The present investigation was carried out to study the effect of Plant Growth Regulators (PGRs) on b iochemical constituents of Cassia angustifolia. The foliar application of PGRs like Gibberellic acid (GA3), Indole acetic acid (IAA) and the growth retardant Abscisic acid (ABA) were applied at the concentrations of 25, 50 and 100 mg/L as foliar spray at 45 DAS up to the flowering stage at the interval of fifteen days. The physiological analyses were carried out on pre-flowering stage (75 DAS), flowering stage (90 DAS) and post-flowering stage (120 DAS). Results revealed that there was positive and highly significant increase in photosynthetic pigments, proteins and phenols due to all the concentrations of PGRs used. The improvement in basic metabolites through PGRs application may probably lead to enhancement in secondary metabolite contents. From the results of the present investigation, we can conclude that all the treatments of PGRs enhance the contents of photosynthetic pigments, proteins and phenols. The most important antioxidant like phenols having pivotal role in abiotic stress tolerance, were significantly increased at pre flowering, flowering and even post flowering stage, due to the application of ABA. This increase of phenols by PGRs treatments may be further enhancing the antioxidant capacity of Senna. The increase in phenolics in the leaves of Senna might be playing important role for stimulating the secondary metabolites in it.