Determining Salinity-Tolerance of Giant Salvinia Using Chlorophyll Fluorescence

Salvinia molesta Mitchell, a floating invasive aquatic plant, is one of the top 10 worst invasive aquatic weeds in the world. It was discovered in the lower Pascagoula River in 2005 and evidence suggests that this non-native species is spreading along the northern Gulf of Mexico. These plants exhibit rapid growth and nutrient uptake rates, allowing them to out compete other plants in similar habitats. Distributional observations suggest that non-native S. molesta is able to survive in salinities of up to 7 ppt in the lower Pascagoula River. The response of S. molesta to three salinity levels (0, 5, 10 ppt) was tested using chlorophyll fluorescence. The health of the plants was measured over a period of one month, using a log scale series of observation intensities (hourly, daily, weekly). Plant responses indicated an acute salinity effect after about 4-6 hrs and then a gradual chronic decline. Compared to initial measurements, the final actual quantum yield (∆F/Fm’) dropped by 5%, 6% and 29%, while the final potential quantum yield (Fv/Fm) dropped 6%, 27% and 39% in the 0, 5, and 10 ppt treatments, respectively. Only plants in the 0 ppt treatment showed significant new growth. Plants in 5 ppt appeared to maintain themselves, but plants at 10 ppt all exhibited signs of severe stress and loss of color, turgor, and tissue viability after 10 d. Tolerance to brackish salinities has been reported in the past, and has implications for the use of the biological control agent, the weevil Cyrtobagous salviniae, that can only tolerate freshwater conditions. IntroductIon Aquatic plants can be grouped into three types: emergent, floating, and submerged (Pieterse and Murphy 1993), with some of the most successful invasive aquatic plants being in the floating group (e.g., Eichhornia crassipes and Salvinia molesta). These plants exhibit rapid growth rates, rapid nutrient uptake rates, are aggressive, and are competitive species that can impact aquatic environments, local economies, and human health (Holm et al. 1977). The impact of these species on a freshwater body is dramatically illustrated by S. molesta, one of the top 10 worst non-native invasive aquatic weeds in the world (Room and Julien 1995, Carley and Brown 2006). Salvinia molesta has become a worldwide problem, with invasions into freshwater bodies in most tropical countries, and was introduced into the United States in 1995 (Julien and Tipping 2002, USGS 2005). Salvinia molesta has a doubling time of 4-6 d (Mitchell and Tur 1975) and was found in the lower Pascagoula River in 2005 (MS DMR 2005). Evidence reported in McFarland et al. (2004) suggests that this non-native species is spreading into the northern Gulf of Mexico (GOM). In Alabama and Mississippi, there are many suitable habitats for native and non-native invasive aquatic plants; there are four river drainage systems along the 121 km (75 mile) coastline of the state of Mississippi alone. The largest of these is the Pascagoula River, which holds the distinction of being the longest un-dammed, natural river remaining in the continental USA and provides habitat for numerous important and endangered salt marsh species (Schueler 2002). Much of this river system remains relatively unimpacted by development, except for the very lower reaches between the towns of Gautier and Pascagoula, MS. Distributional observations during an outbreak in 2005 by personnel with the Mississippi Department of Marine Resources (DMR) suggest that non-native S. molesta was able to survive in salinities of up to 7 parts per thousand (ppt) in the lower Pascagoula River; a similar tolerance has been reported earlier by Divakaran et al. (1980) from growth tests conducted on salinities of 0 to 11 ppt. This has implications for the use of the biological control agent Cyrtobagous salviniae on this infestation, as this weevil can only tolerate freshwater conditions (Thomas and Room 1986, Julien et al. 2002). This observation is distressing in two respects: (1) potential for a portion of the non-native S. molesta population in the Pascagoula River to escape biological control; and (2) a more salinity-tolerant variety of this species could easily spread into similar habitats that abound along the GOM and elsewhere. Pulse amplitude modulated (PAM) fluorescence is a tool to measure photophysiological processes in vivo. While it cannot be used to directly measure the mechanisms of osmoregulation, it has been used successfully to demonstrate the physiological stress resulting from salinity change in a number of aquatic plant species (Ralph 1998, Kamermans et al. 1999, Murphy et al. 2003, Biber 2006). PAM fluorescence has been used in submerged aquatic plants to measure acute stress, such as desiccation (Adams and Bates 1994, Bjork et al. 1999), temperature or salinity shifts (Ralph et al. 1998, Ralph 1999), and even changes in ambient light over short time durations (Beer and Bjork 2000,