PLBs (Protocorm-like bodies) at stage of GI 5 of Chang Daeng Orchid (Rhynchostylis gigantea
var. rubrum Sagarik) cultured in liquid NDM medium with 2% sucrose and 15% coconut water
(CW) were used for treating with colchicines in order to doubling chromosome. The treatments
consisted of five concentrations of colchicine (0, 0.05, 0.10, 0.15 and 0.20%) together with 3
duration of treatment 24, 48, and 72 hours. The results revealed that LD50 lethal dose affected to
50% of colchicine based on regression analysis was 0.14% for 72 hours. PLBs immersed in
colchicines at concentrations 0.20% for 72 hours provided a suitable survival rate of 26% and
induced the highest percentage of tetraploid (60%). The results provide the several different
traits between tetraploid and diploid plants, especially height of the plant. Growth rate and
height of tetraploid plantlets, including leaf shape and size were far lower than those of control.
Anatomical characters of the tetraploid plants revealed the highest stomatal densities at 3.00
cells/mm2 in comparison with control plantlets.