Comparative effect of 28-homobrassinolide and 24-epibrassinolide on the performance of different components influencing the photosynthetic machinery in Brassica juncea L.

BRs are polyhydroxylated sterol derivatives, classified as phytohormones. Plants of Brassica juncea var. Varuna were grown in pots and an aqueous solution (10 M) of two brassinosteroid isomers 28-homobrassinolide (HBL) and 24-epibrassinolide (EBL) of same concentration (10 M) was applied to their leaves. The treatment up-regulated the photosynthetic machinery directly by enhancing water splitting activity, photochemical quenching, non-photochemical quenching, maximum PSII efficiency, actual PSII efficiency, electron transport rate, stomatal movement, stomatal conductance, internal CO concentration, transpiration rate, net photosynthetic rate and carbohydrate synthesis. Moreover, the level of biochemical enzymes (carbonic anhydrase and nitrate reductase), reactive oxygen species (superoxide and hydrogen peroxide) generation, antioxidant enzyme activity and mineral status (C, N, Mg, P, S, K), which indirectly influence the rate of photosynthesis, also improved in the treated plants. Out of the two BR analogues tested, EBL excelled in its effects over HBL.