Physiological and biochemical effects of a tetrahydropyranyl-substituted meta-topolin in micropropagated Merwilla plumbea

In comparison to the most active cytokinins (CKs) previously reported for the micropropagation of Merwilla plumbea, we examined the effect of meta-topolin tetrahydropyran-2-yl (mTTHP-a novel aromatic CK derivative) on in vitro adventitious shoot production, rooting and photosynthetic pigment content of regenerated plants. Its carry-over effect on ex vitro growth, photosynthetic performance and antioxidant enzyme system of this bulbous medicinal plant was also investigated. The treatments with mTTHP and meta-topolin riboside (mTR) gave the highest number of adventitious shoots when compared to thidiazuron (TDZ) application and the control. The highest rooting frequency was observed in mTTHP treatments. Unlike in mTTHP treatments, an increase in mTR or TDZ concentration beyond 0.5 A mu M resulted in a significant decrease in the concentrations of all the photosynthetic pigments quantified. After 6 months of ex vitro growth, regenerated plants from 0.5 A mu M mTTHP treatment had the highest significant total leaf area, total leaf fresh weight and bulb size compared to all mTR and TDZ-treated plants. Plants regenerated from mTTHP or mTR treatments demonstrated a high capacity for energy dissipation in comparison to TDZ-regenerated plants with low photochemical quenching, PSII quantum efficiency and non-photochemical quenching. Despite a significant increase in the antioxidant enzyme activities, malondialdehyde concentration was significantly high in the leaves of TDZ-regenerated plants compared to other CK treatments. This finding indicated a high production of reactive oxygen species beyond the scavenging efficiency of the antioxidant enzymes leading to oxidative stress and subsequent low biomass accumulation in TDZ-derived plants.