Effects of CO2 on Photosynthesis in a
Mid-Atlantic Marsh System

The primary target of CO2 is photosynthesis and in most C3 and some C4 plants this process is stimulated in leaves exposed to elevated CO2. Early observations noted that after some time in elevated CO2, the rate of photosynthesis declined, a phenomenon called acclimation, and it was thought by some that if this occurred under long-term exposure to elevated CO2, there might be no stimulation of growth by elevated CO2 . We have shown that acclimation occurs in the dominant sedge but that it does not obviate a strong response of photosynthesis at the leaf level (Jacob & Drake, 1994). Acclimation caused a 50% reduction in total protein concentration and in Rubisco. There was a correlation between acclimation and rainfall (See Table) such that acclimation reduced photosynthetic capacity less during high rainfall than during drought.

Table 1. The effect of elevated atmospheric CO 2 on net ecosystem gas exchange (NEE) and two parameters related to acclimation -- the maximum carboxylation rate (V cmax ) and maximum photosynthetic electron tranport (J max ). Stimulation is [(E-A)/A]*100 (%). Negative values mean that the value in elevated CO 2 is smaller that the value in normal ambient CO 2 .



Stimulation (%)


Rainfall (cm)


V cmax

J max











Elevated CO2 increased net ecosystem CO2exchange (NEE) in the wetland ecosystem (per unit ground area). There was considerable inter-annual variation in the relative stimulation of NEE associated with variation in rainfall and salinity of the soil water in the marsh. This sustained response of photosynthesis in this ecosystem is due in part to the supply of organic N (Rasse et al., 2003). Net ecosystem gas exchange measured using the open top chamber has compared favorably against the output of an ecosystem model in our study of a Chesapeake Bay wetland (Rasse et al. 2003). Our use of the open top chamber as a cuvette in Florida scrub oak gives results that are very similar to those obtained in the same ecosystem using eddy flux (Dore et al. 2003). Acclimation of photosynthesis was more intense (i.e. V cmax , the maximum carboxylation, was much smaller in plants grown in elevated CO2 ) during periods of drought and this resulted in much lower rates of photosynthesis and in NEE during these years than during wet years (See Figure Below)).

Time course for the effect of elevated atmospheric CO2 on annual Gross Ecosystem Production (GEP). GEP is mediated by annual rainfall. During the critical dry years of 1992, 1995, & 1999, NEE was reduced and the CO2 effect was small. This is consistent with greater acclimation during periods of stress (Table 1)

Research Questions