Effects of salinity on chlorophyll fluorescence and CO2 fixation in C4 estuarine grasses

Abstract

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O2 evolution (J O2) and net rates of CO2 uptake (P N) were measured in the halotolerant estuarine C4 grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO2 concentration ratio (C i/C a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO2 fixation decreased under salinity across species, suggesting there is increased leakage of the CO2 delivered to the bundle sheath cells by the C4 pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (FV/FM) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P N in S. densiflora or D. spicata. This inhibition of P N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C i/C a did not decrease under saline conditions. In growth with and without salt, P N was saturated at ∼500 μmol(quantum) m-2 s-1 while J O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy. © 2007 Institute of Experimental Botany, ASCR.