Salinity restricts light conversion efficiency during photo-acclimation to high irradiance in Stuckenia pectinata

Abstract

Thick beds of Stuckenia pectinata, a submerged macrophyte, were lost from Te Waihora (Lake Ellesmere) in 1968, following an extreme storm event. Subsequently, it has failed to re-establish what is now a turbid coastal lagoon with a rapid light attenuation and fluctuating salinities. We investigated the interacting effects of irradiance and salinity on photosynthesis of S. pectinata in a laboratory experiment. S. pectinata plants were first acclimated to three irradiances: 340 ± 20 μmol photons m−2 s−1, 110 ± 10 μmol photons m−2 s−1, and 45 ± 5 μmol photons m−2 s−1. At each irradiance, half of the plants experienced increasing salinity stress in a stepwise manner (0, 6, 12 and 18 ppt), whereas the other half remained in freshwater. The intrinsic and effective quantum yield of photosystem II, photosynthesis-irradiance relationships, chlorophyll-a and total carotenoids content, and PSII quantum efficiency of regulated heat dissipation were determined. On a chlorophyll-a basis, light-saturated gross photosynthesis was reduced at high irradiance by 12 and 18 ppt salinity, and at medium irradiance by 18 ppt salinity. Coincidently, plants displayed lowered chlorophyll-a to carotenoids ratios and enhanced heat dissipation in the same treatments. The photosynthetic apparatus in leaves of S. pectinata responded to salinity stress as if in acclimation to high light stress. The response of photosynthesis to an interaction of high salinity and high-light reduces the light conversion efficiency of surface reaching leaves, undermining the default shade-avoidance strategy of S. pectinata of etiolation, and thus contributes to the reduced overall photosynthetic capacity and its ability to colonise the lake.