(a) Abscissic acid (ABA) : It is known as growth inhibitor. It is synthesised in leaves and transported to other parts of the plants by phloem. It promotes the dormancy of seeds. ABA inhibits seed germination. The dormancy of seeds is due to abscissic acid. It usually interacts with other plant growth regulators. Dormant seeds germinate when ABA is overcomed by Gibberellins.
(b) Far-red region of light : Phytochrome is regulatory pigment which controls several light dependent developmental process in plants i besides germination. Phytochrome exists in two interconvertible forms : Pr and Pfr. On absorbing red light Pr and Pfr becomes Pr either rapidly by absorbing far-red light or slowly in darkness. Germination promote this. Darkness promotes Pr form formation which induces dormancy and inhibits germination.