Growth, photosynthesis and fertility of Chara aspera under different light and salinity conditions

Abstract The main aim of this study was to investigate if the wide salinity range of Chara aspera C.L. Willdenow is caused by acclimation capabilities alone, or if freshwater and brackish water populations have different salinity optima. In an outdoor experiment, plants collected from one freshwater and one brackish water site (about 8 PSU) in southern Sweden were incubated under different conditions of salinity and light. Elongation rate, final biomass, photosynthesis parameters, pigment composition as well as development of gametangia were determined. Plants collected from freshwater all died at the highest salinity condition tested (20 PSU) possibly due to salinity stress. For most parameters tested, these plants performed best at low salinity. Thus, maximum photosynthetic rate in the absence of photoinhibition (Pmax) and onset of light saturation and index of light acclimation (Ek)were higher at 0 PSU compared to 5 and 10 PSU. The number of gametangia tended to decrease with increasing salinity. In contrast, plants collected from brackish water performed best at intermediate salinities. Thus, final ash-free dry weight (AFDW) was lower at 20 PSU compared to all other salinity conditions and tended to be lower at 0 PSU compared to 5 and 10 PSU. Pmax and Ek as well as the number of gametangia had maximum values at intermediate salinities (5 and 10 PSU). Also pigment composition was influenced by salinity. In both groups, chlorophyll a/chlorophyll b quotients increased with increasing salinity. Chlorophyll a/carotenoid quotients decreased with increasing salinity in the freshwater population, but had maximum values at intermediate salinities in the brackish water population. Apart from salinity, light influenced the physiological performance of both populations. As expected, Pmax, Ek and chlorophyll a/chlorophyll b quotients increased with increasing irradiance during the incubation in both populations, while chlorophyll a/carotenoid quotients decreased. Shoot elongation increased with decreasing irradiance in both populations. This effect was less pronounced at higher salinities, indicating that shoot elongation may be hampered in brackish water. Our results indicate that C. aspera growing in freshwater has optimum conditions at 0 PSU, while plants collected from brackish water have their optimum at 5–10 PSU. As all parameters were recorded after several weeks of incubation, we suggest that genetic differences between the populations rather than physiological acclimation to the field conditions are responsible for these different salinity optima.

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