Micropropagation of Hoya carnosa, H. kerrii, H. parasitica, and H. longifolia using tray-based floating and stationary hydroponic systems

Abstract

Hoya is a wild ornamental plant with medicinal properties that have been extensively exploited from its natural habitat, causing a reduction at an alarming rate. The current study is aiming to fabricate an affordable and simple-to-use hydroponic system for the conservation and mass production of four Hoya species (H. carnosa, H. kerrii, H. parasitica, and H. longifolia). Tray-based floating and stationary hydroponic systems consist of Hoya plants (cuttings) inserted in a thermocol sheet and suspended in MS nutrient solution. Cuttings derived from mature plants were subjected to full MS (Murashige and Skoog), 1/2 MS, 1/3 MS, and 1/4 MS nutrient media for growth optimization. Subsequently, the ½ MS nutrient medium with explants was further augmented with different concentrations (0.5, 1.5, 2.5, 3.5, and 4.5 µM) of auxins like 1-naphthaleneacetic acid (NAA), indole-3-butyric acid (IBA), indole acetic acid (IAA) and IBA (2.5 µM) optimized for rooting. A combination of 1/2 MS + IBA (2.5 μM) proved to be the optimal medium for rooting in Hoya species when compared to all other treatments.Further, improvements in root number were recorded with augmentation of 25, 50, 100, 150, and 200 µM of Zinc Sulphate (ZnSO4). Half MS nutrient solution with 2.5 µM IBA and 100 μM ZnSO4 resulted a maximum of 15 roots in H. carnosa, 20 in H. parasitica, 38 in H. kerrii, and 19 in H. longifolia. The effect of 5.0 µM GA3 (Gibberellic acid) was also evaluated on bud break in rooted plants. Foliar spray of GA3 on acclimatized plants subsequently reduced the bud break time, and the highest reduction of time was observed from 21 d to 7 d in H. parasitica. Transfer of hydroponically raised plantlets was under stress during in vitro to field conditions. Stress markers (MDA, SOD, and CAT) for defense and photosynthetic pigments for better survival of hydroponically grown plants were monitored during acclimatization. MDA content was stable at 60 to 70 d while SOD and CAT activity was stabilized and influenced upon its peak on the same days under the poly-house condition in the propagated Hoya species. Photosynthetic pigments were gradually increased, and maximum values were recorded at 70 d of acclimatization. The present study proposed a reliable protocol for the conservation and mass propagation of most of the Hoya species.