Grafts et al. — 152 — Water in Plants 



life under conditions of low turgor, and 3) that totally unavailable to plants 

 (FuRR and Reeve, 1945). The existence of a definite wilting coefficient 

 or initial permanent wilting point has been contested since Briggs and 

 Shantz (1912) first proposed the term. Furr and Reeve (1945) define 

 the first permanent wilting point of the sunflower plant as permanent wilting 

 of the basal leaves, a moisture status that practically marks the end of 

 vegetative growth. For all useful purposes, this designates the point at 

 which the soil must be irrigated if growth of a crop is to continue. 



They designate ultimate wilting as the soil moisture content at which 

 apical leaves wilt permanently. This stage is usually accompanied by irre- 

 versible wilting and, finally, death of the lower leaves. Between initial and 

 ultimate permanent wilting lies a range of soil moisture contents for the 

 study of which Furr and Reeve describe a standardized procedure. For 

 light and medium textured soils this range is narrow and though it repre- 

 sents a significant shift in the DPD of soil moisture, the actual amount of 

 water involved is relatively small. For heavy soils, because of lack of uni- 

 formity of soil moisture around absorbing roots, slow moisture movement, 

 and strong adsorptive forces, this range may be significantly broad. Further- 

 more, cases are on record where crop yield has been reduced before any 

 appreciable evidences of wilting could be detected, as will be mentioned later. 



At field capacity (maximum moisture content in equilibrium with the force 

 of gravity) the DPD of the soil moisture is between 0.1 and 1.0 atmospheres 

 in most soils (usually less than 0.5 atm.). As water is taken from the soil, 

 the total moisture stress mounts hyperbolically (see Figure 9, Chapter III). 

 Because of the rapid change of stress with decreasing water content, it has 

 proved difficult to reach an agreement as to the significance of plant re- 

 sponses in the field, especially in the range between 5 and 10 atmospheres. 

 Study of soil moisture in this range is difficult because of the rapid increase 

 in DPD with small decreases in moisture content ; also because it is im- 

 possible to maintain a constant water content within this range with plants 

 growing in the soil ; estimates of the absorbing power of plants at permanent 

 wilting range from 4 atmospheres to 20 and higher. 



Evidence regarding the availability of soil moisture for plant growth is 

 conflicting. Apparently from the standpoint of crop production, in light and 

 medium textured soils, moisture is equally available to plants throughout the 

 range from field capacity to permanent wilting (Veihmeyer, 1927; Con- 

 rad and Veihmeyer, 1929; Hendrickson and Veihmeyer, 1942; and 

 Veihmeyer, Edlefsen, and Hendrickson, 1943). Much recent evidence 

 has been advanced, however, to indicate that deep-rooted plants on heavy 

 textured soils may show a marked response to water deficit before the soil 

 mass has reached the permanent wilting percentage (PWP) (Furr and 

 Degman, 1932 ; Furr and Taylor, 1929 ; Aldrich, Lewis, Work, Ryall, 

 and Reimer, 1940; Schneider and Childers, 1941; Davis, 1940; and 

 Wadleigh, Gauch, and Davies, 1943). 



Certain apparent differences in plant response to water deficits are to be 

 expected ; they probably result from inaccuracies in the interpretation of 

 moisture determinations. The fact that, in light soils, water movement and 

 root growth are rapid results in a fairly uniform depletion of moisture 

 throughout the soil mass occupied by roots, and the bulk of the soil reaches 

 the permanent wilting percentage at about the same time. In heavy soils 

 where root growth and moisture movement are slow, the upper zone where 

 the bulk of the roots occurs may become depleted (to or below the PWP) 

 before the lower horizon reaches this point. As a result, most of the absorb- 



