoporphyrin IX (Mg-Proto IX), Protochlorophyllide (Pchlide) of chlorophyll biosynthesis decreased with the increasing disease grade, while Uroporphyrinogen Ⅲ (Uro Ⅲ) was accumulated in silvered leaves and leafstalks. This suggested that chlorophyll biosynthesis was blocked at the step from Uro Ⅲ to Proto Ⅸ. The maximum efficiency of photosystomⅡ(PSⅡ) Photochemistry ((F_v/F_o)), potentially photochemical activity of PSII Fv/Fo, actual PSⅡefficiency (φPSⅡ), photochemical quenching coefficient (q_P) in the silvered leaves decreased, but non-photochemical quenching coefficient (NPQ) in the silvered leaves was reverse. The result showed that summer squash silverleaf disease destroyed the dynamic balance of chlorophyll biosynthesis and degradation. The ratio of chlorophyll biosynthesis decreased and the ratio of chlorophyll degradation increased, so the content of chlorophyll decreased. The change of chlorophyll fluorescence parameters showed that the PSⅡactivity of the silvered leaves was inhibited.

Key words: Summer squash silvered disease, Chlorophyll, Chlorophyll biosynthesis, Chlorophyll fluorescence