Plant growth biostimulant activity of the green microalga Desmodesmus subspicatus

The plant biostimulant activity of the green microalga Desmodesmus subspicatus was investigated, focusing on its potential application to organic crops. Different concentrations of D. subspicatus biomass suspensions were applied to tomato seedlings (Solanum lycopersicum) via a foliar spray. The treatments promoted root growth and foliar area, which increased when the suspension was applied at concentrations of 1 g L-1 and 0.4 g L-1, respectively. To elucidate the major constituents responsible for this potential activity, the biomass of D. subspicatus was submitted to aqueous extraction and dialysis (1 kDa cut-off), giving rise to the non-dialyzable fraction E (12.3%). Size-exclusion chromatography performed on fraction E using a Bio-Gel P-2 column yielded six subfractions. Gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) analyses of the E subfractions revealed the presence of glycosides as major constituents (approx. 81%), including one acidic glycoside, [6-sulfo-alpha-D-quinovopyranosyl-(1 -> 1)-glycerol], and two neutral galactosyl glycosides, [alpha-D-galactopyranosyl-(1 -> 6)-beta-D-galactopyranosyl-(1 -> 1)-glycerol] and [beta-D-galactopyranosyl-(1 -> 1)-glycerol]. Ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) analyses of fraction E resulted in the identification of the phytohormone zeatin (35 mu g g-1). To evaluate the biostimulant potential of fraction E, different concentrations (0.5, 1.0, 1.5, and 2.0 g L-1) were tested for their effects on tomato seed germination, hypocotyl volume, and length. All tested concentrations induced increased hypocotyl lengths and volumes compared with standard treatment (water), at rates equivalent to those in response to a commercial product derived from Ascophyllum nodosum brown seaweed. These results indicated that the biomass suspension and aqueous extract of D. subspicatus showed plant growth biostimulant activity. Glycosides, together with zeatin, may play important roles in biostimulant activity.