Indian Journal of Marine Sciences Vol. 28, March 1999, pp.66-69 Gibberellic acid and 2,4-D as growth regulators in laboratory culture of seaweeds Imelda Joseph* & V S K Chennubhotla Central Marine Fisheries Research In stitute, Post Box 1603, Cochin 682 014, Kerala. In dia Received 25 November 1997; revised 5 October 1998 The growth regulatory effects of gibberellic ac id (GA) and 2,4-D (2,4-dichlorophenoxy acetic ac id) on a green alga, Ul vaf asciata and two species of red algae, Gracilaria corticata and Hypnea valentiae were st udied. For a culture period of 30 day s. the thallus segments showed vary in g growth rates on pretreatment with 6 to 20 mg/I of th e ab ove growth substan ces. Th ese compounds sti mul ated gro wth and increased wet weight of the algae. At hi gher concentrations ( 16-20mg/l) 2,4-D was growth inhibitory, whereas, GA showed significan tl y higher growth at 16-20mg/1 concentrati ons. It is concluded with the present stu dy th at pl an t hormones like GA and 2,4-D could be used for enhancement of algal producti on in culture practices at different concentrations. In view of th e in creasing dem and for seawee ds , it has become imperative to culture them, as the na.tural stock is being heavily ex pl o it ed througho ut the world, and demand for algal produ cts exceed supply . Besides increasing th e total supply , sea weed culture is also attrac ti ve in that it ha s the capac ity of providing high quality r aw material. Use of growth promoting substances or hormones to improve the bioma ss produ ction, in lagoons and brackishwater areas is one of th e variou s modern techniques in alga l culture. Many studies have tried the effect of different growth r eg ulating subs tances like natu ra l au xi ns, gibbereHins, kinetin and o th er synthe ti c phytohormones like 2,4-0 a nd NAA (naphtha le ne acetic ac id) on seve ral seaweed s t. !O. The ' pre sent s tudy was carried out to inves ti gate the effect of growth regulator s, GA a nd 2,4 -0 on the growth of thalli of three selec ted species of seaweeds of economic value, growing luxuriantly alo ng the Kera la coast , by laborato ry culture experiments. Materials and Methods The sea weeds selec ted for the study were VIva fasciata Oelile (C hl orophyceae),Hypnea valentiae (Turn) Mont (Rhodop hy ceae) and Graci/aria corticata JAg. (Rh odop hy ceae). They were co llected from Thiruvananthapuram and Koz hikode districts, during June- Jul y durin g so uth-west monsoon in 1988. The substances tes ted for th eir g rowth regulatory properties were GA and 2,4-0 (Sigma, USA) . The selected specimens were thoroughly washed and trans ported in ice-box with seawater to the laborat ory. It was aerated and maintained in fresh filtered seawater as stock culture. The culture studies were done according to the *Present address:Central Institute of Brackishwater Aquaculture, 141 , Marshalls Ro ad , Egmore. Chennai600 008, India cut-piece' method II for Graci/aria corticata and Hyp/1 ea va/e]1tia e . Cut-pieces of 20 mm and 30 mm were taken res pec ti vely for the above two species. For VIva fas ciata, piec es of thallus l Ox I 0 mm size were cut from the middle re gio n of uniform sized plants. The growth sub sta nce so luti ons were prepared at co nce ntra ti ons of 6, 8, 10, 12, 16 and 20 mg/ I in sterilized seawa te r. These co nce ntr ations were sel ec ted 2 after preliminary experiments with co nce ntrations in the ran ge of 0 . 1 to 10 mg/1. Pretreatment was done for 24 h in the tes t co nce ntra ti ons for the thr ee species and co ntrols were maintained without pretreatment. Th e thalli pieces were then tran sferred to sterile petri-dishes of 90 mm diameter co ntaining 50 ml of modified Erdshr i ebe r culture medium l2 in triplicates. The sa linity and pH of th e medium were maintained at 30 ppt and 7.5 respectively . The cultures were kept on illuminated racks fitted with flu oresce nt lamp s of 40 W, at room te mperatur e ranging from 26° to 28°C. The li ght intensity ranged between 1000 to 1500 lux. Photoperiod was maint ained for 12: 12 LO. The culture medium was renewed on alternate day s and the exper iment was run for 30 day s, since maximum growth occurs during the first 30 day s during the culture period . Experiments were repeated for more re li ability of the results. Linear growth meas urements of ' cut-pieces' of Gracilaria corticata and Hypnea valentiae, from treatments as well as controls were taken to the nearest millimete r on every five days . As the thallus of Viva fasciata is uniformly two layered and thin, it is possible to record areal measurements as an index of growth. These measurements were also taken at 5 day intervals. Results The growth effects of GA and 2,4-0 on cut-pieces of