Chapter IX — 153 — Uptake and Movement 



ing roots may occupy soil at or near the wilting range, whereas the deeper 

 ones may penetrate to depths where there is ample available moisture; the 

 root system may be somewhat restricted ; and crop yield may be reduced 

 before the average moisture content reaches a critical point. There may be 

 no visible evidence of wilting on the part of the plant. Actually, one cannot 

 measure the true PWP of the soil reservoir of a plant unless the whole soil 

 mass is uniformly permeated with roots so that moisture is taken out evenly. 

 Physiological responses have been observed, however, even though wilting 

 was not visible. Wadleigh and Ayers (1945) have reviewed some of this 

 work. 



RosENE (1941) and Hayward and Spurr (1944) have investigated the 

 influence of the DPD of the external medium upon the rate of water entry 

 into roots of water culture plants. Although Veihmeyer and Hendrick- 

 SON (1934) have shown that sunflowers grown in sand cultures did not wilt 

 when placed in sucrose solutions until DPD's of 16 to 20 atmospheres were 

 reached (approximately equivalent to the DPD of the soil at PWP), ab- 

 sorption may be reduced at much lower tensions. The fact that such reduc- 

 tion in water uptake does not affect crop yields until forces from 16 to 20 

 atmospheres are involved indicates that plants normally absorb and transpire 

 more water than they need for carrying on the various functions in 

 which water takes part. The limits of deficit indicated by the results of 

 Veihmeyer and Hendrickson show that plants may endure high stress 

 and still grow normally ; somewhat higher values have even been found 

 occasionally (Richards and Weaver, 1944). 



Though growth of most plants is not markedly hindered at soil mois- 

 ture contents above the PWP, plants vary with respect to root distribu- 

 tion, root hair formation, and other factors that affect their utilization of 

 water. One plant that is commonly known to require frequent irrigation 

 is onion; this is partially caused by limited root growth; Rosene (1941), 

 however, found that DPD's of 4.2 to 5.7 atmospheres represent the limits 

 above which intact roots of onion were unable to absorb water. Isolated 

 roots, dependent upon active absorption could only acquire water from solu- 

 tions having DPD's below 1.8 to 3.3 atmospheres. An OP of 6.5 atmos- 

 pheres limited absorption by intact roots of plants whose leaves were in an 

 atmosphere of 50 per cent relative humidity and at a temperature of 25 ° C. 

 Hayward and Spurr (1944) found that increasing the OP of the culture 

 solution from 0.8 to 4.8 atm. caused an 80 per cent reduction in water ab- 

 sorption by corn roots. An OP of 6.8 atm. stopped absorption completely. 

 The leaves of the plants were in an atmosphere having 70 per cent relative 

 humidity and at 22° C. Tagawa (1934), using beans in culture solutions, 

 found that an OP of 1.94 atm. was sufficient to prevent absorption by ex- 

 cised roots (active absorption) whereas 14.68 atm. were required to pre- 

 vent absorption by whole plants. The relative humidity was 60 per cent 

 and the temperature 28° C. 



In contrast to these findings with culture solution plants, Richards 

 and Weaver (1944), with soils from Southern California, found plants to 

 endure higher stress. At first permanent wilting, values (except for one 

 soil) reached 7.5 to 16 atm., of which 1 to 9.8 atm. represented OP of the 

 soil moisture. Total stress at the ultimate wilting point for 19 out of 24 of 

 these soils ranged from 17.7 to 32.9 atm. (tension 16 to 28.9 atm., OP, 1.8 

 to 7.8 atm.). One soil showed a total moisture stress of 60 atm. at ultimate 

 wilting, a value that seems unaccountably high. 



Unfortunately, the above results can only be regarded as tentative be- 



