14 FUNDAMENTALS OF FRUIT PRODUCTION 



uniform conditions of temperature (about 70°F.) and liumidity (about 

 85 per cent.), conditions under which the evaporating power of the air 

 is low. In other words the plants exhausted the water supply of the 

 soil slowly and because of favorable atmospheric conditions were actually 

 able to use the last of the "available" moisture before transpiration 

 demands overtook absorption. In the field, wilting does not usually 

 occur under such favorable atmospheric conditions — favorable from the 

 standpoint of soil moisture supply. 



It has been found that when atmospheric conditions are such as to promote 

 rapid evaporation, "the departure of observed from calculated soil moisture 

 contents at permanent wilting is extremely marked for all soils; permanent 

 wilting in the open occurs with a soil moisture content from 30 to 40 per cent, in 

 excess of that present when the same or similar plants are wilted in a moist 

 chamber. . . . Marked increase in the evaporating power of the air acceler- 

 ates the outgo of water without producing a proportionate increase in its rate of 

 entrance from the soil. With every increase in transpiration rate above a 

 certain Umit, this rate becomes, therefore, more and more significant as a factor 

 determining the extent to which the soil water may be exhausted by the plant 

 before the advent of permanent wilting. Thus, permanent wilting under high 

 rates of evaporation does not at all indicate that the available soil moisture 

 has been exhausted. Instead, it merely indicates the reduction of the soil 

 moisture content to a magnitude which corresponds to the residue of water left 

 in the soil at the time when excess of transpiration over absorption has brought 

 the entire plant into the permanently wilted condition. Repeated determi- 

 nations, under widely varying conditions but with relatively high evaporation 

 rates, show that the magnitude of this residue is directly related to the intensity 

 of the evaporating power of the air."^^ 



It is these higher wilting coefficients under the comparatively high 

 transpiration rates of midsummer which interest the deciduous fruit 

 grower most frequently. Perhaps the wilting coefficient based upon soil 

 texture and calculated for low transpiration rates is most important in 

 determining whether the plant shall or shall not survive the period of 

 drought, for before death occurs there usually will be a shedding of 

 foliage and other protective measures will be taken to reduce moisture 

 requirements and lower the transpiration rate. On the other hand the 

 effects of drought upon the vegetative activities of the tree during the 

 summer, upon the size of its fruit and upon the abscission of its leaves, 

 flowers and partially grown fruit are exercised during periods of very 

 high transpiration rates. This means that correspondingly high wilting 

 coefficients prevail and that the aim of the grower should be, as far as 

 possible, to maintain the moisture supply of the soil well above these 

 higher amounts. 



Wilt 171(1 Coefficients and Drought Resistance. — Tables 2, 11 and 12 

 compiled by Loughridge, showing the minimum water requirements oi 



