PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



17 



It is not easy to determine from those studies, 

 however, whether the permeability decrease oc- 

 curs only in the presence of a high -n in the 

 external solution or whether there is a definite 

 structural change in the roots that causes the 

 reduced permeability. That is, when the roots 

 are placed in water with a -k of 0, is the permea- 

 bility still lower in those plants that had grown 

 in the higher salinity environment? 



When we measured the pressure-induced water- 

 flow through truncated root systems, we found 

 that, even in distilled water, the waterflow 

 through root systems of osmotically adjusted 

 plants was considerably less than in nonosmotical- 

 ly adjusted plants (44i 4-7-) 



Recently, we have attempted to estimate the 

 average L p for an entire root system. Using the 

 same techniques reported previously (44), and 

 measuring w and P (pressure in excess of at- 

 mospheric) of xylem sap and external solution 

 under controlled fluctuating values of both P and 

 tv of external solution, we have obtained values 

 for L p of the entire root system. It is realized 

 that permeability of the root surface as well as 

 the reflection coefficient for various solutes, varies 

 considerably along the length of the root, because 

 of aging of the cells (1). Also, it is not easy to 

 accurately determine the surface area of an en- 

 tire root system. However, we have worked out 

 the relationship between fresh weight and surface 

 area, using measurements of surface area on in- 

 dividual excised roots of various sizes and ages, 

 and we calculated surface area from fresh weight 

 data. The details of this work have been pre- 

 pared for publication elsewhere. Using this ap- 

 proach, we obtained an average L p for the entire 

 root surface of a red kidney bean (Phaseolics vul- 

 garis L.) plant of 1.0 x lO -6 cm. sec. -1 bar -1 . 



This is not far from the value obtained by 

 House and Findlay (23) for single corn roots. 

 They calculated a value for L p of 0.6 x 10 -6 cm. 

 sec." 1 bar" 1 , and Klepper (28) found an L p of 0.38 

 x 10 -6 cm. sec. -1 bar _1 for corn roots also. Klep- 

 per (28) also found that L p of 0.38 x 10" 6 cm. 

 sec. -1 bar -1 for corn roots also. Klepper (28) also 

 found that L p decreased as tt of the exter- 

 nal solution increased. When bean plants were 

 grown in solutions with 4 bars of added NaCl, 

 the L p for the root surface was reduced to 

 0.1 x 10- G cm. sec. -1 bar -1 , even when placed in 



distilled water. That is, the permeability reduc- 

 tion was the result of a change in the roots them- 

 serves and not just a condition resulting from 

 the high tt of the environment. 



Development Of Water Stress In Osmotically 

 Adjusted Plants 



Water stress in plants usually occurs when 

 the capacity of the soil to deliver water to the 

 plant becomes limiting, or when transpiration 

 is occurring so rapidly that it outruns water 

 absorption by the plant, even in soil containing 

 adequate water. Whether or not transpiration rate 

 exceeds absorption rate depends on the evapora- 

 tive demand of the air and the resistance to 

 water transport to the leaves. When evaporative 

 demand is high, transpiration often exceeds water 

 absorption, and water stress and wilting occur. 

 In osmotically adjusted plants, increased root re- 

 sistance makes such an event more probable since 

 the evaporative demand of the air does not have 

 to be so high for transpiration rate to exceed 

 absorption rate. When this happens, the osmoti- 

 cally adjusted plant incurs a water deficit and 

 suffers the same consequences as the wilted plant 

 in a drying soil. These events are summarized in 

 fig. 2. " 



As 7r so n increases, the * soi i is decreased, but 

 "■plant usually increases also, and * P i an t decreases 

 according^. This maintains the water potential 

 gradient from soil to plant. However, the in- 

 creased root resistance counteracts this tendency 

 to absorb water and, when transpiration is high, 

 this results in an imbalance between transpira- 

 tion and absorption rate, and water stress results. 

 This causes reduced leaf expansion which, in 

 turn, results in less photosynthetic surface, and 

 these two factors both contribute to reduced 

 growth of the plant. Thus, under certain environ- 

 mental conditions, the osmotically adjusted plant 

 can suffer from water stress similar to that suf- 

 fered by a plant in drying soil. This, in effect, 

 might be considered a form of physiological 

 drought. 



Vegetative Growth In Osmotically Adjusted 

 Plants 



Bean plants (Phaseolm vulgaris L.), treated 

 and allowed to adjust osmotically as before (44) , 



