Effect of NaCl Stress on Growth and Photosynthetic Gas Exchange of Watermelon Seedlings

The experiment was carried out with watermelon [Citrullus lanatus(Thunb.) Matsum.et Nakai] cv.'Zaochun-hongyu' to study the effect of different concentration NaCl stress on plant growth,photosynthetic gas exchange parameters,membrane permeability and proline content of seedlings grown hydroponically.The results showed that 9 d after treatment,25 mmol·L-1 NaCl improved watermelon seedlings growth,and NaCl stress beyond 75 mmol·L-1 significantly inhibited its growth.The photosynthetic pigments content in leaves were obviously increased and reached highest value at 100 mmol·L-1 NaCl.With increasing NaCl concentration,the net photosynthetic rate,stomatal conductance,transpiration rate significantly declined,and intercellular CO2 concentration apparently decreased under lower concentration of NaCl,increased beyond 75 mmol·L-1.The stomatal limitation value and water use efficiency increased under lower concentration of NaCl,tended to stabilization for stomatal limitation value and declined quickly for water use efficiency beyond 75 mmol·L-1.With increasing NaCl concentration,membrane permeability and proline content in leaves significantly increased.The research revealed that NaCl stress significantly inhibited the photosynthesis of watermelon leaves,and the reduction of net photosynthetic rate under lower concentration NaCl was the result of stomatal limitation,and non-stomatal limitation was the main reason for the decrease of photosynthetic rate under higher salinity.