Hibiscus tiliaceus (Hau) is a pantropical mangrove associate that usually occurs in coastal ecosystems where substrate salinity is relatively high, but it also inhabits upland habitats in Hawaii. Cuttings from three populations on the island of Oahu, Hawaii, were collected and grown in the glasshouse under two levels of substrate salinity (0 and 335 mOsm kg-1) and three light treatments (0%, 50%, and 90% shade). Photosynthetic gas exchange, biomass allocation, and accumulation were studied in relation to salinity and light. Salinity reduced net CO2 assimilation in the upland population but had no effect or stimulated photosynthesis in the coastal populations, whereas increasing salinity decreased stomatal conductance in all populations and therefore increased water-use efficiency. The degree to which photosynthesis was inhibited by salinity was inversely proportional to the salinity of the source population, indicating a loss of salinity tolerance in upland plants. Light had a stronger effect on leaf area ratio (LAR) and leaf mass per area (LMA), whereas salinity had a stronger effect on leaf water content, internode length, and plant biomass. Salinity reduced total new biomass by 58%, 50%, and 34% in full sun, 50% shade, and 90% shade, respectively, but this response did not differ between populations. Salinity reduced the photosynthesis, but not growth, of upland plants because increased allocation to photosynthetic tissue increased LAR to compensate for inhibition of photosynthesis by salinity.
