The present invention relates to a composition comprising methyl jasmonate and a polysiloxane polyether for increasing the content of glucosinolates in adult plants of the genus Brassica, for example, broccoli. By means of foliar application of the composition on adult plants having a developed cuticle, a significant increase is achieved in the concentrations of glucosinolates (mainly glucoraphanin and neoglucobrassicin), in the floret of the plants, without degrading the organoleptic properties thereof.