| The plant Ageratum conyzoides Linn which originated in Central America and Caribbean, is an herbaceous plant belonging to the family of Compositae. It is now widely distributed in tropic and subtropic areas. A. conyzoides is a normal weed in south China. It is also a drought-tolerant and thermostable, but not cold-resistant and shade-resistant species. However the deep, fertile, humid and well-drined sandy soil is more capable for the growth of A. conyzoides. In present study, the effects of different light intensity, salinity and PEG6000 concentration on seedling growth and some related physiological and biochemical characteristics of A. conyzoides had been conducted in order to investigate this species'growing characters under varied stresses conditions. Based on the stresses treatments, chlorophyll fluorescence technology was used to study leaf chlorophyll fluorescence characteristics of conyzoides speices and evaluate its growth adaptation ability. This study also studied the effects of light, salinity, water content, soil deepth, water potential and termperature on germination of A. conyzoides. Our objectives were to study the growth adaption of A. conyzoides to different environments, thus provide theory and experiment judgement for the controling and utilization of this weed. Research results are shown as follows,(1) When A. conyzoides were planted in A, B and C light intensity environment, higher stomatal conductance (Gs) and transpiration rate (E) were found in B and C light intensity plants, and higher light compensation (LCP) and light respiration rate were found in A light intensity treatment. In low light intensity, net photosynthetic rate (Pn) of shade plants increased faster than unshade plants. In high light intensity, Pn of heavy shade (C light intensity) plants sharply derceased. Pn of A light intensity plants increased rapidly to the maximum and was colse to that of moderately shade (B light intensity) plants. Datas of chlorophyll fluorescence showed that when PPFD was more than 600μmol m-2 s-1, electron transport rate (ETR) of A light intensity plants was higher than 50% shade plants. Photochemical quench (qP) of unshade plants was higher than shade plants. ETR, quantum yield of photosynthesis (Yield) and qP values of heavy shade plants were much lower, which were coincidence with its lower Pn value. Results showed that gas conduction was not the main limiting factor in photosynthesis. Heavy shade treatment had a significant effect on growth of A. conyzoides that it may decrease plant's development. But on the other hand, the lower LCP, higher LSP and ETR in shade plants indicated that this species also had the adaptability to full sun and shade environments.This was in accordance with the character of weeds that they can grow in a wide range of varied light intensity conditions.(2) Influences of shade and salinity on A. conyzoides were that shade treatment could enhance plant height and leaf area. In the early treatments of shade and salinity, chlorophyll content (CHL) increased with the decrease of salinity levels. But in the later determination, no significant differences of CHL were observed among three shade treatments. Proline content was positively correlated with salt stress. In the third measurement, moderate shade plants suffered severely in high salinity level that its MDA content was quitely higher. These results showed that moderate shade plants could grow well in a wide range of salt stress. The varied trends of chloroplyll content, proline and soluble protein indicated different reactions of A. conyzoides to light intensity and salinity treatment in plants growth and development. Combined with 2-factors analysis, we concluded that shade treatment could relief salt stress to some extent. Further, high light intensity had a stronger inhibitory effect on plant growth than low light intensity when multiple stressors of shade and salinity were applied. (3) When treated with different concentration of PEG6000 (PEGc), drying rate of A. conyzoides increased as PEGc varied from H to K. Soluble protein, proline and MDA content were positivly correlated with PEGc, they added as the increase of PEGc. In terms of photosynthetic parameters, Pn value under high PEGc was obviously higher than H PEGc in the treatments of medium and later periods.In the meanwhile, trends of Gs and E values were in accordance with variation of Pn. Yield also displayed a gradient variation, which was according with PEGc, that it was smaller in severe water stress. ETR was larger in lower water stress. Plant water content decreased as the dehydration of A. conyzoides under low water potential, thus resulted in the increasing of drying weight. Membrane-lipid was seriously damaged in high PEGc than in low PEGc. Therefore, soluble sugar, soluble protein and proline content were increased to enhance the productivity and adaptation to adversity when suffered water stress. High PEGc had a serious damage than in low PEGC. Datas of photosynthesis and chlorophyll fluorescence parameters indicated that water stress had negative effects on PSII transformation efficiency to A. conyzoides plants.(4) Results of seed germination shown that germination rate (GR) of A. conyzoides seeds in darkness was significantly lower than in light treatment. GR in 150mM salt was more than 98%, then decreased rapidly as salinity raised, and became 0 when salinity was 350 mM. The largest excavated rate (ER) was observed in 0.2 cm depth of sandy soil, then ER decreased as burial depth increased except that on the surface. Under different water potential treatments, GR increased with the increasing of water potential, and the most proper water potential was ranged from -0.1 to -0.3Mpa, in which GR was higher than 95%. The influence of different temperature was that, GR was quite smaller in low and high temperature. High GR was obtained in the proper temperature of 20 to 25℃. Thus, it could be concluded that A. conyzoides seeds needed light to promote germination. Furthermore, temperature, depth of soil, soil water content and water potential were important factors in influencing seed germination and the spreading of A. conyzoides plants. On the other hand, water content on surface was lower and light in deeper sandy soil was weaker, so the seeds could not fulfill the after ripening or break seed dormancy, thereby caused a lower ER.In conclusion, A. conyzoides has a wide range of environmental adaptability and higher reproductive potential. But overall, it prefers habitat of abundant light and humidity condition. Our researches had a comprehensive understand of the growth characteristics of A. conyzoides, which provided experimental basics and theoretical directions for further management of this weed, and for the utilization of it in biological controling and as flowers in horticulture. |
PDF Full Text Request