Effects of iso-osmotic Ca(NO₃)₂ and NaCl stress on respiration and metabolism of reactive oxygen species in roots of tomato seedlings

Soil secondary salinisation is a problem resulting in a decreased productivity of protected vegetable production in China. In protected greenhouses, salts accumulated in soil are mainly as Ca²⁺ and NO₃^-, which are different from those in coast saline soil as Na⁺ and Cl^-. The effects of different types of salts accumulated in soils on plant leaf growth and root morphology have been well documented; however, little has been done on different types of salts affecting root growth and its metabolism. In this research, we used a commercial tomato hybrid Solanum lycopersicum L. cv. Hezuo903 in hydroponic culture to investigate the effects of two types of salts, Ca(NO₃)₂ and NaCl, on root respiration and metabolism including KCN-resistant respiratory rate (V_alt), cytochrome respiratory rate (Vcyt), root production rate of <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j.issn.1008-9209.2012.10.311-I001.jpg"/>, H₂O₂ concentration and activities of superoxide dismutase (SOD), guaiacol peroxidase (GPX) and catalase (CAT).The seedlings were transplanted into 10 L plastic containers containing aerated full nutrient solution. Fourteen days after being transferred to the hydroponic medium, the salt treatments were applied by adding 80 mmol/L Ca(NO₃)₂ or 120 mmol/L NaCl to the nutrient solution and were maintained for 12 d. Four trays (16 plants) were included in each treatment as well as in the controls. Root respiration was measured as oxygen consumption in a 2-mL closed cuvette using a Clark type oxygen electrode. Salicylhydroxamic acid was used as an inhibitor for alternative respiration and KCN was used as an inhibitor for cytochrome respiration.The results showed that both root and shoot growth of tomato plants were severely inhibited by salt stress. Compared to the control plants, the shoot and root dry masses in Ca(NO₃)₂ -stressed plants were reduced by 42.5% and 33.5% respectively, less than in NaCl-stressed plants by 63.2% and 53.8% respectively. Salt stress decreased the total respiratory rate (V_T) due to the decrease of V_cyt, particularly under NaCl stress, but increased V alt, which was again more pronounced under NaCl stress. After 12 d of treatment, the ratio of V_alt to V_T increased by 51.2% in Ca(NO₃)₂ -treated plants and 63.5% in NaCl-treated plants, respectively, while this ratio remained unchanged at 26.1% in control plants. The production rate of <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j.issn.1008-9209.2012.10.311-I002.jpg"/> and H₂O₂ concentration were increased by salt stress, especially under NaCl stress. This was also the case for the activities of SOD, CAT and GPX during the first 8 d of NaCl treatment, while the activities of these enzymes under Ca(NO₃)₂ treatment were increased all the time. After 12 d of treatment, the MDA content increased by 48.3% in Ca(NO₃)₂ -stressed plants and by 122.8% in NaCl-stressed plants respectively.In conclusion, NaCl stress cause much severer inhibition to plant growth than Ca(NO₃)₂ stress, which may be resulted from its damage effects with a higher lipid peroxidation level and lower respiratory rate, though its elevated KCN - resistant respiration effect can compensate partly such detrimental effects. It is also interesting to note that in companion with the enhancement of <inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j.issn.1008-9209.2012.10.311-I003.jpg"/> and H₂O₂ production, KCN-resistant respiratory rate and its ratio to total respiratory rate increase significantly in salt-stressed plants, suggesting that the increased KCN-resistant respiration might be able to enhance reactive oxygen species scavenge in salt-stressed plants, especially in NaCl-stressed plants.