Effects of applied NaCl on Opuntia humifusa

Plant morphology in the field in Virginia, USA, and growth responses to applied NaCl in a glasshouse were determined for two populations of Opuntia humifusa (Rafinesque) Rafinesque, one from an inland site and the other from the marine strand, which differed in potential exposure to salinity. Cladode dimensions, plant height and rooting depth varied significantly between the populations. Application of NaCl in 50 m M increments up to 400 m M every 3 days for 6 weeks caused the cladodes to decrease up to 30% in thickness, the decrease being about 20% more at 50 to 150 m M NaCl for plants from the inland site than from the marine strand. Inhibition by 150 m M NaCl of the maximum rate of net CO₂ uptake and of the total CO₂ uptake over a 24-h period was greater for the inland population. Growth, especially of roots, was inhibited by applied NaCl, with a decrease in biomass above 200 m M NaCl for plants from the inland site and not until 400 m M for those from the marine strand. Although the root Na⁺ level was the same for plants from the two populations, reaching a maximum of about 8 mg (g dry weight)^?1 at 200 m M NaCl, the cladode Na⁺ level was two-fold higher for plants from the marine strand than from the inland site. Thus, exclusion of Na⁺ from the cladodes is not the reason for the greater NaCl tolerance of O. humifusa from the marine strand, which is a habitat that can experience periodic episodes of high salinity.