Physiological Mechanisms Of The Different Responses To Cinnamic Acid For Cucumis Sativus L. And Cucurbita Ficifoli B.Seedlings

Cucumber (Cucumis sativus L.) is an important economic vegetable crop. However, monocropping becomes popular while it was planted in the intensive, large-scale and specialized manner. A reduction in both crop yield and quality often occurs when it was cultivated successively on the same soil, which has greatly limited cucumber production. As one of the most important factors responsible for soil sickness, autotoxicity attracted extensive attention of researchers in recent years. It has been documented that cucumber has autotoxic potential, and more than 10 kinds of growth inhibitory substances including benzoic acid and cinnamic acid from its root exudates have been isolated and identified. Those substances could significantly inhibit plant growth by influencing physiological process such as ion absorption, water utilization, photosynthesis and respiration etc. In addition, it was found that figleaf gourd (Cucurbita ficifolia Bouche), a plant often used as rootstock for cucumber, does not have autotoxic potential. Interestingly, the growth of figleaf gourd was even promoted by some autotoxins at low concentration. Base on these phenomena, this study investigated the different responses of roots of these two species to cinnamic acid, in respect of the metabolism of reactive oxygen species (ROS), metabolism of water in plant, nutrient ion uptake and the growth in order to clarify the mechanism of the autotoxic effect of cinnamic acid on cucumber. The main results are as follows:1) NADPH oxidase activity in plasma membrane, production of ROS (including H₂O₂ and O₂^-) and antioxidant enzyme activities increased sharply after 3 days treatment of 0.25mM CA .Meanwhile, MDA content, an indicator of membrane lipid peroxidation, was also enhanced by CA. In contrast, these parameters of figleaf gourd were not altered significantly.2) ATPase ( including PM-H⁺-ATPase, V-H⁺-ATPase and V-H⁺-PPase) activities and nutrient ion uptake of cucumber roots were inhibited significantly after 3 days treatment of CA, and the inhibitory effects were enhanced with increasing concentrationof CA. Then the ATPase activities increased along with prolonging time of treatment. In comparison, no significant changes were found in figleaf gourd. The results indicated that decreased ATPase activities might playe important roles in nutrient ion uptake of cucumber roots.3) The water potential decreased firstly and then increased under the treatment of CA in cucumber roots. Difference between control and 0.25mM CA treatment, however, was not noticeable in figleaf gourd plant, indicating no obvious effect of CA on metabolism of water in figleaf gourd plant. CA did not result in the increased water potential in figleaf gourd either.4) CA inhibited the growth of cucumber seedling in terms of fresh weight and dry weight, and the inhibitory effect increased with the increasing concentration. The plant morphology and roots shape were altered to some extent. Those phenomena did not occur in figleaf gourd, and the growth rate was higher than cucumber. The evidence showed here indicated that a significant different response existed between growth of cucumber and figleaf gourd.In conclusion, CA could trigger an oxidative damage, and finally leading to inhibition of the nutrient absorption and plant growth.