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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Soil TT Increased 



Plant TT 

 Increased 



SoilY 

 Decreased 



Plant T 

 Decreased 



AT(soil-plant) 

 Maintained "** 



Water Uptake 

 Reduced 



I 



Transpiration Exceeds 

 Absorption 



WATER STRESS 



Root Resistance 

 Increased 



High Transpiration 

 Rate 



n 



Evaporative 

 Demand of 

 Air High 



Figure 2.— Hypothesis to explain how osmotically ad- 

 justed plants can suffer from water stress under con- 

 ditions of high evaporative demand. ($= water poten- 

 tial; 7r=osmotic pressure). 



were used for analysis of the effect of osmotic 

 stress on growth and development of the vege- 

 tative plant body. Five plants were harvested 

 from both control and salt-treated groups of 

 plants several times during the growing period. 

 Fresh weight (fig. 3) and dry weight (fig. 4) 

 both were affected immediately after receiving 

 the increased salt concentration in the external 

 solution. All parts of the plant were affected; 

 that is, there was a general decrease in growth 

 rate in the osmotically stressed plants. The rate 

 of leaf initiation also was reduced (fig. 5). This 

 is not in agreement with Livne and Levin (34). 

 who indicated that 3 bars of NaCl added slowly 

 (thus, presumably allowing osmotic adjustment 

 to occur) to pea plants did not affect the rate of 

 appearance of new leaves. 



The leaf area of osmotically adjusted plants 

 becomes drastically reduced also (38, 42, 47). 

 This usually is the most noticeable effect of in- 

 creased salinity in the root medium (42). De- 

 ceased leaf expansion usually is accompanied by 

 increased leaf thickness, that is, the leaves are 

 more succulent in the osmotically adjusted plants 



(46), provided the plants are growing in an en- 

 vironment that does not promote development of 

 water stress as indicated earlier. The following 

 sequence of events probably occurs. In response 

 to the elevated n of the root medium, plant cell 

 7T increases, water enters the cell in response to 

 this increased jr CPU but the cell wall does not yield 

 normally, that is, the cell does not readily ex- 

 pand. This suggests to the writer that the in- 

 crease in ttcph causes a decrease in cell wall ex- 

 tensibility. If this is so, then one should expect 

 the turgor pressure to increase in the nonyielding, 

 osmotically adjusted cells. Does that happen? 



The literature indicates that such a situation 

 has occurred many times, but little attention has 

 been given to the increased turgor pressure. For 

 example, Boyer (6) found that turgor pressure 

 in cotton leaves subjected to an increase of 8 bars 

 in the ?r of the external medium was double the 

 turgor pressure in leaves of nonsalinized plants. 



Gale and others (15) also found slightly higher 

 turgor pressures in leaves of salt-treated cotton 

 plants, and Meiri and Poljakoff-Mayber (39) 

 found higher turgor pressures in leaves of bean 

 plants subjected to increased salinity. This has 

 consistently been observed in our laboratory as 

 long as the environment of the plant does not 

 favor high transpiration rates (43, 44, 46, 51). 

 That this higher turgor pressure in osmotically 

 adjusted plants is indicative of more rigid cell 

 walls is supported by the observation that the 

 dry weight per unit area of the leaf is increased 

 considerably in the osmotically adjusted plants 

 (46). This, of course, might indicate thicker cell 

 walls if the cell number per unit leaf area was 

 the same in each case, or it might indicate a 

 greater number of cells per unit leaf area in the 

 osmotically adjusted plants. Both conditions have 

 been reported. 



Nieman (42) reported that the number of cells 

 per unit leaf area was the same in leaves from 

 control and osmotically adjusted bean plants, 

 while Meiri and Poljakoff-Mayber (38) reported 

 that there are more cells per unit area in leaves 

 from salt-treated bean plants. 



We conducted experiments similar to those 

 cited above, and the treatments were replicated 

 in two different greenhouses (56). In one green- 

 house, we obtained results that agreed with Nie- 

 man (42) and in the other we obtained results 



