Assessment of cell wall histochemistry of velamentous epiphytic roots in adaptive response of micropropagated plantlets of Vanda tessellata (Roxb.) Hook. ex G. Don

Abstract

Velamentous roots of orchids are desiccation resistant and help in anchorage and absorption of water and nutrients. The specialized multi-layered, spongy epidermis ‘velamen radicum’ is meant for epiphytic adaptation in economically and ecologically significant ‘grey orchid’ Vanda tessellata. The development of velamen tissue during various stages of plantlet growth determines its efficacy in mechanical protection and nutrient absorption. The present study aimed to analyze the development of cell wall histochemistry (lignin, cellulose, pectin, and suberin) in the epiphytic roots of V. tessellata during various stages of micropropagation. Sequential histochemical evaluation of roots revealed the interaction between developmental stages and cell wall composition of velamentous tissues, exodermis, endodermis, and vascular bundles. Bright-field microscopic analysis indicated that the cell wall composition of velamen layers and xylem vessels were lignin-rich, and tend to appear bright pink when stained with safranin and phloroglucinol. Cellulose and polysaccharides were intensely stained by ferric chloride and light green. Exodermis and endodermis were rich in pectin and suberin as evidenced by Sudan black staining. The highly sophisticated in vitro heterotrophic environment did not allow the physiological processes of cell wall depositions. The roots developed considerable lignins, cellulose, pectins, and suberin in the cell wall when exposed to a partially controlled ex vitro environment. Cell wall compositions were more significant under in vivo conditions as the roots accumulated the highest amount of lignins, cellulose, pectins, and suberin in velamen tissues, xylem vessels, exodermis, and endodermis. The study thus highlighted the pivotal role of cell wall histochemistry of epiphytic roots in adaptive responses such as the development of mechanical support and water retention capacity of tissue culture raised Grey orchid to survive successfully under stressful in vivo conditions.