Promoting Effect of Pectic-Oligosaccharides Produced from Pomelo Peel on Rice Seed Germination and Early Seedling Growth

Pectic-oligosaccharides (POS), one of the natural biostimulants, can regulate plants growth that the bio-stimulating activities are related to their molecular size. This study assessed the promoting effects of POS derived from pomelo peel with different molecular sizes, including POS-L, POS-M, and POS-S, with a degree of polymerization (DP) > 5 approximately 91.7%, 60.5%, and 38.6%, respectively, on rice seed growth. Rice seeds were soaked in 1% (w/w) POS solution for 3 days before being sown in a petri dish for 5 days, and water was used as the control. POS treatments considerably decreased germination time by 17–19%. They also improved the seedling root and shoot length at 5 days after sowing as compared to the control. Seeds treated with POS-L exhibited the highest growth with root and shoot lengths of 68.9 and 51.6 mm, respectively, while those of seeds treated with water were 9.3 and 34.7 mm, respectively. The amylase activity of seedlings at 5 days after sowing was increased by 8.9 and 12% when treated with POS-L and POS-M, respectively. Consequently, soluble sugar levels were also increased, with glucose and sucrose appearing as major sugars. After 2 days of soaking until the fifth day of sowing, seeds and seedlings treated with POS-L and POS-M had significantly lower H 2 O 2 accumulation than the control. According to our observations, the molecular size of POS played an important role in rice seed growth activation, i.e., larger size POS showed a more favorable impact on plant growth and metabolism.