An Experimental Validation Of Buffer Capacity As A Decisive Index Of Alkali-stress Intensity In Salt-alkaline Mixed Stress

The components of salts in natural salt-alkalinized soils are very complex, and salinization and alkalization frequently co-occur. Consequently, in essence, their injuring effect is not caused by salt-stress or alkali-stress but by salt-alkaline mixed stress. It is necessary for researching salt-alkaline mixed stress to determine index reflecting salt or alkali stress intensity. We found that buffer capacity of mixed salts system could reflect alkali-stress intensity in a salt-alkaline mixed stress. The objective of the study was to prove this conclusion.The salt-alkaline mixed stress condition with the same salinities and same pH values but different buffer capacities (same salt-stress intensity but different alkali-stress intensity) were established by mixing two neutral salts (NaCl and Na2SO4) and two alkali salts (NaHCO3 and Na2CO3) in various proportions, in which the buffer capacity defined as the millimolar amount of H+ needed to drop the pH of 1L of treatment solution to the same pH as the control by titration with HCl. The seedlings of Chloris virgata and xiaobingmai (Triticum aestivum-Agropyron intermedium) were treated under the conditions of salt concentration 100mM, pH 9.6, and buffer capacity 20 - 198 mmol H+, and salt concentration 250mM, pH 9.6, buffer capacity 54 - 354mmol H+ respectively. The relative growth rate (RGR) and other physiological indexes of stressed seedlings were determined to decide that buffer capacity is a valid index of determine the intensity of alkali-stress in a salt-alkaline mixed stress.The results showed that at the same salinities and pHs, the RGR, photosynthetic pigment contents and net photosynthetic rates of Xiaobingmai and C. virgata decreased with increasing buffer capacity. This indicated that buffer capacity was a dominant factor inhibiting photosynthesis. However, at the same salinity and pH, buffer capacity did not affect K+ content. The K+ uptake may only depend on the Na+/K+ ratio in treatment solution, and not be related to high pH or buffer capacity; this should be researched in depth. The organic acid contents in C. virgata and xiaobingmai both increased significantly with increasing buffer capacity, the result also indicates that the greater the buffer capacity the more significant was the effect of high pH by alkali-stress.C. virgata can grow well under high buffer capacity (316 mmol H+) and high pH (pH 9.67), while xiaobingmai seedlings all died at low buffer capacity (98 mmol H+ buffer capacity) and pH 9.60, indicating that C. virgata is an alkali-resistant halophyte, and xiaobingmai is a less alkali-resistant glycophyte. Xiaobingmai and C. virgata are glycophyte and halophyte respectively, and their differences of alkali-resistant capability are evident, but their physiological response all proved that at the same salinities and pHs, the buffer capacity is a valid index of determine the intensity of alkali-stress in a salt-alkaline mixed stress. Basis on the results, it is scientific, reasonable and feasible that buffer capacity is used to estimate the intensity of alkali-stress in a salt-alkaline stress. This maybe closely correlated with the mechanism of plant resistance to alkali-stress. The pH adjustment outside roots is a key physiological mechanism for plants resisting alkali-stress, and buffer capacity is a index which to measure the pressure of pH adjustment. The greater buffer capacity is, the harder pH adjustment is. In other word, the greater buffer capacity of soil solution is, the stronger its alkali-stress action to plants is. The pH adjustment is easy or hard for plant, which lies on the buffer capacity out root. Although pH is an important stress factor of salt–alkalized soil, the pH does not reflect the intensity of alkali-stress, and buffer capacity is an essentially measure for the high pH action on plants. We can use it to estimate the intensity of alkali-stress in salt-alkaline mixed stress.