Early plant growth: identifying the end point of the seedling phase.

• Despite the importance of seedling establishment in plant biology, there is no consensus on what constitutes a 'seedling'. Here we examined aspects of early plant development that could be used to mark the transition from the seedling to the postseedling phase. • Using a hypogeal species (pea, Pisum sativum) and an epigeal species (sunflower, Helianthus annuus), we investigated whether the utilization of cotyledon-stored mineral nutrients coincides with any changes in relative growth rate (RGR). We also examined how the timing of cotyledon removal at different points during early development affected subsequent growth. • For both species, the timing of RGRmax , the exhaustion of cotyledon reserves, and the attainment of independence from cotyledons all roughly coincided (though exhaustion of seed reserves was not observed in the epigeal species because the cotyledons absorbed external nutrients). • We conclude that because the point of attainment of RGRmax is a distinctly identifiable event, it is a more reliable marker for defining the end of the seedling stage than either the exhaustion of mineral reserves, or the cessation of dependence on cotyledon reserves.