Effect of Aluminum Chloride on the Organogenesis of Two Types of Cymbidium In Vitro

The primary morphogenic pathway leading to the whole plant regeneration involves shoot organogenesis followed by root organogenesis in vitro (Malepszy, 2009). Apart from plant growth regulators, many treatments have been applied to improve the efficiency of regeneration of explant. One of them is incubation of cultures for a certain time under stress condition (low and high temperature, drought, salinity, metal). These types of stress have been found to have a positive effect on regeneration of plants (Puijalon et al., 2008). Aluminum (Al) is the 3rd most abundant element in the Earth’s crust (after oxygen and silicon), accounting for roughly 7% by mass. In soil, Al ions can be toxic to plants, but in combination with other minerals, it increases plant growth by enhancing phosphorus availability and activating the genes associated with abiotic stress (Noor et al., 2019). The effect of Al on plant growth, both toxic and beneficial, depends on the concentration and varies with species, physiological age, and growth conditions (Bojórquez-Quintal et al., 2017). Aluminum chloride (AlCl3) can produce metallic stress condition when added to culture media (Gallego et al., 2002). It enhanced shoot regeneration in date palm (Al-Mayahi, 2019), and increased micro-tuber and tuberous root production in Gloriosa superba L. (Subiramani et al., 2019). However, whether it can be used in orchid in vitro culture has not been reported yet. Cymbidium species are highly valued in the flower market due to its attractive foliage, flower color and pleasant aroma. So, a high quality plantlet is always on demand. Based on morphological and ecological characters, the genus Cymbidium can be differentiated into two types, protocorm-forming and rhizome-forming (Shimasaki and Uemoto, 1987). The protocorm and protocormlike body (PLB) forming type of Cymbidium are epiphytic, mostly common in tropical or subtropical regions and the rhizome-forming type includes terrestrial or saprophyte, which is widely distributed in oriental regions. The organogenetic pathways of PLB-forming and rhizome-forming types of Cymbidium are different (Ogura-Tsujita et al., 2007). The PLBs of PLB-forming Cymbidium are developed from apical meristem culture in vitro and developed shoots and roots within short period. In case of rhizomeforming types, rhizomes are developed directly from apical meristem culture in vitro and started forming more branches. However, shoot formation of a rhizome-forming type of Cymbidium is difficult compare with of PLB-forming type under an in vitro condition because rhizome has long dormancy period than PLBs (Shimasaki and Uemoto, 1987). In the present study we investigated the metallic stress effects of AlCl3 on in vitro cultures of two types of Cymbidium to identify its optimum concentration for regeneration of PLBs or rhizomes from inoculated PLB or rhizome, respectively, and formation of shoots and roots.