| In recent years, many studies show that the soil fertility declined after successive rotation because of the accumulation of toxic substances(mainly as allelochemicals such as phenolic acid). Autotoxicity of plant allelochemicals not only have adverse effects on their own growth and development, but also affect growth in the surrounding plants. More frightening thing the caused impact on the physical and chemical properties of the soil, which will affect the absorption and growth of plants further. Phenolic acids, the typical allelochemicals after successive rotation, it can affect the growth and distribution of their own or other plants, even microorganisms is the main factor leading to successive rotation obstacles. The damage of phenolic acids to plants begins from the injury to the root system(including the morphological and physiological aspects). Plant roots absorbing the necessary nutrients and moisture from the soil, havean impact on the growth and development of the aboveground(stem and leaf). The accumulation of dry matter of plants will be greatly impacted, which seriously hinders the sustainable development of agriculture and forestry.Objective: The purpose were to explore the effects of phenolic acids accumulation on photosynthesis and biomass of poplar and to reveal the inhibitory effect and physiological mechanism of phenolic acids on poplar growth from the perspective of photosynthesis.Methods: The research of phenolic acids can provide insight into understanding the mechanism of successive rotation obstacle. Researches related to successive rotation was mostly on crops and medicinal materials, such as peanut, corn, rehmannia, radix pseudostellariae and strawberry. Photosynthesis of plant will be limited and the chlorophyll fluorescence yield will be affected in phenolic acid environment. So far, the researches on successive rotation of poplar are mainly in physical and chemical properties of forest soil and rhizosphere effect. Researches on effects of successive rotation on photosynthesis of poplar are limited. In this study, one-year-old I-107(Populus euramericana cv.?Zhonglin46‘) seedlings were used to simulate successive rotation of poplar. Physiological parameters, such as photosynthesis and chlorophyll fluorescence under different concentrations of phenol acid, were measured to study the process of photosynthesis which was helpful to understand the cause of decline of the biomass under continuous cropping. Photosynthetic performance, PSII photochemistry efficiency, chlorophyll content etc. in Poplar seedlings treated with different concentrations of phenol acids were studied in the Forestry Experiment Station of Shandong Agricultural University.Results:(1) When the phenolic acids being applied to the culture medium, root vigor of poplar was decreased. Especially under high concentrations of phenolic acids, root vigor decreased rapidly and remained at low levels, indicates that root absorption ability and synthesis ability, reduction and oxidation ability have been greatly weakened.(2) With the increase of the concentration of phenolic acids, the content of chlorophyll(Chl) decreased; under the same light intensity, net photosynthetic rate(Pn) shows decrease significantly accompanied as the quantum efficiency decreases significantly due to an increase in phenolic acids concentration. The decrease of dark respiration rate(Rd) indicates that the physiological activity of poplar leaves decrease with the increase of the concentration of phenolic acids.(3) In the daily variation of photosynthesis process, all poplar showed different levels of photosynthetically "lunch break" phenomenon, and the reasons for that were different, such as CK ~ T2 is due to stomatal limitation(ie, due to the disruptions of photosynthesis raw material-CO2 supply caused by stomatal aperture), high acid concentrations(T3 ~ T5) is due to non-stomatal limitation(ie, acid lead to the damage of photosynthetic apparatus of poplar leaves, so mesophyll cells decreased photosynthetic capacity).(4) With the increase of acid concentration, the minimal fluorescence(Fo) increased significantly, which means PS Ⅱ reaction center destroyed or irreversibly inactivated. The maximum fluorescence(Fm), PSⅡ actual photochemical efficiency(ΦPS Ⅱ), maximum photochemical efficiency(Fv/Fm) decreased, indicating acid-treated leading to the photoinhibition of poplar produces light inhibition. Photochemical quenching(qP) decreased at the same time, indicating that the light energy(absorbed by chlorophyll) distributed to photochemical reactions was decreased. And the non photochemical quenching quenching(NPQ) and de-epoxidation degree of xanthophyll cycle increased, indicating that excess energy dissipated by NPQ and xanthophyll cycle increased significantly, showing that the poplar started up their own protection mechanisms under acid stress from damage or to prevent or reduce damage of PSⅡ.(5) With the increase of the concentration of phenolic acids, the content of malondialdehyde(MDA) increased gradually, which showed that the membrane system of the photosynthetic apparatus was damaged and the membrane lipid peroxidation increased. The activity of superoxide dismutase(SOD) and peroxidase(POD) increased first and then decreased in the leaves of poplar. At low concentrations of phenolic acids, the activity of SOD and POD increased to reduce the damage of photosynthesis by scavenging reactive oxygen.(6) The ammonium nitrogen(NH4+-N) content and leaf nitrogen content index(NDCI) showed decreasing trends with the increase of phenolic acid content, which indicated that phenolic acids was not conducive to the accumulation of nitrogen in poplar.Conclusions: Phenolic acids decreased root vigor of poplar, meaning that the absorption ability, synthetic ability, and oxidation and reduction ability was weakened. Light adaptation and use efficiency of strong light decreased, showing that photoinhibition occurd. Phenolic acid lead to the damage of PS II reaction center, and the electron transfer of PS II center reduced. |
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