Objective This study aimed to provide guidance on pretreatment methods for plant leaves during field sampling, preservation, and experimentation by detecting changes in the contents of total phenol, flavonoid, and tannin in plant leaves after different pretreatments.

Method The leaves of herbaceous plants, shrubs, broadleaved trees, and coniferous trees were selected as research objects. The drying pretreatment methods included vacuum freeze-drying, oven drying, microwave drying, natural sun drying, and natural shade drying. The storage pretreatment methods involved storage temperatures of −20 and −80 ℃ and storage durations of 15 and 30 d. The effects of different pretreatment methods on contents of total phenol, flavonoid, and tannin in plant leaves were compared.

Result (1) After different drying treatments in the leaves of 8 plant species, the maximum content of total phenol in leaves mostly appeared in vacuum freeze-drying and microwave drying; the maximum flavonoid content of leaves mostly appeared in vacuum freeze-drying, oven drying and microwave drying; the maximum tannin content of leaves mostly appeared in vacuum freeze-drying and microwave drying. (2) The contents of total phenol, flavonoid and tannin in leaves of 8 species decreased with the increase of storage time, and the contents of secondary metabolites measured at −80 ℃ were mostly higher than those stored at −20 ℃.

Conclusion In experiments detecting the content of foliar secondary metabolites, we recommend to complete the measurement as soon as possible after sample collection. Vacuum freeze-drying or microwave drying methods can be used for sample drying pretreatment. If long-distance transportation is required after sampling from field experiment sites, and storage conditions at −80 ℃ cannot be guaranteed, short-term transportation and storage at −20 ℃ is acceptable for measurement of the content of foliar secondary metabolites. Microwave drying, as a fast, safe, and efficient drying method, has strong applicability in the pretreatment of plant samples in laboratories.