Effects of salinity on chlorophyll fluorescence and CO2 fixation in C4 estuarine grasses
2007
Maricle, B. R. | Lee, R. W. | Hellquist, C. E. | Kiirats, O. | Edwards, G. E.
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.
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