Physiological effects of a novel aromatic cytokinin analogue in micropropagated Aloe arborescens and Harpagophytum procumbens

Globally, there is a continuous effort geared toward improving micropropagation protocols with much emphasis on the type of cytokinins (CKs). We investigated the effect of meta-topolin tetrahydropyran-2-yl [mTTHP—a novel derivative of the aromatic cytokinin meta-topolin (mT)] on shoot proliferation, photosynthetic pigment content, phytochemicals and antioxidant activity of two widely used medicinal plants, Aloe arborescens and Harpagophytum procumbens. In terms of shoot proliferation, besides mTTHP and mT (at equimolar level) showing comparable effects, both topolins were significantly better than the control and benzyladenine riboside (BAR) in micropropagated A. arborescens. In H. procumbens, mT-treated cultures were the most responsive treatment at 2.5 μM when compared to the control. At 5.0 μM concentration, mTTHP and meta-topolin riboside (mTR) demonstrated a comparable effect on shoot proliferation. To a certain extent (particularly at low concentrations), mTTHP had a better rooting stimulatory effect when compared to the other CKs in both investigated species. In micropropagated A. arborescens, 2.5 μM mTTHP showed the best antioxidant activity (ferric reducing power) which was significantly higher than the control and all BAR-treated plants. Although 2.5 μM mTTHP exhibited a significant antioxidant activity (in β-carotene based assay) when compared to mTR at equimolar level in H. procumbens, it was not significantly different from the treatment with 2.5 μM mT and the control. As evidenced in the current study, the efficacy of the novel cytokinin may be species-specific and beneficial for few physiological parameters. It is conceivable that mTTHP is another viable alternative topolin with the extra advantage of inducing rooting at a low concentration.