RESPONSE OF FRUIT PLANTS TO CONDITIONS OF SOIL 81 



of the "June drop" of the Washington Navel orange in CaHfornia indi- 

 cates that a large part of this drop is due to abnormal water relations 

 during that part of the day when transpiration is at its highest. ^^ 



"During the day the fruits (of the Washington Navel orange) decrease in 

 water content as much as 25 to 30 per cent. It has been definitely established 

 that under severe conditions when the atmospheric pull is high the leaves actually 

 draw water back out of the young fruits to maintain themselves. But this supply 

 is not sufficient and they decrease in moisture content also. The combined 

 effect of this tremendous loss from leaves and fruits results in tensions in the 

 water-conducting systems of the tree. These tensions as well as the water deficits 

 have been found to be at their maxima when environmental conditions are most 

 severe, that is, between 10 a.m. and 3 p.m. 



"Meteorological records show that the atmospheric humidity of the interior 

 valleys is quite low during the growing months, relative humidities of 15 per cent, 

 being not uncommon. Such humidities may and do occur without marked 

 increase in air temperature. In other words, it is possible for extremely dry 

 weather to occur without the characteristic hot-norther. 



"Experiments have been performed in the laboratories at Berkeley in which 

 this process of abscission of leaves on cut branches has been induced by artificial 

 means. The process itself has been studied and found to consist in the gelatini- 

 zation and dissolution of the cell walls resulting in complete separation of the 

 cells. . . . 



" The major part of the June drop occurs early in the season and has to do with 

 blossoms and small fruits. It is caused by a stimulus to abscission arising from 

 abnormal water relations within the plant due to peculiar climatic conditions. 



"Further evidence that the cause as indicated is substantially correct lies 

 in the fact that in certain orchards which are provided with efficient windbreaks 

 and interplanted with alfalfa and heavily irrigated, the water deficits in leaves 

 and fruits have been found to be much reduced. Such orchards have less drop 

 and are notable for their comparatively large yields. The Kellogg orchard at 

 Bakersfield is planted to alfalfa and is shielded by a fairly efficient windbreak. 

 Meteorological measurements made in this orchard and on the desert to windward 

 show that the climatic complex is greatly ameliorated. . . . The alfalfa tran- 

 spires at a tremendous rate and literally bathes the trees in a moist atmosphere. 

 The windbreak retards the movement of this relatively moist air away from the 

 vicinity. The vaporization of water from soil and plants tends to lower the 

 temperature of the air. As the soil is largely shaded, the high soil temperatures 

 are reduced, which temperatures operate to cut down root absorption at the time 

 of day when water loss from the leaves is greatest. . . . 



"It thus seems probable that under the prevalent practice of clean cultivation, 

 during the middle of the day when transpiration is greatest the root absorption 

 is actually reduced, resulting in water deficits in all parts of the tree. 



" Not only are clean cultivated orchards subjected to higher soil temperatures, 

 but inasmuch as the root system tends constantly toward the surface layers, it is 

 much reduced by the annual spring plowing which shears off many of the fibrous 

 feeders, thus reducing the root area just before blooming and at the very time 

 the trees are under the greatest strain. "^^ 



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