Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 1: Spartina BiologyTable 2. Gross and net photosyn<strong>the</strong>tic rates collected for high- and lowsalinity,flooded-treatment plants at moderate PPFD (1100 μmol m -2 s -1 ).The mean gross photosyn<strong>the</strong>tic rate <strong>of</strong> O 2 evolution and net photosyn<strong>the</strong>ticrate <strong>of</strong> CO 2 fixation ± SD (n) is given for each species/treatmentcombination.Species SalinityGross PS rates Net PS rates(μmol O 2 m -2 s -1 ) (μmol CO 2 m -2 s -1 )S. alterniflora 0‰ 40.7 ± 1.0 (3) 11.0 ± 0.8 (3)30‰ 34.4 ± 9.0 (3) 2.9 ± 2.6 (3)S. anglica 0‰ 40.4 ± 5.5 (4) 2.2 ± 1.7 (4)30‰ 41.7 ± 2.6 (4) 5.9 ± 4.4 (4)S. densiflora 0‰ 38.3 ± 12.9 (4) 5.3 ± 7.2 (4)30‰ 46.3 ± 1.7 (4) 3.3 ± 11.4 (4)S. patens 0‰ 48.2 ± 2.5 (3) 16.1 ± 11.4 (3)30‰ 42.0 ± 1.6 (3) 3.0 ± 2.1 (3)D. spicata 0‰ 41.7 ± 13.2 (3) 10.3 ± 2.7 (3)30‰ 41.0 ± 3.3 (4) 6.3 ± 7.0 (4)electron sinks are induced. Some possibilities include(Demmig-Adams and Adams, 1992): CO 2 pump activitymay increase to compensate for bundle sheath CO 2 leakage;this would use additional ATP generated by <strong>the</strong> Mehlerreaction, and was potentially reflected in lower PSII yieldsin S. alterniflora and S. patens under high salinity (Table 1).Alternatively, an increase in photorespiration could helpsustain gross photosyn<strong>the</strong>sis rates if CO 2 levels drop in <strong>the</strong>bundle sheath allowing O 2 to react with RuBP. Aconsequence <strong>of</strong> photorespiration is producing products likePGA and ammonia that need reductive power fromphotochemistry. Increased reduction rates <strong>of</strong> nitrate, sulfate,or phosphate within chloroplasts could utilize excess lightenergy. Fur<strong>the</strong>r work will be needed to see if nitrate orsulfate reduction or photorespiration rates increase withincreasing salinity. Finally, energy not used inphotochemistry may be dissipated by nonphotochemicalquenching (NPQ) mechanisms, where excess light energy islost as heat. Maximum amounts <strong>of</strong> NPQ increased withsalinity in S. patens and D. spicata (ANOVA, p≤0.073), butin no o<strong>the</strong>r species (ANOVA, p≥0.570; Table 1).Under high light, rates <strong>of</strong> light-harvesting (grossphotosyn<strong>the</strong>sis) will invariably be larger than carbon fixation(net photosyn<strong>the</strong>sis) rates. Therefore excess energy must besafely dissipated before reaction centers are damaged(Demmig-Adams and Adams 1992). Dark-adapted F V /F Mratios <strong>of</strong> chlorophyll fluorescence can indicate damage to <strong>the</strong>PSII reaction center (Krause and Weis 1991). F V /F M ratioswere not significantly reduced by salinity in any species inthis study (ANOVA, p≥0.165; Table 1), suggesting excessenergy was efficiently dispersed before reaction centers weredamaged. Prevention <strong>of</strong> photoinhibition is likely to beimportant in determining plant resistance to environmentalstresses that reduce carbon fixation relative to lightharvesting rates (Demmig-Adams and Adams 1992).The c i /c a values <strong>of</strong> <strong>the</strong> plants in this study ranged from0.35 to 0.61 (Table 1). The c i /c a value <strong>of</strong> S. anglicasignificantly decreased with increasing salinity at moderatePPFD (ANOVA, p=0.032). The c i /c a values <strong>of</strong> all o<strong>the</strong>rspecies did not change in response to increasing salinity(ANOVA, p≥0.421). In previous studies, c i /c a values did notchange with increasing salinity in C 4 grasses (Bowman et al.1989, Meinzer et al. 1994). C 3 leaf c i /c a values tend to behigher than corresponding C 4 values and are more sensitiveto increasing salinity (Brugnoli and Lauteri 1991).Increasing salinity may decrease C 3 c i /c a values by as muchas 0.48 (Farquhar et al. 1982). In <strong>the</strong> present study, c i /c adecreased by 0.20 in S. anglica, and by 0.22 in S. densiflora(nonsignificant change) with 30‰ salinity, but only as muchas 0.07 in <strong>the</strong> o<strong>the</strong>r species (Table 1). We believe this to be<strong>the</strong> first report <strong>of</strong> salinity-induced decreases in C 4 c i /c avalues. One factor contributing to salt sensitivity in C 4 plantsmay be <strong>the</strong> susceptibility <strong>of</strong> CO 2 leakage from bundle sheathcells under increasing salinity. Increasing salinity appearedto decrease c i /c a in S. anglica, but it was <strong>the</strong> most resistantspecies to salinity in terms <strong>of</strong> CO 2 fixation. This suggestsCO 2 pump activity does not increase in S. anglica withincreasing salinity. Rates <strong>of</strong> photorespiration or nitratereduction may increase instead to use excess light energy.Excess energy dissipation in response to salinity may be anarea for future investigation in marsh halophytes.This work illustrated how light harvesting and CO 2uptake relate to sediment salinity in C 4 marsh grasses.Excess energy dissipation may also increase in times <strong>of</strong>salinity stress. Additionally, NPQ mechanisms increase inmany plants as a result <strong>of</strong> external salinity. Portableflourometers can easily be used in <strong>the</strong> field to determine invivo NPQ and thus in vivo salt stress in some plants.However, photochemistry does not appear to be affected bysalt, so fluorescence yield measurements in <strong>the</strong> light or darkadaptedF V /F M measures will not reflect salt stress. Incontrast, gas-exchange methods appear to be far moresensitive to salinity.ACKNOWLEDGMENTSThe authors thank C. Cody, P. Rabie, K. Patten, S.Hacker and M. Figueroa for assistance and plants. Thisproject was partially funded by <strong>the</strong> Betty W. HiginbothamTrust and a Padilla Bay NERR Research Assistantship. Thisresearch was also supported by NSF IBN0076604, EPA R-82940601, and NSF DBI-0116203.REFERENCESBowman, W.D., K.T. Hubick, S. von Caemmerer, and G.D. Farquhar.1989. Short-term changes in leaf carbon isotope discriminationin salt and water stressed C 4 grasses. Plant Physiol. 90:162-166.Brugnoli, E., and M. Lauteri. 1991. Effects <strong>of</strong> salinity on stomatalconductance, photosyn<strong>the</strong>tic capacity, and carbon isotope discrimination<strong>of</strong> salt-tolerant (Gossypium hirsutum L.) and saltsensitive(Phaseolus vulgaris L.) C 3 non-halophytes. PlantPhysiol. 95:628-635.-57-