Biophysical and morphological leaf adaptations to drought and salinity in salt marsh grasses

Salt marsh species are adapted for growth in different intertidal zones that are characterized by different environmental conditions. In this study, leaf energy budgets were constructed for 13 species of salt marsh grasses to investigate the biophysical effects of drought and salinity on the interception and dissipation of solar energy. Twelve Spartina species plus Distichlis spicata were grown under controlled soil water potential conditions in a greenhouse. Additionally, Distichlis spicata plants were measured along transects across salt pans in field settings. Latent heat flux decreased by as much as 65% in response to decreasing soil water potential, leading to increases in leaf temperature of up to 4°C. Consequently, radiative and sensible heat losses increased with decreasing water potential. Sensible heat flux increased as much as 336% under decreasing water potential. Latent heat loss appeared to be an important mode of temperature regulation in all species, and sensible heat loss appeared to be more important in high marsh species compared to low marsh species. High marsh species are characterized by narrower leaves than middle and low marsh species, leading to a smaller boundary layer and providing higher conductance to sensible heat loss. This may be an adaptation for high marsh species to regulate leaf temperature without access to large amounts of water for transpirational cooling. Additionally, high marsh species had higher tolerance against drought- or salinity-induced oxidative stress. It was concluded that adaptations for growth in drought- or salinity-affected sediments include biophysical, biochemical, and morphological traits that optimize heat exchange with the environment and prevent oxidative damage.