<str<strong>on</strong>g>Proceedings</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>Third</str<strong>on</strong>g> <str<strong>on</strong>g>Internati<strong>on</strong>al</str<strong>on</strong>g> <str<strong>on</strong>g>C<strong>on</strong>ference</str<strong>on</strong>g> <strong>on</strong> <strong>Invasive</strong> SpartinaChapter 1: Spartina BiologyTable 2. Gross and net photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>tic rates collected for high- and lowsalinity,flooded-treatment plants at moderate PPFD (1100 μmol m -2 s -1 ).The mean gross photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>tic rate <str<strong>on</strong>g>of</str<strong>on</strong>g> O 2 evoluti<strong>on</strong> and net photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>ticrate <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 fixati<strong>on</strong> ± SD (n) is given for each species/treatmentcombinati<strong>on</strong>.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)electr<strong>on</strong> 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 additi<strong>on</strong>al ATP generated by <str<strong>on</strong>g>the</str<strong>on</strong>g> Mehlerreacti<strong>on</strong>, and was potentially reflected in lower PSII yieldsin S. alterniflora and S. patens under high salinity (Table 1).Alternatively, an increase in photorespirati<strong>on</strong> could helpsustain gross photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>sis rates if CO 2 levels drop in <str<strong>on</strong>g>the</str<strong>on</strong>g>bundle sheath allowing O 2 to react with RuBP. Ac<strong>on</strong>sequence <str<strong>on</strong>g>of</str<strong>on</strong>g> photorespirati<strong>on</strong> is producing products likePGA and amm<strong>on</strong>ia that need reductive power fromphotochemistry. Increased reducti<strong>on</strong> rates <str<strong>on</strong>g>of</str<strong>on</strong>g> nitrate, sulfate,or phosphate within chloroplasts could utilize excess lightenergy. Fur<str<strong>on</strong>g>the</str<strong>on</strong>g>r work will be needed to see if nitrate orsulfate reducti<strong>on</strong> or photorespirati<strong>on</strong> rates increase withincreasing salinity. Finally, energy not used inphotochemistry may be dissipated by n<strong>on</strong>photochemicalquenching (NPQ) mechanisms, where excess light energy islost as heat. Maximum amounts <str<strong>on</strong>g>of</str<strong>on</strong>g> NPQ increased withsalinity in S. patens and D. spicata (ANOVA, p≤0.073), butin no o<str<strong>on</strong>g>the</str<strong>on</strong>g>r species (ANOVA, p≥0.570; Table 1).Under high light, rates <str<strong>on</strong>g>of</str<strong>on</strong>g> light-harvesting (grossphotosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>sis) will invariably be larger than carb<strong>on</strong> fixati<strong>on</strong>(net photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>sis) rates. Therefore excess energy must besafely dissipated before reacti<strong>on</strong> centers are damaged(Demmig-Adams and Adams 1992). Dark-adapted F V /F Mratios <str<strong>on</strong>g>of</str<strong>on</strong>g> chlorophyll fluorescence can indicate damage to <str<strong>on</strong>g>the</str<strong>on</strong>g>PSII reacti<strong>on</strong> 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 reacti<strong>on</strong> centers weredamaged. Preventi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> photoinhibiti<strong>on</strong> is likely to beimportant in determining plant resistance to envir<strong>on</strong>mentalstresses that reduce carb<strong>on</strong> fixati<strong>on</strong> relative to lightharvesting rates (Demmig-Adams and Adams 1992).The c i /c a values <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> plants in this study ranged from0.35 to 0.61 (Table 1). The c i /c a value <str<strong>on</strong>g>of</str<strong>on</strong>g> S. anglicasignificantly decreased with increasing salinity at moderatePPFD (ANOVA, p=0.032). The c i /c a values <str<strong>on</strong>g>of</str<strong>on</strong>g> all o<str<strong>on</strong>g>the</str<strong>on</strong>g>rspecies did not change in resp<strong>on</strong>se 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 corresp<strong>on</strong>ding 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 <str<strong>on</strong>g>the</str<strong>on</strong>g> present study, c i /c adecreased by 0.20 in S. anglica, and by 0.22 in S. densiflora(n<strong>on</strong>significant change) with 30‰ salinity, but <strong>on</strong>ly as muchas 0.07 in <str<strong>on</strong>g>the</str<strong>on</strong>g> o<str<strong>on</strong>g>the</str<strong>on</strong>g>r species (Table 1). We believe this to be<str<strong>on</strong>g>the</str<strong>on</strong>g> first report <str<strong>on</strong>g>of</str<strong>on</strong>g> salinity-induced decreases in C 4 c i /c avalues. One factor c<strong>on</strong>tributing to salt sensitivity in C 4 plantsmay be <str<strong>on</strong>g>the</str<strong>on</strong>g> susceptibility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 leakage from bundle sheathcells under increasing salinity. Increasing salinity appearedto decrease c i /c a in S. anglica, but it was <str<strong>on</strong>g>the</str<strong>on</strong>g> most resistantspecies to salinity in terms <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 fixati<strong>on</strong>. This suggestsCO 2 pump activity does not increase in S. anglica withincreasing salinity. Rates <str<strong>on</strong>g>of</str<strong>on</strong>g> photorespirati<strong>on</strong> or nitratereducti<strong>on</strong> may increase instead to use excess light energy.Excess energy dissipati<strong>on</strong> in resp<strong>on</strong>se to salinity may be anarea for future investigati<strong>on</strong> in marsh halophytes.This work illustrated how light harvesting and CO 2uptake relate to sediment salinity in C 4 marsh grasses.Excess energy dissipati<strong>on</strong> may also increase in times <str<strong>on</strong>g>of</str<strong>on</strong>g>salinity stress. Additi<strong>on</strong>ally, NPQ mechanisms increase inmany plants as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> external salinity. Portableflourometers can easily be used in <str<strong>on</strong>g>the</str<strong>on</strong>g> 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 <str<strong>on</strong>g>the</str<strong>on</strong>g> light or darkadaptedF V /F M measures will not reflect salt stress. Inc<strong>on</strong>trast, 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 <str<strong>on</strong>g>the</str<strong>on</strong>g> 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. v<strong>on</strong> Caemmerer, and G.D. Farquhar.1989. 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