Intraspecific Variation in Salt Tolerance and Morphology in Panicum hemitomon and Spartina alterniflora (Poaceae)

Abstract

Nineteen clones of Panicum hemitomon, a fresh marsh dominant, and 25 clones of Spartina alterniflora, a salt marsh dominant, were collected from the coastal marshes of Louisiana and Texas. Plants were deacclimated from field conditions under uniform, nonsaline conditions in the greenhouse for four to six vegetative generations and morphological variables were then measured. Genotypes of each species were subjected to a salinity screening protocol where salinity was increased in weekly increments of 2 per mil (g salt/L solution) for P hemitomon and 10 per mil for S. alterniflora using a commercial sea salts mix. Plants were harvested when there was 50% death of aboveground tissue, defined as the lethal salinity level. Both species displayed highly significant intraspecific variation in lethal salinity level, ranging from 7.6 per mil to 12.0 per mil for P. hemitomon and from 83 per mil to 115 per mil for S. alterniflora. Both species also displayed significant intraspecific variation in many morphological variables, as well as in leaf-rolling index, leaf expansion rates, aboveground, belowground, and total plant dry weight, and belowground-to-aboveground biomass ratio. An ANOVA of principal component scores further illustrated intraspecific variation in both species expressed as a single principal component made up of lethal salinity level and covariable-adjusted total plant dry weight. In S. alterniflora none of the plant morphological variables was significantly correlated with salt tolerance, whereas leaf rolling at 35 per mil accounted for 38% of the variation in lethal salinity level among genotypes. Conversely, in P. hemitomon leaf rolling was not significantly correlated with salt tolerance, but two morphological variables, leaf length and leaf length x width, accounted for 44% and 46%, respectively, of the variation in lethal salinity level among genotypes. Therefore, morphological traits related to plant size appear more important in explaining genotypic variation in salt tolerance in the glycophyte P. hemitomon than in the facultative halophyte S. alterniflora. The ability to identify superior salt-tolerant genotypes within both species has significant implications for marsh restoration and creation projects throughout their ranges.