Effects of NAA and <i>Ecklonia maxima</i> Extracts on Lettuce and Tomato Transplant Production

<i>Ecklonia maxima</i> and the commercial biostimulants produced from it contain various plant growth regulators that are responsible for the growth stimulation recorded in many crops. Auxins are one of the major plant growth regulators contained in <i>E. maxima</i> extracts. The aim of this research was to evaluate the growth-promoting effect of a seaweed extract from <i>E. maxima</i> on lettuce and tomato transplant production under nursery conditions, and to compare the effect of this extract with an equal concentration of synthetic auxin. Two doses of natural or synthetic exogenous auxins (50 or 100 μg L⁻¹) were supplied to the substrate through the irrigation water with an ebb and flow system, 4, 11, and 18 days after sowing. A commercial biostimulant based on <i>E. maxima</i> extract was used as a source of natural auxin, while 1-naphthaleneacetic acid (NAA) was used as a synthetic auxin. Seedlings supplied only with water were used as a control. Tomato seedlings treated with 100 μg L⁻¹ of natural auxins from <i>E. maxima</i> extract produced the tallest plants (+22%), with a higher leaf number (+12%), a wider leaf area (+44%), and a stronger stem (+12%), whereas lettuce seedling growth was promoted by all the treatments, but with a greater effect with increasing auxin supplementation and when using <i>E. maxima</i> extract, compared to NAA. The results showed that the supplementation of exogenous synthetic auxin (NAA), or an <i>E. maxima</i> extract containing natural auxins, can have a growth-promoting effect on lettuce and tomato seedlings. This effect was more evident on lettuce than tomato. The biostimulant produced from <i>E. maxima</i> extracts improved seedling quality and promoted shoot and root growth more than the NAA used as a synthetic source of auxins.