Vegetative response to saltwater intrusion into coastal freshwater wetlands is governed by the combined effects of increased salinity and porewater sulfide concentrations. I conducted a series of experiments to address the primary hypothesis that growth of Panicum hemitomon is adversely affected by an interaction between salinity and sulfide stresses associated with saltwater intrusion, and the mechanisms for decreased growth are alterations in the metabolic and morphological adaptations needed for a plant to survive in a flooded environment.
I exposed marsh sods to a factorial treatment arrangement of three salinities (0, 2, and 4 ppt) and three porewater sulfide concentrations (0, 0.5 and 1 mM) for 19 and 39 weeks. While salinity and sulfide both decreased relative growth rates in P. hemitomon, the salinity-induced growth inhibitions were more severe, particularly with regards to the belowground tissue. Additionally, there was a sulfide-induced stimulus in the production of adventitious tissue that was completely inhibited by elevated porewater salinities.
After 19 weeks, salinity at 4 ppt and elevated sulfide concentrations were deleterious to overall plant growth. A sulfide-induced growth stimulation in adventitious root production was inhibited at elevated salinities. After 39 weeks, elevated salinity at all concentrations was so stressful that the long-term effects of sulfide became inconsequential. Root respiration under anaerobic conditions was higher under elevated sulfide, but this stimulation was also eliminated at higher salinity. A 12-week hydroponic exposure to elevated salinity and sulfide showed opposite effects of stressor treatment, with salinity stimulating and sulfide inhibiting root ethanol production.
A 3-month field experiment intended to validate the growth chamber experiments supported the sensitivity of P. hemitomon belowground tissue to saltwater flooding, and potential reductions in the capacity to form aerenchymatous tissue for root tip aeration. I concluded from these data that the loss of Panicum hemitomon from the fresh marshes of coastal Louisiana is caused by both reduced growth and a reduced ability to adapt metabolically and morphologically to the highly-reduced edaphic conditions of a saltwater-flooded marsh.