| With the destruction of ozone layer,ultraviolet-B radiation has being enhanced at the earth's surface.The enhanced UV-B radiation would affect plants in various extents. Up to now,the effects of UV-B Radiation on organisms have been studied extensively. However,the investigated materials in these studies were mainly herbaceous plants, very few woody plants were explored,Furthermore,the conclusions from these studies have many inconsistencies.One-year seedlings of Camptotheca acuminata were potted and grown in the open.The plants received an enhanced UV-B radiation of 5.0μW·cm-2was applied with an artificial lamp system,which approximated the predicted enhanced UV-B reaching the earth surface when stratosphere ozone was depleted by 20%in the Xi'an, and the control plant received the ambient UV-B.During the experimentation,the effects of enhanced UV-B radiation on physiological and ecological characters were measured.The results show as follows:(1)Enhanced UV-B radiation induced increase of flavonoids,soluble proteins, camptothecin(CPT)and hydroxy camptothecin(HCPT)accumulation in short time.It suggested that C.acuminata seedlings could enhance the resistance to UV-B radiation and alleviate the damage of UV-B radiation by increasing content of secondary metabolites and enzymes related activity.Phenylalanine ammonia-lyase(PAL,EC4.3.1.5),as one of the critical enzymes in the synthesis of flavonoids,its activity was regulated by UV-B radiation.The PAL activity in leaves increased under UV-B radiation,it accelerated the transformation from L-phenylalanine to transcinnamic acid,thereby providiong more precursors for synthetizing flavonoids,and forming more UV-B-absorbing compounds.The flavonoids content increased significantly during enhanced UV-B radiation at daytime, and then declined at night when seedlings released from the stress of UV-B radiation. Meanwhile,the difference of flavonoids content between control and UV-B treatment decreased gradually.The activity of PAL showed a similar trend with the content of flavornoids.Tryptophan decarboxylase(TDC;EC 4.2.1.28)and tryptophan synthase (TRPS;EC 4.2.1.20)had been shown to be correlated with the accumulation of CPT under natural conditions.However,the activities of the two enzymes showed discriminating trends with increasing content of alkaloids in UV-B treated seedlings.(2)Long-time UV-B radiation inhibited the growth of plants and the plant weight, while the width of stem increased.Effects of enhanced UV-B radiation on leaf morphological structure and the contents of photosynthetic pigment and flavonoids in C.acuminata were investigated.The results showed as follows:①The contents of chlorophyll and flavonoids,the ratio of chlorophyll a to chlorophyll b(Chla/Chlb) increased in earlier stage and decreased in later stage under enhanced UV-B radiation compared with which growing under natural light,while carotenoids content increased constantly.②The leaf shape asymmetry occurred because of skewing,the photosynthetic pigments distribution in leaves were uneven,the adaxial epidermis was thickened,the chloroplast structure was distorted.③Further studies with scanning electron microscope(SEM)observation revealed that the number of epidermal hairs and glandular trichomes increased,glandular trichomes turned shorter and wider, stomatal apparatus in abaxial epidermis were coverd by waxy and aperture decreased.(3)The activities of NR and GS in C.acuminata leaves exposed to enhanced UV-B Radiation increased firstly,and then decreased.Enhanced UV-B radiation reduced the contents of NO3--N in leaves.Above mentioned results showed that enhanced UV-B radiation influenced the absorption of nitrate,assimilation of nitrogen and synthesis of amino acids,and might further affect the synthesis of proteins.As for the metabolism of proteins,the contents of soluble proteins in leaves of treated plants increased firstly,and then decreased compared with the control plants.The contents of soluble proteins in leaves of treated plants were lower than those of control plants at the later stage.The contents of soluble proteins in treated plants increased at first,was probably due to the synthesis of some resistance proteins induced by UV-B radiation. Maybe because of the increase in proteolytic enzyme activities,the soluble proteins contents of treated plants decreased.In a word,under UV-B radiation,the nitrogenous metabolism in leaves was in disorder.(4)Plants interact with their environment by producing a diverse array of secondary metabolites,one of which is alkaloid.The results indicated that CPT and HCPT are involved in the C.acuminata resistance against short-time environmental stress.HCPT is influenced greatly by the environmental factor.CPT,however,is a universal tactic.So HCPT is relatively unstable,while the CPT is more stable.HCPT and CPT accumulation were under developmental and environmental regulation,the contents of HCPT were higher than control occasionally,it did not show a very sensitive response to enhanced UV-B radiation.(5)Enhanced UV-B radiation significantly affected plant resource allocation in present study.The soluble sugar contents of leaves and stem increased in earlier stage and decreased in later stage,change tendency of sugar contents in root showed a similar trend with the conrol,however,content leaf was lower.The enhanced UV-B radiation in long-term increased carbon contents in stem and in root,while in leaves reduced in the final time.
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