Comparative Toxicity Of NaHCO₃ And NaCl Stress On Tabacco (Nicotiana Tabacum Linn.)

The salinization and alkalization of soil severely reduces agricultural yields and productivity, and even barren. Over the past 50 years, extensive research has gradually revealed the toxic mechanisms of salt, but this toxic mechanism is based mostly on neutral NaCl stress. In recent years some studies have examined that alkaline salts (NaHCO3, Na2CO3) can induce much stronger destructive effects on plants than neutral salts (NaCl), but also some researchers have began to realize that the alkaline salt stress and neutral salt stress are two distinct kinds of stresses for plants, therefore, further study of the toxic mechanism of alkaline salt will help us to understand enhancing toxicity of alkaline salt. In this paper, using the Scanning Ion-selective Electrode Technique (SIET), Laser Scanning Confocal Microscopy Technique and Transmission Electron Microscopy Technique, the toxic mechanism of NaHCO3 was investigated with tobacco (Nicotiana tabacum Linn.) in order to reveal the reasons that NaHCO3 stress can produce more serious harm to plants than NaCl stress and differences from NaHCO3 stress and NaCl stress. The main conclusions were as follows:(1) Na+ absorption and compartmentation:When NaCl and NaHCO3 were added to the plant growth medium, NaHCO3 shock led to a significant increase in the net H+ efflux (5.42 pmol cm-2s-1 compare to NaCl shock (2.41 pmol cm-2s-1), at the same time, pH value of the intracellular vacuole increased from 5.34 to 6.64, the vacuole was alkalized. The seedlings were treated 3 day by the various salts and alkaline solutions, NaHCO3 stress induced increased Na+ efflux than NaCl stress, but this efflux was not inhibited by sodium vanadate and Na+ efflux was a positive correlation with pH of solution. These results indicated that NaHCO3 adding induced H+ extrusion and vacuole alkalization, which leaded to increase membrane permeability. Increased Na+ efflux was not due to the increase of activity of PM Na+/H+ antiporter, but because of the high sodium concentration in the cytoplasm. It was possible that NaHCO3 adding destructed ion selective absorption and leaded to failure of ion compartmentation.(2) Na+ accumulation and parts of toxicity:The various salt stress from one month showed the root/shoot ratio, the transpiration rate, the CO2 absorption rates and the biomass were significantly decreased under 100mM NaHCO3 stress than 100mM NaCl stress, but Na accumulated ratio of old leaves was 1.11 significantly lower in the NaHCO3 Stress than Na accumulated rate of 1.22 under the NaCl Stress. These results suggested that lower root/shoot ratio and transpiration rate inhibited the upward transport of water and accumulation of Na. Parts of toxicity was the root under NaHCO3 stress, it is different Parts of toxicity was old leaves under NaCl stress. (3) Cell wall metabolism and antioxidant enzyme activity:The seedlings were treated 48h by various saline and alkaline solutions in dark, soluble sugar of the root was negative correlated with pH of solution. When pH of solution was greater than 8.5, HC1 and HC2 content of cellulose was positively correlated with pH value of solution. These results indicated that alkali accelerated the decomposition of soluble sugar and out of cell. At high pH, increased hemicellulose percentage was due to decreased pectin percentage. Because of cell viability of the root has been lost within 24h, so the decrease in pectin content was mainly from the alkali hydrolysis of pectin. The antioxidant enzyme activities in the roots were greater impacted than the antioxidant enzyme activities in the leaves when tobaccos were treated 24h by salt (400mM NaCl) of and alkali (200mM NaHCO3, 100mM Na2CO3) stress. The different salt and alkali affected on activity of different antioxidant enzymes. With increased Na+ concentration, CAT and GR activity decreased more rapid; SOD, APX and POD activity were greater inhibited with pH changes.(4) Organelle damage and programmed cell death:The protoplasts of tobacco were treated by various salt and alkali solution, NaHCO3 stress caused more broken and vacuolization of mitochondria, transmission electron microscopy analysis showed that endomembrane system has been damaged before the plasma membrane were damaged, the cytoplasm had lysed regions, mitochondria with dilated or absent cristae. The programmed death of protoplast occurred in all treatment, the high apoptosis rate been found in 200mM NaHCO3 stress and 200mM NaCl stress, it was respectively 10.34% and 7.57%. With the pH increased, Cell apoptosis was completely replaced by broken. These results indicated that alkaline salt could dissolute plasma membrane in high pH environment.(5) ROS response and function of exogenous ethylene:NaHCO3 stress inhibited root elongation, root hair length of tobacco and also significantly reduced the number of root hairs, but the ROS response was not obvious. Although sodium vanadate and ACC also inhibited root elongation, but can promote the development of root hairs, reduced apical ROS response. Lower NaHCO3 stress was weakened by Sodium vanadate and ACC, but sodium vanadate was effective in the short time and ACC was effective in the relatively long time. Sodium vanadate and ACC could inhibited PM ATPase activity and also inhibited absorption of Na+. ACC reduced H+ efflux to avoid further cytoplasmic alkalinization in NaHCO3 solution although they inhibited the root growth.