Effect of Salinity on Leaf Ionic Content and Photosynthesis of Taxodium distichum L.

Abstract

Bald cypress (Taxodium distichum L.) seedlings were subjected to flooding with salinity levels ranging from 0-140 moles m-3 NaCl. Imposition of salinity was designed to simulate the increase in salinity level which some of Louisiana's extensive cypress forests are currently experiencing due to rapid subsidence in the Mississippi River Deltaic Plain. The effect of salt water on leaf tissue ion concentrations and subsequent changes in net photosynthesis were measured. Leaf concentrations of Na, K, Ca and Mg increased substantially as salinity of floodwater increased. Net photosynthesis declined from 7.6 ,umol.m-2s-1 to 0.9 ,umol.m-2s-1 when salinity increased from 0-140 moles m-3 NaCl. Despite the parallel reduction in both stomatal conductance and net photosynthesis, the leaf internal CO2 concentrations remained relatively constant over the entire range of salt concentrations. These results suggest that elevated floodwater salinity causes excess accumulation of several ions in the bald cypress leaf tissue. Consequently, increase in leaf ionic content is primarily the cause of reductions in photosynthesis which is accompanied by simultaneous diffusional limitations caused by stomatal closure. INTRODUCTION Significant parts of coastal forests including bald cypress (Taxodium distichum L.) stands are currently being subjected to saltwater intrusion caused by rapid subsidence and compaction of recent Mississippi River alluvial sediment. Salinity in streams flowing through the extensive cypress forests bordering Lake Pontchartrain has been reported as high as 50 moles m-3 NaCl (Salinas et al., 1986). The continuous land subsidence in wetland coastal areas and the projection indicating faster sea level rises in the future (Hoffman et al., 1983) may result in elimination of many salt-sensitive, but otherwise flood-tolerant species by exposing them to high salinity. Despite the potential impact of saltwater intrusion on survival, productivity and species composition of coastal vegetation, there is little information on the extent of salt tolerance and the physiological characteristics of these species. Assessment of physiological response of bald cypress seedlings to salt stress is important in order to identify the level of tolerance and possible adaptation/acclimation to salinity conditions. Salt stress reduces growth through several effects including water stress, excess ion and biochemical relations (Greenway and Munns, 1980). Presumably, plants can survive sublethal salt stress as long as they are able to photosynthesize positively. The photosynthetic rates of glycophytes, however, declines in response to salinity. Such response has been attributed to stomatal closure as well as direct effect of excess ion content on photosynthetic processes (Seemann and Critchley, 1985). Reports of photosynthetic response of bald cypress to salinity is limited to those of Pezeshki et al., (1986) and Pezeshki et al., (1987). The former studied the photosynthetic response of different leaf age group under salinity and the latter reported photosynthetic responses to a wide range of floodwater salinity. Generally, significant declines in net photosynthesis have been observed as a result of both flood and salt stresses in bald cypress. In our continuous efforts to further investigate the response of this key wetland tree species to increased inundation and salinity, this study was conducted to examine the effects of floodwater salinity on leaf ionic contents and the subsequent effects on photosynthesis.