Growth, callus formation and plant regeneration in color morphs of Gracilaria domingensis (Gracilariales, Rhodophyta) cultured under different irradiance and plant growth regulators

The effects of plant growth regulators and irradiance on growth, callus formation and the process of direct and indirect regeneration were studied in axenic cultures of the green and red morphs of Gracilaria domingensis. Apical and intercalary segments from both color morphs were cultivated in ASP 12-NTA solid medium (0.5% agar) supplemented with auxins (indole-3-acetic acid, IAA; 2,4-dichlorophenoxyacetic acid, 2,4-D) and cytokinins (kinetin; 6-benzylaminopurine) in concentrations ranging from 0.5 to 50.0 mu M and under 50 and 150 mu mol photons m(-2) s(-1). In experiments to induce direct and indirect regeneration, explants and basal calluses isolated from apical and intercalary segments were cultivated in ASP 12-NTA liquid medium for 4 wk and then Von Stosch liquid medium for 3 wk under the same conditions as those noted for solid medium culture. The response to plant growth regulators and irradiance was dependent on the explant and color morph. In general, 2,4-D inhibited growth processes, while IAA and 6-benzylaminopurine were stimulatory. Neither plant growth regulator nor the two photon flux densities (PFD) influenced callus formation, although basal callus formation was greater than intermediate or apical callus formation. Compared to indirect plant regeneration, the percentage of direct plant regeneration was higher, and IAA (50.0 mu M), 2,4-D (0.5 mu M) and 6-benzylaminopurine all stimulated direct plant regeneration when color morphs were cultured under 150 mu mol photons m(-2) s(-1). Our results indicated that plant growth regulators and PFD played a regulatory role in growth and direct regeneration. However, the color morphs of G. domingensis showed little potential for regeneration from callus, with direct regeneration being more suitable for micropropagation.