Oligocarrageenan foliar spray promotes the growth and yield of pechay (Brassica rapa ssp. chinensis) under postflooding stress

Flooding driven by climate change and poor infrastructure severely compromises the growth and yield of vegetables such as pechay ( Brassica rapa ssp. chinensis ). While oligocarrageenan is known to stimulate growth and enhance tolerance to abiotic stress, its curative potential after flooding remains largely unexplored. This study evaluated the effects of flooding and the use of oligocarrageenan as a curative foliar spray on the growth and yield of pechay. Oligocarrageenan concentrations (0, 60, 120, and 240 ppm) were applied as foliar sprays onto 14-day-old pechay plants grown under well-watered and flooding stress conditions. Despite being flooded, pechay plants have high survival rates (86.67–100%). However, flooding caused reductions in shoot and root length and biomass. Oligocarrageenan, as a curative treatment, improved these parameters under the flooding setup, with 240 ppm oligocarrageenan as the most effective concentration. Under well-watered conditions, 120 ppm oligocarrageenan was the optimal concentration. The yield responses provided the most robust evidence of the curative potential of oligocarrageenan; specifically, the foliar spray of 240 ppm oligocarrageenan mitigated flooding stress, resulting in a 769% increase relative to that of the flooded controls at 25 DAT. Flooding stress consistently induces higher TSS accumulation in pechay, although under well-watered conditions, a 60 ppm oligocarrageenan application optimizes the TSS. Temporally, the untreated control plants maintained elevated TSS concentrations at 25 DAT, whereas 60 ppm oligocarrageenan increased the TSS concentration at 35 DAT regardless of the water condition. These findings demonstrate that oligocarrageenan foliar spray improves the growth, yield, and quality of pechay postflooding stress.