Proceedings of the Third International Conference on Invasive ...

Chapter 1: Spartina Biology<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartinain <strong>the</strong>se environments is thought to be oxygen transport to<strong>the</strong> roots and rhizosphere, facilitated by a system <strong>of</strong> gasspaces (aerenchyma) connecting leaves to root tissues (Tealand Kanwisher 1966, Hwang and Morris 1991, Arenovskiand Howes 1992, Howes and Teal 1994). The presence andfunctioning <strong>of</strong> aerenchyma is well documented in plantstolerant <strong>of</strong> flooded conditions, and generally results in asupply <strong>of</strong> oxygen for aerobic respiration as well as radialoxygen loss to <strong>the</strong> environment (reviewed by Jackson andArmstrong 1999).Once oxygen has reached submerged tissues inemergent plants like Spartina, it has at least three possiblefates (Fig. 1). Oxygen can be released into <strong>the</strong> rhizosphere,support mitochondrial respiration, or be used in sulfideoxidation processes. The strength <strong>of</strong> <strong>the</strong>se competing oxygensinks can be important in <strong>the</strong> ecophysiology <strong>of</strong> wetlandplants since it influences how oxygen is budgeted insubmerged tissues (Sorrell 1999).Anoxic estuarine sediments represent a strong externaloxygen sink and <strong>the</strong>y can overwhelm plant oxygen transportprocesses. Therefore, Spartina grasses also must exhibit astrong capability for anaerobic respiration to sustainmetabolism when oxygen supplies are low. The enzymealcohol dehydrogenase (ADH) catalyzes <strong>the</strong> final reaction infermentative ethanol syn<strong>the</strong>sis. The ability to respire underhypoxic conditions is important for life in waterlogged soils,so ADH activity in <strong>the</strong>se plants appears to be an adaptationfor anoxia tolerance (Crawford 1967).To understand <strong>the</strong> mechanisms conferring success inlow intertidal zones, aspects <strong>of</strong> oxygen transport andmetabolic characteristics related to anoxia tolerance andrhizosphere oxidation were investigated in greenhouseSpartina plants. The four Spartina species introduced intoFig. 1. Atmospheric oxygen can be transported through wetland plants tosubmerged tissues. Once oxygen reaches <strong>the</strong> roots (inset), it has severalpossible fates. Measurements <strong>of</strong> <strong>the</strong> indicated processes and enzymeactivities may indicate how oxygen is allocated in <strong>the</strong> roots <strong>of</strong> flood-tolerantplants.Washington estuaries provide a good system for studyingestuarine zonation since <strong>the</strong>y represent a range from highmarsh to low marsh species. The low marsh species Spartinaalterniflora and S. anglica were studied and compared to <strong>the</strong>high marsh species S. densiflora, S. patens, and <strong>the</strong> nativeDistichlis spicata.Plants are aerobes. Therefore, survival in waterloggedsoils requires a supply <strong>of</strong> oxygen to support tissuessubmerged in anoxic sediments (Crawford 1982). However,many additional physiological processes can be affected by<strong>the</strong> supply <strong>of</strong> oxygen to submerged tissues in wetland plants.Measures <strong>of</strong> <strong>the</strong> mechanisms shown in Fig. 1 may allow oneto estimate how much oxygen is used in aerobic respirationand how strong external oxygen sinks may be. In <strong>the</strong> presentstudy, rates <strong>of</strong> oxygen transport were measured andcompared to rates <strong>of</strong> aerobic respiration. Highly reducingmudflat conditions may inhibit aerobic respiration processesand induce alternative anaerobic respiration pathways.Therefore, rates <strong>of</strong> aerobic respiration were measured as wellas activities <strong>of</strong> <strong>the</strong> aerobic respiration enzyme cytochrome coxidase and <strong>the</strong> anaerobic respiration enzyme alcoholdehydrogenase. Tolerance <strong>of</strong> estuarine mudflat conditionsmay also require mechanisms to detoxify hydrogen sulfide, aphytotoxin produced under anaerobic conditions.Consequently, rates <strong>of</strong> sulfide oxidation processes were alsomeasured in root tissues. The results <strong>of</strong> this study areexpected to provide a physiological explanation to helpdefine differences between high marsh and low marshfunctional types and <strong>the</strong>ir relationship to estuarine zonation.MATERIALS AND METHODSSpartina plants were collected from field sites andsubsequently maintained under greenhouse conditions.Spartina alterniflora plants were collected in Willapa Bay,Washington and S. anglica was collected from nor<strong>the</strong>rnPuget Sound, Washington. Additionally, S. patens plantswere obtained from <strong>the</strong> Gulf Coast <strong>of</strong> northwest Florida andS. densiflora plants were obtained from <strong>the</strong> Odiel SaltMarshes, southwest Spain. The native Distichlis spicata wascollected in nor<strong>the</strong>rn Puget Sound, Washington.Greenhouse temperatures were 26°C during <strong>the</strong> day and18°C at night. Natural lighting provided a photosyn<strong>the</strong>ticphoton flux density (PPFD) averaging 200 micromolesquanta per square meter per second (200 μmol quanta m -2sec -1 ) during daylight hours with peaks around 1,100 μmolquanta m -2 sec -1 on sunny days. Daughter tillers from fieldcollectedplants were potted individually in a 50/50(vol./vol.) sand/potting soil mixture and were watered tosaturation twice weekly with modified Hoagland nutrientsolution (Epstein 1972). Freshly potted plants were selectedfor uniformity in size and randomized between flooded anddrained treatments. At least four replicate plants were grownin each treatment. Plants were allowed 60-80 days in <strong>the</strong>ir-48-