Structural Adaptation and Antioxidant Response of Guarianthe bowringiana (O’Brien) Dressler & W. E. Higgins (Orchidaceae Juss.) Seedlings During Ex Vitro Acclimatization

Abstract

Guarianthe bowringiana is one of the oldest samples cultivated at NBG’s orchid unit glasshouses since 1970s. An efficient protocol for asymbiotic in vitro seed germination of G. bowringiana has previously been established. Given that acclimatization is a crucial step in micropropagation, this study assesses the structural adaptation and antioxidant response of G. bowringiana seedlings during ex vitro acclimatization to ex vitro conditions. The leaf surface micromorphology of the G. bowringiana juvenile plants propagated in vitro from seeds as well as the leaves of adult plants cultivated in glasshouse were analyzed using scanning electron microscopy. The levels of lipid peroxidation (TBARS level), superoxide dismutase (SOD) activity, and the photosynthetic activity were monitored for seven days from the transfer of seedlings from the in vitro cultivation vessels as they are markers indicating the response of the leaves of in vitro propagated G. bowringiana plants to oxidative stress during the early stages of acclimatization to ex vitro conditions. During the initial 2 days of the monitored acclimatization period (0–7 days), the level of photosynthetic pigments (chlorophyll a , b , and carotenoid content) increased, followed by an insignificant increase during the successive period (by the seventh day) of acclimatization. At the same time, the level of the tested antioxidant enzyme (SOD) exhibited an increasing trend throughout the acclimatization period. The SOD activities in the leaves of G. bowringiana seedlings were significantly affected when they were transferred from in vitro to ex vitro conditions due to drought stress. Thus, it was revealed that in the early stages of acclimatizing to the altered environments, G. bowringiana seedlings exhibited a rapid increase in photosynthetic pigments, superoxide dismutase activity, and lipid peroxidation levels after being transferred to ex vitro conditions. Comparison of the leaf micromorphologies of G. bowringiana plants grown under in vitro and those grown under ex vitro conditions revealed that leaf development had undergone significant changes during acclimatization to the altered conditions. In vitro to ex vitro transfer leads to a transient decrease in photosynthetic parameters.