Physiological and Biochemical Responses of Tobacco at Different Growth Stages to Alternaria alternata

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ABSTRACTTobacco brown spot, caused by Alternaria alternata (Fries) Keissler, is one of the main leaf diseases of tobacco. It is common in the mature stage of tobacco but rarely occurs in the early growth stage. Therefore, it is also called ‘mature spot’ by tobacco farmers. However, the physiological and biochemical mechanisms underlying the responses of tobacco plants at different growth stages to A. alternata remain unclear. In this experiment, A. alternata was inoculated into two tobacco varieties, Yun87 and HongHuaDaJinYuan (HD), at the immature and mature stages respectively. The disease index, stomata, reactive oxygen species (OFR and H2O2) and main defence enzymes (SOD, POD, PPO, CAT and PAL) of tobacco at the two growth stages were analysed. Results showed that the disease index of tobacco plants in the immature stage was lower than that of tobacco plants at the mature stage at 5 days after inoculation. Stomatal opening was not significantly different between the immature and mature stages, but the stomatal apertures of HD and Yun87 decreased by 1%–2.33% at 12 h after inoculation. Moreover, the OFR and H2O2 contents in the mature stage were significantly higher than those in the immature stage. SOD activity was higher in the immature stage than in the mature stage, whereas POD activity was higher in the mature stage than in the immature stage. PPO, CAT and PAL activities were not significantly different between the immature and mature stages. Reactive oxygen species and defence enzymes began to respond at 12 h after inoculation. Specifically, SOD and PAL activities peaked at 12 h, whereas CAT activity peaked at 24 h. The response levels and rates of H2O2 content, SOD activity and PAL activity were higher in the immature stage than in the mature stage. Correlation analysis revealed that the strength of the effect of these factors was different. In general, tobacco resistance to brown spot disease was higher in the immature stage than in the mature stage, and this resistance was mediated by the response rates and levels of stomata, reactive oxygen species and defence enzymes.