The flavonoid content determines the quality of Ginkgo biloba that can be increased by using of plant growth regulators. The objective of study was to observe the effect of 5-aminolevulinic acid (ALA), a key precursor in the biosynthesis of porphyrins and a new plant growth regulator, on photosynthetic rate, chlorophyll and soluble sugar content, flavonoid accumulation, and flavonoid enzyme activity in G. biloba leaves. The ginkgo seedlings were grown in greenhouse conditions under low levels

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... 0 and 100 mg l-1) of foliar application of ALA. Photosynthetic rates of leaves increased significantly at day 4 in response to both ALA concentrations and remained elevated as compared to control for further 12 days. Chlorophyll and soluble sugar contents were significantly increased by day 4 and continued to increase by day 16; however, Chl a/b ratio remained unchanged. Total polyphenols, flavonoids, and anthocyanins, phenylalanine ammonia-lyase (PAL), chalcone synthase (CHS) and chalcone isomerase (CHI) activities were increased from day 4 to 16 after ALA treatment. The increase in chlorophyll and soluble sugar contents, and activities of flavonoid enzymes (PAL, CHS and CHI) were likely to be closely associated with improvement of the accumulation of total polyphenols, flavonoids, anthocyanins and advance of leaf quality by ALA treatment. Foliar treatment with a low concentration of ALA therefore, might provide a useful means of improving pharmacological properties of G. biloba leaves.