The relationship between ethylene and Polygalacturonase(PG) was thoroughly investigated in two aspects of the effect of ethylene on PG function and the regulation of PG on ethylene synthesis.

本课题从乙烯对PG作用的影响和PG对乙烯的反馈调节两方面研究了乙烯与PG之间的关系。

youdao

But, like a lot of these films, this one has a parental guidance rating of "PG-13."

VOA: special.2011.06.20

The MPAA has never revealed its guidelines for what qualifies a movie for a G, PG, PG-13, R or even the dreaded NC-17 rating, but Hollywood producers have a second sense for the tipping points between categories.

WSJ: Roger Ebert on Movie Ratings

They know a G film is harmless and a PG almost certainly is, and a PG-13 may or may not be.

WSJ: Roger Ebert on Movie Ratings

Scenes of smoking have become increasingly rare in non-R-rated movies, according to the MPAA. Of the movies portraying smoking in recent years, 72% were rated R, 21% were rated PG-13, and 6% were rated PG.

CNN: Should smoking trigger an R rating?