INFLUENCE OF SALT STRESS ON STOMATAL, BIOCHEMICAL AND MORPHOLOGICAL FACTORS LIMITING PHOTOSYNTHETIC GAS EXCHANGE IN PAULOWNIA ELONGATA x FORTUNEI

The effect of salt stress on Paulownia elongata x fortunei seedlings grown ex vitro as hydroponic culture at three levels of salinity - 100 mM l(-1), 200 mM l(-1), 300 mM l(-1) sodium chloride (NaCl) solution was investigated. The results showed that growth parameters like ratios fresh/dry biomass (FM/DM) and total leaf area (LA) were enhanced under middle salinity level (200 mM l(-1) NaCl). Net photosynthetic rate (P-N), transpiration rate (E) and stomatal conductance (g(s)) decreased gradually, but total respiration rate (R-d) increased under low and middle salinity level (200 and 300 mM l(-1) NaCl). With increasing salinity the values of the intrinsic components of photosynthesis were reduced - V-c (max) (from 13.25 to 11.81 mu mol m(-2)s(-1)), J(max) (from 11.11 to 9.07 mu mol photons m(-2)s(-1)), and V-TPU (from 2.39 to 2.11 mu mol m(-2)s(-1)), and the differences were statistically significant. Stomatal limitation to gas exchange conductance (S-L) increased from 11.22% to 21.24% and 15.88% at 100 and 200 mM l(-1) NaCl. Water use efficiency (WUE) was subsequently enhanced from 3.12 for the control plants to 30.60 for 300 mM l(-1) NaCl treatment. It is concluded that salinity causes a decline in photosynthetic gas exchange in Paulownia elongata x fortunei leaves through its intrinsic and stomatal components, and that the apparent plasticity represented by the leaf area modulation is unlikely to be the mechanism by which this species avoids salt stress.