The seedlings of wheat were treated by salt-stress (SS, molar ratio of NaCl : Na 2 SO 4 = 1 : 1) and alkali-stress (AS, molar ratio of NaHCO 3 : Na 2 CO 3 = 1 : 1). Relative growth rate (RGR), leaf area, and water content decreased with increasing salinity, and the extents of the reduction under AS were greater than those under SS. The contents of photosynthetic pigments did not decrease under SS, but increased at low salinity. On the contrary, the contents of photosynthetic pigments decreased sharply under AS with increasing salinity. Under SS, the changes of net photosynthetic rate (P N ), stomatal conductance (g s ), and transpiration rate (E) were similar and all varied in a single-peak curve with increasing salinity, and they were lower than those of control only at salinity over 150 mM. Under AS, P N , g s , and E decreased sharply with rising salinity. The decrease of g s might cause the obvious decreases of E and intercellular CO 2 concentration, and the increase of water use efficiency under both stresses. The Na + content and Na + /K + ratio in shoot increased and the K + content in shoot decreased under both stresses, and the changing extents under AS were greater than those under SS. Thus SS and AS are two distinctive stresses with different characters; the destructive effects of AS on the growth and photosynthesis of wheat are more severe than those under SS. High pH is the key feature of the AS that is different from SS. The buffer capacity is essentially the measure of high pH action on plant. The deposition of mineral elements and the intracellular unbalance of Na + and K + caused by the high pH at AS might be the reason of the decrease of P N and g s and of the destruction of photosynthetic pigments.
Seedlings of Chloris virgata were treated with varying (0-160 mM) salt-stress (SS; 1 : 1 molar ratio of NaCl to Na 2 SO 4 ) or alkali-stress (AS; 1 : 1 molar ratio of NaHCO 3 to Na 2 CO 3 ). To compare these effects, relative growth rates (RGR), stored energy, photosynthetic pigment contents, net photosynthetic rates, stomatal conductance, and transpiration rates were determined. Both stresses did not change significantly the photosynthetic parameters of C. virgata under moderate stress (below 120 mM). Photosynthetic ability decreased significantly only at high stress (160 mM). Thus C. virgata, a natural alkali-resistant halophyte, adapts better to both kinds of stress. The inhibition effects of AS on RGR and energy storage of C. virgata were significantly greater than that of SS of the same intensity. The energy consumption of C. virgata was considerably greater while resisting AS than while resisting SS.Additional key words: carotenoids; chlorophyll; leaf area; net photosynthetic rate; relative growth rate; stomatal conductance; transpiration rate; water content. --The salinization and alkalization of soil is a widespread environmental problem. Although the world's land surface occupies about 13.2×10 9 ha, no more than 7.0×10 9 ha are potentially arable and only 1.5×10 9 ha are currently cultivated. Of the cultivated lands, about 0.34×10 9 ha (23 %) are saline and another 0.56×10 9 ha (37 %) are sodic (Läuchli and Lüttge 2002). Saline and sodic soils exist in over 100 countries, and cover about 10 % of total arable lands. In saline and sodic soils, Na + , Ca 2+ , Mg 2+ , and K + are the main cations of dissoluble mineral salts, and Cl -, SO 4 2-, HCO 3 -, CO 3 2-, and NO 3 -are the corresponding main anions (Läuchli and Lüttge 2002). These ions all come from neutral salts or alkaline salts. We can further classify natural salt stress, in terms of the salt characteristics, into neutral salt stress, alkaline salt stress, and mixed salt stress. The alkaline salt stress and the neutral salt stress are two distinct kinds of stresses for plants and should be called alkali-stress (AS) and saltstress (SS), respectively (Shi and Yin 1993). Alkaline salts (NaHCO 3 and Na 2 CO 3 ) induce much stronger destructive effects on plants than neutral salts (NaCl and Na 2 SO 4 ) (Shi and Yin 1993). When salinized soil contains HCO 3 -and/or CO 3 2-, which raise the soil pH, plants suffer damaging effects of both SS and AS. However, relatively little attention has been given to AS. There are some reports about high pH of calcareous soils (Brand et al. 2002, Nuttall et al. 2003, alkaline soil (Hartung et al.
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