WHOLE-PLANT GAS-EXCHANGE RESPONSES OF SPARTINA-ALTERNIFLORA (POACEAE)TO A RANGE OF CONSTANT AND TRANSIENT SALINITIES

A two-chamber-system was used to study whole-plant gas exchange respon ses of Spartina alterniflora to long-term and transient salinity treat ments over the range of 5 to 40 ppt NaCl. Lower photosynthetic rates, leaf water vapor conductances, belowground respiration rates, and high er aboveground respiration rates in plants adapted to 40 ppt NaCl were observed. Area-specific leaf weight increased with salinity, although the salt content of leaf tissues did not. A reduced rate of gross pho tosynthesis and higher aboveground respiration rate in 40-ppt NaCl pla nts significantly lowered the net whole-plant CO2 gain below that of 5 -ppt NaCl plants, while the net CO2 gain of 25-ppt NaCl plants was int ermediate. Within 6 hr of increasing the salinity of 5- and 25-ppt NaC l plants by 20 and 15 ppt NaCl, S. alterniflora responded by reducing leaf water vapor conductance, which in turn reduced the photosynthetic rate. This response was reversed by returning the plants to their ori ginal salinity, which indicates that S. alterniflora adjusts water los s and gas exchange in response to transient salinity stress by regulat ing stomatal aperture. On the other hand, decreasing salinity of the g rowth media of plants cultured at 25 and 40 ppt NaCl had little or no effect on gas exchange characteristics. This suggests that S. alternif lora adapts to constant salinity through fixed, salinity-dependent str uctural modifications, such as stomatal density.