Cytokinin-like activity of some species of Rhodophyta from tropical and subtropical waters

The use of seaweed extracts as biostimulants is increasing worldwide to improve agricultural production, since they are natural and environmentally friendly compounds. Seaweeds biosynthesize plant hormones, including cytokinins which play an important role in plant growth and development. Cytokinin-like (CK) activity was analysed in eight species of Rhodophyta, including agarophytes (Gracilaria spp., Gelidium), and carrageenophytes (Chondracanthus, Hypnea). Comparison of CK-activity of different samples of the same species (e.g., field and cultured samples, different color and morphological variants, and reproductive phases) is described. Dried samples were extracted for 24 h at 10(degrees)C in 100 mL 80% ethanol, filtered and rinsed with 80% ethanol. The extracts were purified using C18 Sep-Pak cartridges and Oasis MCX columns and separated into fractions by descending paper chromatography. CK-like activity was determined using the soybean callus bioassay with activity compared to kinetin standards (1, 5, 10 and 50 mu g L-1). The green strain of Hypnea psedomusciformis was more active than the brown strain with most activity co-chromatographing with zeatin. Tetrasporophytes and female gametophytes of Gracilaria birdiae had CK-like activity that co-chromatographed with zeatin, and female gametophytes had an additional peak that co-chromatographed with riboside-5'-monophosphate. The highest CK-like activity (equivalent to 50 mu g L-1 kinetin) occurred in G. cervicornis which co-chromatographed with isopentenyladenosine. Morphological variants of Hypnea had similar CK-like activity but the "nigrescens" variant had higher activity (equivalent to 10 mu g L-1 kinetin) than the "musciformis" variant. These results indicate that Gracilaria cervicornis and Hypnea pseudomusciformis ("nigrescens" variant) have potential as biostimulants in agriculture.