Effects of Autotoxicity on Seed Germination, Gas Exchange Attributes and Chlorophyll Fluorescence in Melon Seedlings

Abstract

In recent years, continuous cropping is more and more common in melon production, which shows significant adverse effects on melon cultivation. Autotoxicity is a critical factor for continuous cropping obstacle. The purpose of this study was to investigate whether the changes of chlorophyll fluorescence parameters could sensitively reflect the degree of melon autotoxicity and its damage. The root aqueous extract of melon plant was used to simulate the autotoxicity. The effects of autotoxicity on seed germination, seedling morphology, photosynthesis and chlorophyll a fluorescence transient (OJIP) were determined. The results showed that autotoxicity significantly inhibited seed germination and subsequent growth. The content of chlorophyll and carotenoid, photosynthetic rate, stomatal conductance, water-use efficiency and transpiration rate decreased significantly in melon seedlings under autotoxicity. The OJIP test parameters showed slight improvement or no obvious change at low treatment concentration (0.01 and 0.02 g·mL−1) and deteriorated rapidly at high concentration (0.03 and 0.04 g·mL−1), which was consistent with the concentration-dependent characteristics of autotoxicity. Under high autotoxicity stress concentration, most parameters related specific fluxes or activities per photosystem II (PS II) reaction centers (RC), such as absorption flux per active RC (ABS/RC), dissipated energy flux per active RC (DIo/RC) and other parameters, increased significantly in OJIP test. At the same time, most parameters related yields or flux ratios and performance index, such as maximum quantum yield of primary photochemistry (φPo), performance index on absorption basis (PIabs) and other parameters, decreased significantly. Photoinhibition induced by autotoxicity affected both the donor and the acceptor side of PS II, and then inhibited the growth of plants, decreased of photosynthetic pigment content. OJIP test parameters can be used to measure effectively the effect of autotoxicity on melon seedlings.