Effects of UV-B on Chlorophyll Biosynthesis in Greenhouse Nectarine Leaves
In order to clarify the key steps in the response of chlorophyll(Chl)biosynthesis to ultraviolet B(UV-B)irradiation in greenhouse nectarine leaves,the 7-year-old nectarine'Zhongyou5'was used as the test material,which was cultivated in greenhouse from 7 days after flowering.The appropriate dose of UV-B(about 1.44 kJ·m-2·d-1screened by previous experiments)was supplemented at 8:30-9:30 every morning until the fruit was harvested.The dynamic changes of precursor substances,intermediate product content,Chl content and key enzyme activity in the process of Chl biosynthesis in leaves during the growth and development period were investigated with normal growth and unsupplemented UV-B irradiated plants as the control.The results showed that in the first stage of Chl synthesis,UV-B irradiation increased the accumulation of δ-aminolevulinic acid(ALA)by increasing the activity of glutamate-l-semialdehyde-2,1-aminomutase(GSA-AM),a key enzyme in the synthesis of precursor ALA,which laid a good material foundation for the beginning of biosynthesis.In the second stage,UV-B increased the activity of bile pigment deaminase(PBGD),promoted the synthesis of hydroxymethyl bile pigment,and provided sufficient substrates for the synthesis of subsequent products uroporphyrinogen Ⅲ(Urogen Ⅲ),so that the content of Coprogen Ⅲ and Proto Ⅸ in leaves increased significantly during the whole growth period.After UV-B irradiation,the activities of key enzymes such as Mg-chelating enzyme(MgCH)and Mg-protoporphyrin Ⅸ monomethyl ester cyclase(MgPEC)in the third stage were significantly increased,and the content of intermediate products Mg-protoporphyrinⅨ(MgP Ⅸ),Mg-protoporphyrin Ⅸ methyl ester(MgPME),bivinyl protochlorophyllate,protochlorophyllate(Pchlide),chlorophyllate a(Chlide a)and chlorophyllate b(Chlide b)were all increased,the chlorophyll content of the product was also significantly higher than that of the control.It can be seen that the main key steps of UV-B irradiation affecting Chl biosynthesis are the synthesis of precursor ALA,the synthesis of hydroxymethylcholanogen,the conversion of Urogen Ⅲ to CoprogenⅢ,the insertion of Mg2+into Proto Ⅲ and the following four steps,and the effect of UV-B on Chl biosynthesis has a long-term accumulation.
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