Paclobutrazol enhances tall fescue salt tolerance via physiological and root system architecture modulation

Salinity represents a major global constraint on crop productivity. Promoting the cultivation of tall fescue in saline environments offers not only nutritional advantages for livestock but also enhances its potential for ornamental use. In this mesocosm study, we examined the effects of paclobutrazol (PBZ) on tall fescue performance under salt stress, focusing on key physiological traits to evaluate salt tolerance. Under high salt stress, paclobutrazol application increased the total number of lateral roots by 85%, from 18.39 to 34.04, and widened their growth angle by 24%, from 24.10° to 29.88°, fundamentally enhancing topsoil exploration. This reconfigured root system supported a 65% increase in tiller number, from 16.39 to 27.11, compared to plants under salt stress alone. Physiologically, paclobutrazol treatment elevated root proline concentration by 3.2-fold and sustained the maximum quantum efficiency of photosystem II (Fv/Fm) at 0.7877, a value 23% higher than the salt-stressed control (0.7052) and equivalent to non-stressed plants. Although genes associated with abscisic acid (ABA) biosynthesis were also induced, no significant increase in ABA accumulation was detected. Principal component analysis discretely distinguished the traits into root system, photosynthetic parameters, and stress indicators. NaCl induced stress-associated shifts, while PBZ-treated plants occupied an intermediate position, and showing improved root traits, photosynthesis. A strong positive correlation with control indicates PBZ partially mitigates salinity effects, maintaining functions near non-stressed levels. These synergistic improvements culminated in a 98% increase in aboveground biomass yield, producing 33.75 g per mesocosm compared to 17.06 g under salt stress alone. We conclude that paclobutrazol promotes a stress-tolerant phenotype by enhancing root exploration, strengthening osmotic regulation, and stabilizing photosynthetic performance, making it a promising agronomic tool for securing forage yield in saline soils.