Comprehensive effects of exogenous salicylic acid and light on chlorophyll fluorescence parameters and photosynthetic oxygen evolution in Ulva prolifera

Ulva prolifera is one of the causative species of green tides. Salicylic acid (SA) alleviated the high-temperature damage by up-regulating a portion of antioxidant-related proteins, however, little is understood regarding the impacts of SA under high light in this species. In order to study the regulation effects of SA on U. prolifera under different light conditions, we grew this species under three SA concentrations (0, 2.5, 5.0 µg mL-1) and three light levels [high light (HL): 400 μmol(photon) m-2 s-1, medium light (ML): 160 μmol(photon) m-2 s-1, and low light (LL): 70 μmol(photon) m-2 s-1]. After 3 d, the growth, chlorophyll fluorescence parameters, photosynthetic oxygen rate production, and superoxide dismutase (SOD) activity of U. prolifera were investigated. We found that: (1) The relative growth rate (RGR) of U. prolifera increased with the increase of light, no matter if with or without SA; under the three light levels, the highest RGR was observed in the presence of 2.5 µg(SA) mL-1. (2) U. prolifera grown under HL conditions showed the highest values in nonphotochemical quenching and total content of cartenoids. (3) Under three light levels, the dark respiration rate, the net, maximal, and gross photosynthetic oxygen rate significantly decreased by SA. (4) The highest value of SOD activity was observed under LL in the presence of SA (5.0 µg mL-1). In conclusion, the effects of light on the physiology in U. prolifera could be mediated by SA in a concentration-dependent manner.