Study on the effect of explant size on direct shoot regeneration capability of seaweed Kappaphycus alvarezii(Rhodophyta, Solieriaceae) in Vietnam

Direct regeneration is a biological technique used to create a large quantity of seaweed seedlings from mother plants. In this experiment, Kappaphycus alvarezii, ranging in size 0.25-2.0 cm, was cultivated in a Provasoli's Enriched Seawater (PES) medium supplemented with 5 mg/l of Indole-3-acetic acid combined with 1 mg/l of N6-benzyladenine. After 60 days of the experiment, the size of the explant significantly affected the direct regenerative capability of Kappaphycus alvarezii. The 0.25 cm-sized explants showed the lowest survival and regeneration rates 13.33±3.34%, with an average shoot count of 3.08±0.14 shoots/explant and an average shoot length of 1.16±0.56 mm. The 0.75 cmsized explants exhibited high regenerative efficiency, with the best shoot formation time (12 days of cultivation), a shoot formation rate of 83.33±3.34%, an average shoot count of 3.77±0.18 shoots/explant, an average shoot length of 6.37±0.55 mm, and the highest growth rate of 1.13±0.02% per day. The 1.5 and 2 cm-sized explants produced the highest shoot count (8.05±0.32 and 11.54±0.25 shoots/explant, respectively), corresponding to an average shoot length of 1.45±0.27 and 1.21±0.12 mm, and growth rates of 0.72±0.08 and 0.70±0.04% per day, respectively. The results indicated that the 0.75 cm-sized explants are considered the best choice for the clonal propagation of K. alvarezii seaweed. The regenerated seaweed was then transferred to a nursery tank and showed a good adaptation with a survival rate of 100% and a growth rate of 1.79±0.32%/day.