Chiarello cells were exposed to changing and constant salinities which ranged from 1 atm to 6 atm of NaC1, Na2SO4 and NaC1+KC1 respectively. In case of changing salinity an increments of salt producing 0.5 atm pressure were added periodically over previous salt concentration till it reaches to desired final concentration, whereas in constant salinities the total salt concentration was added in single increment. Chlorella cells growing under former conditions showed some adaptive abilities over those growing under latter conditions. Cell division was enhanced at higher levels of mixed changing salinity though it was inhibited at the iso-osmotic concentrations of constant salinities. There was an inhibition in cell division under single salt treatment irrespective of constant or changing salinities though the inhibition was higher in former than the latter treatments. Cultures receiving sulphate type of salinity was more toxic than chloride type.
Accumulation of intracellular Na + in all the treatments of changing salinities was less than that of Constant salinities. Intracellular K+ was higher in charging salinity of mixed salts treatments only. Chlorophyll content was increased in all the treatments of changing in comparison with constant salinities, showing higher ratio of chlorophyll a/b in former than latter treatment.
Respiration and photosynthesis on cell basis was enhanced in all the treatments of changing salinities whereas marked inhibition was observed in constant salinities. However, on chlorophyll basis enhancement in photosynthesis and respiration was observed only under mixed salt treatments. Exposure of chlorella cells to the higher salinities of single salt achieved in one increment, caused complete inhibition in nitrate reductase and nitrite reductase activities whereas those reaching to these concentration through changing salinites showed varied responses. Cells growing in the cultures supplemented with lower levels gradually increasing concentrations of single salts showed an inhibition in the activities of these enzymes on dry weight basis, though there was a promotion on the organism basis. In mixed salt treatments activities of these enzymes were enhanced on both the above mentioned parameters. Nitrate reductase of chlorella cell was found to be more sensitive to salt stress than nitrite reductase.
The experiments performed with ammonium grown cells demonstrate that the decrease rate of nitrate assimilation at low salinities was due to the inhibition in the synthesis of enzymes whereas at the higher salinities both the synthesis and the enzyme activity were inhibited. The level of total proteins was found to be higher in gradually increasing mixed salt treatments whereas it was lower in all the other treatments. Salinity is not a constant factor under field condition. It tends to increase or decrease depending upon the quality of irrigation water and soil conditions. In the present study, wheat and cotton plants were grown under changing and constant salinization at par with saline soils of Pakistan and the effects of both the types of salinities on growth, ionic composition, biochemical changes, enzyme behavior and reproductive physiology were studied. Growth of the plants was effected differently depending upon the nature of experiments. The salt effects were dependent on the rate of salinization and the ultimate levels of salinities. At lower levels of changing salinity some enhancement in growth was observed, Wheat was highly sensitive to higher concentrations of constant salinities as d result no seedling survived at these levels, whereas cotton managed to grow. Water content '. plant height and biomass production was reduced by increasing salinities in both the plants. Reduction in biomass was more under constant as compared to changing salinities. The ionic composition of root and leaves of both the test plants show a remarkable increase in Na +, C1- and SO-24 contents. It was further observed that Ca+2 and Mg+2 contents decreased variably under salt stress. An increase in K+ content in wheat leaves and roots was observed at higher levels of changing salinities, whereas under constant salinities its increase was observed only at lower level of salinities. In cotton leaves the K+ and Mg +2 contents were increased upto highest changing salinities whereas under constant salinities this increase was observed only in the roots. Both the chlorophyll a and b contents of wheat and cotton were increased under low levels of changing salinities. Increase in Chi a was more pronounced as compared to Chi b. Reduction in both the Chi a and b was observed at higher levels of changing slinities. However, under constant salinities, the chlorophyll contem was reduced significantly at all the levels of salinities. The reduction in chlorophyll content was more in wheat as compared to the cotton. Proline and total sugars of the leaves and roots of both the wheat and cotton were increased with increasing concentrations of changing and constant salinities. Proline and DNA contents of both the wheat and cotton leaves were increased under lower concentrations of changing salinties. Whereas proline, RNA and DNA contents decreased both in the leaves and roots of rest of the treatments. Changes in the metabolic activities were observed alongwith the changes of water content and ionic composition of plants exposed to various types of salinities. While comparing the effects of changing and constant salinities on IV starch synthetase (x I, 4 glucanglucosyltransferase) of developing wheat grains, there was found a difference in adaptation. Specific activity of this enzyme was enhanced significantly at lower concentration of changing salinity and with the increasing concentrations of salts, the observed increase was reduced though remained greater than control. However, under constant salinities the enhancement of enzyme activity was observed at lower levels of salinity only. This increase in the activity could be due to possible replacement of K+ by Na + for activating the enzyme under salt stress.
The activity of Cel1ulose synthetase (B I, 4, D glycosyItransferase) in developing cotton bolls was enhanced at all the levels of changing salinities and at low level of constant salinities. This enhancement was observed after 10 days of flower opening. In vitro activity of nitrate reductase of both the wheat and cotton was more salt sensitive as compared to In vivo under constant as wel1 as changing salinities. The phosphatase activities were influenced variously in wheat and cotton leaves under changing and constant levels of salinities. Both. the plants exhibit enhanced activity of acid phosphatase under former treatment as compared to latter. On the contrary, alkaline phosphatase activity was enhanced under latter as compared to former treatment. Pollen behaviour is an important factor for crop production under saline environment. ,Pollen grains were collected from the flowers produced upon the plants exposed to different levels of changing and constant salinities. Reproductive phase of the wheat was found most sensitive both under changing and constant salinities as compared to the cotton. There was a linear reduction with increasing concentrations of salts showing hundred percent inhibition in pollen germination of wheat grown at higher levels of changing salinities. Even viable pollens were not capable of germination under higher level of salinities. The degree of inhibition in pollen viability and germination was greater under constant salinities. Seed formation was not recorded at salinities where hundred percent inhibition in pollen germination was observed. Number of seeds/ear and total yield was reduced with increasing concentrations of salinity of the rooting medium.
This reduction was highly significant even at lower levels of constant salinities. Reduction in the yield was also due to retardation in filling rate of developing grains and translocation of photosynthates.
Though cotton plants were also found to be sensitive at reproductive phase but the degree of inhibition in pollen viability and germination was comparatively less than wheat under both the treatments. Pollen collected from the cotton plants growing at highest salinity levels remained viable and capable of germination. Number of cotton bolls initiated under changing salinities was higher over constant salinities but the number of mature cotton bolls was reduced. The number of cotton seeds/boll was reduced greatly with increasing concentrations of salts in both the treatments. The yield of cotton seed and lint was though increased at low levels of changing salinity showing linear decrease with increasing concentrations of salt. Fiber quality of cotton were changed including short fibers, upper quartile length, mean length and maturity coefficient. Under both the saline treatments the oil content was increased per hundred gram of seed weight with the exception to higher level of constant salinity. Ionic composition of seeds were also influenced under salt stress. Na + and C1 contents of wheat and cotton seeds were increased at all the levels of changing and constant salinities. Decrease in K+ content was observed with increasing concentrations of salt under both the treatments. Higher accumulation of Na + and C1 was observed in cotton seed coat as compared to inner parts of the seeds (cotyledons + embryo) slight increase in K+ content was observed at lower levels of changing salinities in cotton seeds.
The seeds of wheat and cotton produced on the plants exposed to Changing salinities germinated better in the levels of salinity similar to those in which they had been produced than the seeds from untreated control plants or those which had received constant salinitie. Percentage improvement in seed germination under such cases was higher in cotton as compared to wheat.