Studies On Mechanisms Of Alleviative Effects Of Rare Earth Ions On Longan (Dimocarpus Longana Lour.) Under Acid Rain Stress

Longan was used to study the chlorophyll fluorescence characteristics, active oxygen metabolism, antioxidant systems, pollen germination and fruit setting and their regulation of rare earth under the simulated acid rain stress. The main results are as follows:1．Under acid rain stress, contents of chlorophyll a, b and carotenoid decreased while Chla/Chlb increased. The sensitivity of the pigments to acid rain was Chlb>Car>Chla. Treatments of La³⁺ and Ce³⁺ enhanced the content of leaf photosynthetic pigment which alleviating the degradation of acid rain stress and the optimum concentration was 20 mg·L^-1 , while the Chla/Chlb was not influenced. It is presumed that the rare earth ions may couple in the chlorophyll porphyrin rings forming Chl-La complexes and protect chlorophyll by preventing from replacement of Mg²⁺ by H⁺.2．Curves of fast chlorophyll fluorescence induction kinetics showed that, under acid rain stress, minimal fluorescence production in longan leaves increased, while the maximal fluorescence production and variable fluorescence production decreased, resulting in the decrease of maximal quantum field of PSâ…¡photochemistry (Fv/Fm). Furthermore, acid rain stress led to the inactivation of longan leaves PSâ…¡reaction centers, the inhibition of transfer of linear electrons, and the decrease in content of PSâ…¡acceptor. The less influence of acid rain stress on the oxygen evolving complexes (OEC) of the PSâ…¡donor side was found. The main reason for decreasing of electron transfer rate was the inhibition of electron transfer from Q_A to Q_B which causing the mass accumulation of Q_A and the marked damage to photosynthetic apparatus was caused three days after acid rain stress. Treating with La³⁺ and Ce³⁺ reduced the proportion of inactive reaction centers and enhanced the probability of the electron transfer to other electron acceptor over Q_A through enhancing the amount of active reaction centers of PSâ…¡in leaf to alleviate the damage of reaction centers and acceptor side of leaf PSâ…¡under acid rain stress.3．Energy pipeline models of reaction centers in PSâ…¡of longan leaf after treatments of acid rain stress and rare earth elements regulation were obtained. The models revealed that, under acid rain stress, photo-energy absorption and energy dissipation in PSâ…¡reaction centers of longan per unit area leaf model increased, while the excited energy capture and electron transfer decreased in that. Absorbing and excited energy in per active PSâ…¡reaction centers increased, electron transport per PSâ…¡reaction centers was blocked apparently, and resulted in an increase of energy dissipation. Reasons for the decreasing photosynthetic rate in longan leaf under acid rain stress were also analyzed by the models. We presumed that the decreasing photosynthetic rate was more likely resulted from the impairment of electron transfer than from the decreasing content of chlorophyll.4．Lots of free radicals (O₂·ˉand H₂O₂) in longan leaf were induced under acid rain stress, which attacked the cell membranes and induced the membrane lipid peroxidation. When the stress was taken away, the content of free radicals and the degree of membrane lipid peroxidation still remained at a high level. Treatments of La³⁺ and Ce³⁺ reduced the content of free radicals in leaf, protecting the cell membrane and alleviating the oxidation damage of longan leaf cell membrane under acid rain stress. It is probably that the rare earth io n exerted a function of scavenging free radicals directly.5．Treatments of La³⁺ and Ce³⁺ can effectively scavenge the reactive oxygen species induced by acid rain and alleviate the damage of membrane lipid and antioxidative system in longan leaf by enhancing the activities of antioxidative enzymes (SOD, CAT, APX, MDAR, DHAR and GR) and contents of antioxidants (AsA and GSH) and affecting AsA-GSH cycle. The rare earth ion might exert a function similar with calcium signal and play a positive role in AOS messenger induced antioxidative system in longan leaf. It was affirmed that treatments of rare earth ions enhanced linear electron transfer of PSâ…¡reaction centers in longan leaf, producing a large number of NADPH to promote AsA-GSH cycle and enhance APX activity. Under physiological condition, the best application concentration of rare earth ion was 30 mg·L^-1 , while under stress condition, it declined to 20 mg·L^-1 .6．In the media lacked of calcium, La³⁺ and Ce³⁺ enhanced the germination rate of longan pollen in vitro and alleviated the negative effect of low pH (acid stress) and lack of calcium, revealing that rare earth ions could replace the biological function of calcium to some degree. Furthermore, low concentration (0．1-1μl·L^-1 ) of La³⁺ and Ce³⁺ enhanced the concentration of free calcium in pollen cells markedly, causing calcium oscillation and the increase of Ca²⁺-ATPase and H⁺-ATPase activities which were downstream target enzyme regulated by calcium signal, alleviating the negative effect of acid rain on the pH condition and ion balance in pollen cells, and enhancing the pollen germination rate. On the contrary, high concentration (10μmol·L^-1 ) of rare earth ions aggravated the damage of pollen under acid rain stress and inhibited the germination.