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Growth and Photosynthetic Responses of Scaevola sericea, A Hawaiian Coastal Shrub, to Substrate Salinity and Salt Spray

Growth patterns, water relations, and photosynthetic traits in Scaevola sericea plants grown under different levels of substrate salinity and salt spray were studied. Scaevola sericea is a dominant shrub species in coastal strand ecosystems throughout the tropical and subtropical Pacific and Indian Oceans. Seventy-two cuttings from two coastal sites on the island of Oahu (Hawaii) were grown in a greenhouse under six treatments that resulted from the combination of three levels of substrate salinity (0, 100, and 335 mOsm kg-1) and two levels of simulated salt spray (0 and 1200 mg salt m-2 d-1). Several characteristics of S. sericea were strongly affected by substrate salinity but only weakly affected by salt spray. New stem and leaf biomass per plant decreased by ca. 65% as substrate salinity increased from 0 to 335 mOsm kg-1; photosynthetic rates decreased by only 20% over the same salinity range. Leaf sap osmolarity increased 300 mOsm kg-1 as substrate salinity changed from 0 to 335 mOsm kg-1, allowing the maintenance of a constant soil-to-leaf osmotic potential gradient favorable for water uptake even at higher salinity levels. Carboxylation capacity, determined by the initial slope of net CO2 assimilation--intercellular CO2 concentration relationships, remained constant for plants grown under different levels of salinities. The δ13C of leaves increased from -29.2‰ to -26.3‰ with increasing salinity and was associated with lower stomatal conductances but nearly unchanged photosynthetic rates. Scaevola sericea is capable of substantial growth and physiological responses, which apparently are required to maintain a positive carbon balance in coastal habitats characterized by large temporal and spatial variations in substrate salinity and salt spray levels.