Main Allelochemicals Isolated And Identified From The Leachates Of Ageratina Adenophora Sprengel (Asteraceae) And Their Action Mechanisms On Upland Rice (Oryza Sativa) Seedlings

Ageratina adenophora Sprengel (Synonym: Eupatorium adenophora Sprengel) (Asteraceae), originally found in Mexico and Costarica, is a noxious invasive alien weed, and it has become widespread in many countries and regions. In these invaded countries and regions, this weed has caused serious economic loss and threatened native biodiversity, ecosystem integrity and even the health of human and livestock. Therefore, understanding the invasive mechanisms is essential to carry out the sustainable control of this weed. Guided with the review of the invasive mechanisms of alien plants and the spread characters of A. adenophora, this research paper has been conducted mainly on two parts, which were the isolation and identification of the main allelochemicals from the leachates of A. adenophora, and the physiological and morphological impacts of these allelochemicals on upland rice (Oryza sativa L.) seedlings. The main results were showed as follows,1. Considering the interference of osmotic pressure and pH, the allelopathic potential of the leachates from the whole aerial part of A. adenophora on the growth of upland rice and alfalfa (Medicago sativa L.) were investigated under room temperature. The results showed that, the A. adenophora leachates inhibited the root and hypocotyl growth of rice and alfalfa significantly, and the effects increased with the increase of concentration i.e. concentration dependent. However, the osmotic pressure and pH according to the different concentrations of leachates accordingly interfered the growth of receptor plants obviously. When the osmotic pressure was 55 mosm/L, the interference-index to allelopathic action of the leachates reached 18.6% and 27.5% on root and hypocotyl growth of upland rice, and 32.7% and 79.8% of alfalfa, respectively.2. With the methods of chromatographic fractionating and bioassay-guided and the techniques of Nuclear Magnetic Resonance (NMR) and GC-MS, the main allelochemicals of the leachates from the whole aerial part of A. adenophora were isolated and identified. The results showed that two compounds were isolated as the main allelochemicals, and they were 4,7- dimethyl -1-(propan-2-ylidene) -1,4,4a, 8a-tetrahydronaphthalene-2, 6(1H, 7H)-dione (DTD) and 6-hydroxy-5-isopropyl- 3, 8-dimethyl-4a, 5, 6, 7, 8, 8a-hexahydronaphthalen-2(lH)-one (HHO). The IC₅₀ (concentration for 50% inhibitory effects) of DTD on the root elongation of upland rice and alfalfa were 0.979 and 0.714mM, and the IC₅₀of HHO on the root elongation of upland rice and alfalfa were 0.680 and 0.660 mM.3. Under hydroponics conditions, impacts of the allelochemicals DTD and HHO at different concentrations (0.5,1 and 1.5 mM) on the contents of malondialdehyde (MDA), activities of antioxidant enzymes in roots, and the contents of chlorophyll (CHL) in leaves of upland rice seedlings, were investigated. The results showed that, after exposure to the two allelochemicals, the contents of MDA and the activities of peroxidase (POD) increased significantly, whereas the contents of CHL decreased obviously with time and increasing concentrations. The activities of superoxide dismutase (SOD) were increased when treated with these two allelochemicals at the concentrations of 0.5 and 1 mM, butdecreased significantly at the dose of 1.5 mM after 48 h.4. Using the same methods of treatment described above, impacts of the two allelochemicals at three concentrations (0.5, 1 and 1.5 mM) on the contents of abscisic acid (ABA), indoly-3-acetic acid (IAA), and zeatin riboside (ZR) in roots of upland rice seedlings, were further investigated. The results showed that, the contents of ABA were increased when treated with the DTD at the concentrations of 0.5 and 1 mM, but with a decreased tendency at the dose of 1.5 mM. After exposure to the HHO, the contents of ABA increased significantly with time and increasing dose. The contents of IAA and ZR decreased obviously with time and increasing concentrations, with the treatments of the two allelochemicals.5. With the techniques of paraffin dissection, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), impacts of the allelochemicals DTD and HHO on the morphologies, cell anatomical structures and ultrastructures of the upland rice primary root tip were investigated. The results showed that, treated upland rice seedlings were stunted, with leaves in etiolation and some physiological speckles. Treated roots of upland rice were stunted and swollen, and lacked lateral roots. Observations from the SEM and paraffin dissection showed that, appearance and anatomical structures of the treated upland rice root tip cells changed obviously, such as the increased width and reduced elongation of cortex parenchyma cells, and the large numbers of slough off epidermis cells. Ultrastructural changes in the treated upland rice root tip cells showed that, the action of DTD and HHO was multifaceted from the changes of magnitude, position, and shape of organelles, such as the lost karyotheca, increased vacuoles, and hollow dictyosomes. The impact of HHO on the morphology and structure of upland rice primary root tip was more obvious than that of DTD.