Effects of Two Oligosaccharide Biostimulants on Growth, Physiological Traits and Hormone Levels in Tomato Seedlings

Cultivating high-quality seedlings is an important step in crop production. Commercial formulations containing chitosan or brown-algae oligosaccharides are used as plant biostimulants. This study was conducted to evaluate the effects of oligosaccharide biostimulants on the growth of tomato seedlings. We evaluated the effects of biostimulant treatment on growth indexes, photosynthesis, lipid-peroxidation indicators, antioxidant enzyme activities and relative expression levels of growth-related genes in tomato seedlings. The biostimulant application significantly increased the plant height, plant width, stem diameter and root–shoot ratio of tomato seedlings. In addition, photosynthetic rate, transpiration rate, stomatal conductance, maximum quantum yield of photosystem II (Fv/Fm) and chlorophyll content were also significantly increased. Superoxide dismutase (SOD) and peroxidase (POD) activities increased, whereas malondialdehyde (MDA) levels decreased after treatment, indicating changes in antioxidant activity and lipid peroxidation under the tested greenhouse conditions. Further study found that the biostimulant application was associated with higher gibberellin 4 (GA4) and GA7 and lower indole-3-acetic acid (IAA)levels in sampled stem tissue, and the expression of GA- and IAA-related genes also changed, which was consistent with the observation that the biostimulant application significantly improved the growth of seedlings. These results associate growth responses to the two commercial formulations with changes in GA- and IAA-related indicators. They do not establish hormonal causation or improved stress tolerance, because no defined stress challenge was included.