Etiolation reduces phenolic content and polyphenol oxidase actiivity at the pre-culture stage and in-vitro exudation of phenols from mango explants

The effect of etiolation on phenol content and polyphenol oxidase (PPO) activity at the pre-culture stage and on post-culture phenolic exudation and explant survival was studied in shoot tips of eight mango (Mangifera indica L.) cultivars, viz., Amrapali, Mallika, Langra, Bhadauran, Tommy Atkins, Eldon, Sensation, and Alphonso. Etiolation treatment resulted in marked decline in PPO activity and phenol content at the pre-culture stage and phenolic exudation at the post-culture stage, which in turn increased the explant survival in Murashige and Skoog's culture medium. Etiolated shoots had low PPO activity (0.496 ΔA400 g-1 min-1) and phenol content (11.19 mg g-1 fresh tissue wt) at the pre-culture stage, exuded a smaller amount of phenols into the culture medium, and had higher explant survival (36.1 %) compared to non-etiolated shoots, which had higher PPO activity (1.081 ΔA400 g-1 min-1) and phenolic content (18.27 mg-1 fresh tissue wt), exuded a greater amount of phenols into the medium (0.250 mg mL-1 medium), and had low survival of explants (7.7%). Among the cultivars, Bhadauran exhibited the highest PPO activity (1.468 ΔA400 g-1 min-1) and phenol content (27.74 mg g-1 fresh tissue wt), exuded the greatest amount of phenols into the culture medium (0.340 mg mL-1 of medium), and had the minimum explant survival (3.4%) in contrast to Tommy Atkins, which exhibited the lowest PPO activity (0.582ΔA400 g-1 min-1) and phenol content (8.17 mg g-1 fresh tissue wt), exuded the smallest amount of phenols into the medium (0.095 mg mL-1 of medium), and had the highest explant survival (38.6%). Explant survival, in general, was very high (59.0%) initially (3rd day after culture initiation) but declined drastically thereafter to 0.9% after the 21st day of culture. The interactions between etiolation x cultivar, etiolation x time, cultivar x time, and etiolation x cultivar x time, were significant for explant survival. A strong positive correlation existed between PPO activity and phenol exudation (r = 0.912) and phenol content and exudation (r = 0.892), whereas the correlations between PPO activity and explant survival (r = -0.872) and phenol content and explant survival (r = -0.899) were strongly negative.