Research studies reported in this thesis were performed to investigate the effects of salinity and sodicity on gas exchange and growth of different wheat varieties having contrasting salt tolerance. The research was conducted to identify the traits responsible for imparting salt tolerance or salt sensitivity in wheat. Experiments were conducted under saline and sodic field conditions and in greenhouse in solution culture with acidic and alkaline reaction.
There was a broad variation in salt tolerance in various wheat varieties used in different experiments. Salt tolerance mostly depended on the ability of different cultivars to exclude Na⁺ and Cl^- and to retain K⁺. Different wheat varieties responded differently to salt stress. SARC-1 and Kharchia-65 showed better performance than the salt-sensitive variety Punjab-85 in solution culture as indicated by their ability to maintain higher leaf area, dry weight and relative growth
rate, at 150 mol m^-3 NaCl stress. At 150 mol m^-3 NaCl stress, hundred grain weight in Kharchia-65 was lower than SARC-1 and PAK-81 although its leaf area and dry weight was the highest. This suggested the presence of some inhibitory mechanism which hindered the translocation of assimilates from the leaves to the seeds at high salinity.
LU26S proved to be the most salt-tolerant variety and gave the highest grain weight per spike under saline and sodic soil conditions as indicated by its lower Na⁺ uptake, higher K⁺/Na⁺ ratio, higher Pn, higher dry weight of shoots and spikes and better grain development ( 100 grain weight). High Na⁺ uptake, lower K⁺/Na⁺ ratio, lower dry weight of main shoots and spikes and lower 100 grain weight seemed to be the main reasons for salt-sensitivity in Punjab-85.
Punjab-85 which was the most salt-sensitive variety in the glasshouse and saline soil conditions showed medium tolerance to sodicity possibly due to smaller effects on its yield components. Kharchia-65 is a salt-tolerant variety but it did not prove tolerant to sodic conditions indicated by its failure to maintain flag leaf area and dry weight under very dense sodic soil conditions. Pooled data for grain weight per spike for the ten wheat varieties tested under sodic soil conditions showed little correlations with Na⁺, K⁺ contents of the flag leaves and K⁺/Na⁺ ratios. Greater uptake of Na⁺ and lower uptake of K⁺ was noted in the wheat varieties sown in sodic soil than those in the saline soil.
NaCl salinity decreased the rate of net photosynthesis by affecting stomatal and non-stomatal factors. In the salt-sensitive variety, the effect of age seemed to be due to non-stomatal factors which seem to be influenced by excessive concentrations of Na⁺ and Cl^- in the leaves.
Fitting a rectangular hyperbola model for Pn/I response curves gave a good fit but the calculated values for Pn_max were 32-66% higher as compared to actual observed values. Under salt stress, overall photosynthetic productivity was higher in LU26S than in Punjab-85 as indicated by its higher Pn_max , higher a, lower le and lower observed~ , particularly in the older leaves.
The experiment conducted on SARC-1 variety under alkaline pH conditions provided some evidence that the effects of salinity on Pn, growth and uptake of toxic ions are greater at high pH of the growth medium (pH 8.5) than at low pH (pH 5.8). Percentage decreases in most of the growth parameters by salinity, and increases in concentrations of toxic ions (Na⁺ and Cl^-) were also greater at high pH than at low pH. These effects were not so severe when the same experiment was repeated with LU26S and Punjab-85 at pH 8.
High salinity caused an increase in Na⁺ and Cl^-, and a decrease in K⁺, Ca²⁺, Mg²⁺, K⁺/Na⁺ ratio, Zn²⁺, Fe²⁺ and Mn^{2 +}. High pH induced an increase in Na⁺, K⁺, Cl^-·, K⁺/Na+ and a decrease in Zn²⁺, Fe²⁺ and Mn²⁺. Generally, weight of grains per plant and 100 grain weight was comparatively greater in LU26S.