Factors influencing the growth of seagrass seedlings: A case study of Posidonia australis

•Growth of P. australis not significantly enhanced by hormones or nutrients.•Initial seedling size determined subsequent growth.•Literature review revealed little evidence for hormone enhancing seagrass growth.

Seagrasses of the genus Posidonia are known to be particularly vulnerable to disturbances because damaged plants are slow to recover and recolonisation by seedlings is rare. Reestablishment of denuded areas by propagules is therefore likely to rely on maximising the survivorship of seedlings or vegetative fragments. This study investigated methods for enhancing growth rates of P. australis seedlings using hormones and nutrients, which may increase establishment success of plants used for rehabilitation. Laboratory experiments compared seedlings treated with different combinations and concentrations of hormones (auxin, cytokinin, gibberellic acid) and nutrients (N, P, K). Growth rates of healthy seedlings were not significantly enhanced by any treatment and negative effects of 30 mg/L auxin and nutrients were evident. Any positive effects of hormones were limited to improving survival of small seedlings with the addition of low concentrations of auxin (5 mg/L) or cytokinin (2.5 mg/L), and 0.1 or 20 mg/L gibberellic acid. A review of hormone effects on seagrasses found few consistent results among studies, in part due to differences in methods, species and hormones used. The few studies that were comparable tended to have conflicting results and we found relatively little evidence for positive effects of hormones on the growth of any seagrass. A final experiment demonstrated that the greatest influence on growth of P. australis seedlings was the initial size of seedlings (just after being released from the fruit). Large seedlings produced significantly larger plants (1.5–2.3 fold for various above and below ground measures) over 232 d than did initially small seedlings. Rehabilitation programmes should therefore use the largest seedlings available to help maximise the chances of success.