Plant growth regulators are organic compounds other than nutrients they modify plant physiological processes. These are called bio-stimulants or inhibitors. They inhibit enzyme system and also effect plant senescence but they increase yield in plants indirectly.

some organic molecules like phenolic compounds , flavonoids and it.s derivatives were be used as activators for some plants growth. this study under construction.

As far as I remember, PGR were used in cotton cop for advancing leaf senescence and to induce leaf shedding to facilitate harvesting the mature bolls. If you check Australian and USA literature in the 60s and 70s, you may get some good leads.