Effects of Salinity on the Photosynthesis of Two Poaceous Halophytes

Abstract

To explore the difference of C4and C3photosynthetic pathways in responding to high salinity and its potential role in determining vegetation zonation in coastal saltmarshes, two Poaceous halophytes, C4Spartina alternifloraand C3Phragmites australis, were studied with different salinities. The results showed that proper salinities were required for both species to maintain optimal photosynthesis. The maximal net assimilation rates ofS. alternifloraandP. australisappeared with 400 and 100 mmol L−1NaCl, respectively. Both species exhibited similar responses of stomatal conductance towards the set salinities, and chlorophyll contents inP. australiswere even promoted by 100–400 mmol L−1NaCl. Although no significant changes in the maximal quantum yield of photosystem II were found in either species treated by NaCl, instant quantum yield of photosystem II ofP. australiswas significantly decreased, whereas that ofS. alternifloraincreased. We suggest that the zonation betweenS. alternifloraandP. australisis not exclusively caused by the CO2‐concentrating mechanism of the C4pathway and stomatal conductance is not a restricting factor for either species. It might be that the electron transfer from photosystem II and the energy use efficiency being limited in the C3P. australisgrown under high salinity.