16 



MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



Table 2. — Osmotic adjustment of Phaseolus vul- 

 garis L. plants growing in solutions subjected 

 to increasing concentration of NaCl. tt = osmotic 

 pressure 





ir of plant in 



it of plant 





it of nutrient 



nonsalinized 



in salinized 



A7r between 



solution 



solution 



solution 



plants 



Bars 



Bars 



Bars 



Bars 



1 



7.7 



8.5 



0.8 



2 



6.4 



7.1 



.7 



3 



6.2 



8.5 



2.3 



4 



6.8 



8.9 



2.1 



4 



6.9 



8.3 



1.4 



4 



6.6 



9.5 



2.9 



4 



6.6 



10.5 



3.9 



4 



5.8 



9.7 



3.9 



in the salinized solutions exhibited fluctuations 

 from 8.3 to 10.5 bars. However, within a week 

 after the maximum salinity level had been 

 reached, the plants apparently were fluctuating in 

 concert so that the difference in it of the plants 

 was the same as the difference in w of the growth 

 solutions. 



Even though there have been numerous reports 

 of osmotic adjustment in plants accompanying 

 increasing n in the growth medium, there are 

 no reports of whether the adjustment is reversi- 

 ble. We have found that the osmotic adjustment 

 is, in fact, reversible (fig. 1). The plants were 

 subjected to an increasing NaCl concentration 

 in the growth solution at a rate of 1 bar every 2 

 days. Plants were sampled every 2 days also. Just 

 as there is a lag in adjustment as it is increased, 

 so is there a lag as w of the growth solution is 

 then decreased back to zero. Nevertheless, the 

 adjustment was almost completely reversed with- 

 in a day of return of the plants to nonsalinized 

 solution. 



Root Resistance In Osmotically Adjusted Plants 



Despite osmotic adjustment that occurs in 

 plants, which should maintain the water potential 

 gradient from growth solution to leaves (3, 33), it 

 often has been observed that salinity toxicity 

 symptoms resemble drought symptoms (2, 4). 

 That is, it appears that water transport to leaves 



may be reduced in osmotically adjusted plants 

 under certain conditions. How this might occur 

 can be seen from consideration of the generalized 

 flow equation : 



J« = L/3 (A*) (3) 



where J<» = waterflow across the root (cm. 3 cm. -2 

 sec." 1 ), L/3 = hydraulic conductivity coefficient of 

 the root surface (cm. sec -1 bar -1 ), and A* = the 

 water potential gradient from the external solu- 

 tion to the interior of the root (bars). Reduced 

 waterflow into the plant could occur under a con- 

 stant A* if Lp is reduced. It has been suggested 

 many times that permeability of roots may be 

 reduced by increased salinity or decreased water 

 content in the root environment (27, 28, 30, 37). 



12 3 4 



Growth Solution ATT 



(bars) 



Figure 1. — Difference in osmotic pressure (Air) between 

 salinized and control plants vs. difference in osmotic 

 pressure of nutrient solutions in which the plants were 

 growing. Osmotic pressure of the growth solution was 

 increased every 2 days and, upon reaching 4 bars, was 

 then decreased every 2 days. On the alternate days, a 

 plant was sampled from the adjusted solution and 

 from the nonadjusted or control growth solution 

 (7r = 0.3 bars, but called zero here). 



