Morpho-physiological Responses Of Populus Alba ×P. Berolinensis Leaves To Nitrogen Dioxide/Exogenous Nitrogen-or Sulfer-containing Compounds

NO2 is one of main pollutants in urban atmosphere. Recent sharp increase in motor vehicle amount is aggravating NO2 pollution in the atmosphere. Some plant species have the capacities of adsorbing and depositing NO2. There are significant differences in responses to NO2 bettwen plant species. Therefore, under the background of heavy NO2 pullution, the effects of NO2 and simulated nitrogen deposition on leaf morphology, photosynthesis, and nitrogen metabolism were investigated in the hybrid poplar (Populus alba × P berolinensis) seedlings grown in Harbin city and the potential roles of leaf-spraying sodium sulfide (Na2S) and sodium hydrogen sulfite (NaHSO3) were also studied.(1) By using gas fumigation, gas exchange, and scanning electron microscope techniques, the effects of NO2 on stomatal dynamics and photosynthetic characters were investigated in the P. alba × P. berolinensis leaves. The results showed that 4 μl·l-1 NO2 treatment for 48 h had significantly negative impacts on stomatal function, photosynthesis, and dark respiration. Under a saturated light (1200μmol·m-2·s-1), net photosynthetic rates (Pn) of the fumigated leaves had no significant correlation with leaf temperature (R2=0.0151); in addition, dark respiration rates (Rdark) were correlated positively with leaf temperature. Under light transition (from a saturated light to the darkness), the leaves were rapidly changed from photosynthetic status to dark respiration status, NO2 significantly disturbed the gas exchange-time induction process.(2) Effects of leaf (4.8 mmol·L-1) NaHSO3-or (5.0 mmol·L-1) Na2S-spraying on photosynthetic parameters of the hybrid poplar leaves were studied. Results showed that NaHSO3 treatment for 30 days had no significant impacts on nitrate reductase activity (NRA, P > 0.05), chlorophyll fluorescence induction kinetics, and gas exchange rate induction; Na2S treatment had positive impacts on them. Moreover, temperature could disturb leaf responses to the spraying treatments.(3) The morphological and physiological characters of the leaves were studied under NO2 treatment and leaf spraying of Na2S solution. Results showed that leaf Na2S-spraying had an alleviated effect on the NO2-caused damage:30-day sprayed leaves increased thicknesses of palisade tissue by 40% and spongy tissue by 60%, respectively, as compared with that of the CK; in addition, when NO2 treated for 48 h, the Pn, maximal quantum yield of PSⅡ (Fv/Fm), efficiency of open PSⅡ reaction centers (Fv’/Fm’), and effective quantum yield of photochemical energy conversion in PSⅡ (ΦPSⅡ) of the sprayed leaves were increased by 14.86%,13.64%, 19.15%, and 25.71%, respectively. NRA and hydrogen sulfide (H2S) contents of the sprayed leaves were significantly higher than that of CK (P<0.05). Based on the above results, leaf Na2S-spraying might be through the H2S signal pathway to enhance the seedlings’ tolerance to NO2.(4) The effects of leaf spraying (NaHSO3 4.8 mmol·L-1) on photosynthesis of NO2-treated hybrid poplar were investigated. Results showed that NO2 treatment led to a significant decline in Pn, Fv’/Fm’, OPSII, and leaf nitrate-N, and partial disfunction of stomata was also observed. The spraying alleviated the NO2-caused damage to some extent by decreasing excessive light energy pressure, enhancing photochemical energy dissipation, and maintaining a relatively high photochemical efficiency and carbon assimilation rate, as well as improved stomatal function and decreased accumulation of leaf nitrate-N. It is concluded that leaf NaHSO3-spraying has a positive impact on the hybrid tree responses to NO2, which might be through enhancing stomatal function or alleviating the NO2-caused damage to photosynthetic apparatus.(5) The effects of leaf nitrate- or mixed nitrite (NaNO3 20 mmol·L-1 or mixed NaNO2(Na+ concentration-20 mmol·L-1, NO2- concentration-100 μmol·L-1) spraying on leaf morphology, pigments content, photosynthetic parameters and nitrogen assimilation were studied. Results showed that stomata was mainly distributed on lower leaf surface; leaf spraying led to burning symptoms, declines in contents of Chla, Chlb, and Chl(a+b) but a slight increase in contents of carotenoid and anthocyanin. NaNO3 or NaNO2 spraying had no substantial influence on light energy absorption by the reaction center and electron transfer rates; however substantially affected the gas exchange rates (Pn and Ci)-time induction under light transition from sub-saturated light to the darkness, without significant difference in the curve between NaNO3 and NaNO2 sprayed leaves.