The combined effects of salinity and sulfide on the growth and physiology of the freshwater marsh plant Panicum hemitomon J. A. Schultes

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 investigate the growth and physiological responses of the freshwater marsh plant Panicum hemitomon to increased salinity and sulfide, with the overall goal of understanding how these stresses operate in concert and individually. Specifically, the following questions were addressed: (1) What are the individual and combined effects of salinity and sulfide on the growth of Panicum hemitomon? (2) By what mechanisms are exposure to salinity and sulfide detrimental to the physiological processes affecting plant growth? (3) Are these stresses synergistic in their modification of such physiological processes, and which, if either, is dominant in its effect?;This research addressed 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. Specific objectives were: (1) To determine the effect of salinity and sulfide stress on the growth of Panicum hemitomon, as defined by production of new biomass and culms. (2) To determine if the growth responses of Panicum hemitomon to salinity and sulfide are related to adaptations in plant morphology and fermentative and respiratory metabolism. (3) To determine if the results of controlled experiments on the effect of salinity and sulfide on Panicum hemitomon are accurate predictors of the growth and physiological response of plants under field conditions.;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 cellular 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 was subsequently initiated to further explore the response of physiological pathways associated with anaerobic fermentation capacity. Data from that experiment 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 was complicated by drought-induced hypersaline conditions, but data did support the sensitivity of P. hemitomon belowground tissue to saltwater flooding, and reductions in the capacity to form aerenchymatous tissue for root tip aeration. These data led to the conclusion 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.