PARENT SESSION
Oral Session 87: Photosynthesis and Respiration III: Hydrologic Controls.
Presiding: B Bond
Wednesday, August 4, 1:30 PM to 5:00 PM, Meeting Room D 137.

Effects of salinity on carbon isotope discrimination and photosynthesis in C4 estuarine grasses.

Maricle, Brian^*,1, Kiirats, Olavi¹, Edwards, Gerald¹, Lee, Raymond¹, ¹ Washington State University, Pullman, WA, United States

ABSTRACT- Salinity-induced effects on photosynthetic performance may have important effects on plant zonation and productivity in estuaries. In this study, stable carbon isotope measurements were used to assess effects of salinity on stomatal conductance and photosynthesis in estuarine C₄ grasses. Leaf ¹³C and gross and net photosynthetic rates were measured in Spartina alterniflora, S. anglica, S. patens, S. densiflora, and Distichlis spicata. ¹³C values of leaf tissue decreased with increasing salinity in greenhouse and growth chamber experiments. Models for C isotope discrimination predict that ¹³C can decrease in salt-stressed C4 plants with increases in CO₂ leakage from bundle sheath cells or decreases in internal CO₂ concentrations from decreased stomatal conductance. Quantum efficiency measurements indicated no increase in bundle sheath leakage with increasing salinity in S. anglica. Gas exchange measurements indicated a decrease in internal CO₂ concentrations in S. anglica, suggesting that lowered internal CO₂ concentration were responsible for the increased discrimination against ¹³C. We believe this to be the first report of C4 ¹³C values changing in response to salinity as a result of changes in internal CO₂ concentrations. While gas-exchange and stable isotope data showed that internal CO₂ concentrations were decreased at high salinity levels, fluorescence yield data showed that light-harvesting processes were not affected in any species tested. Nonphotochemical quenching mechanisms increased with salinity in S. patens and D. spicata, indicating some of the excess energy was lost as heat. Therefore, excess excitation energy is diverted away from the photosynthetic reaction centers to prevent photoinhibition. F_V/F_M ratios were not significantly decreased by salt, suggesting there was no damage to PSII reaction centers. These results suggest that decreased productivity in some salt marsh plants in response to salinity is not a result of decreased photosynthesis.

Key words: Distichlis, photosynthesis, Spartina, stable isotopes