1917] Hodgson: Abnormal Water Relations in Citrus Trees 39 



a small colony of about seven hundred acres of orange orchard 

 located eight miles southeast of Bakersfield and surrounded on two 

 sides by typical desert of the southern San Joaquin valley, with its 

 characteristic semixerophytic flora. Extreme climatic conditions, as 

 above mentioned, are operative there but the Washington Navel 

 orange matures early and is of excellent quality, although crops are 

 small because the drop referred to is excessive. 



WATER RELATIONS AND ABSCISSION 



It has long been recognized that abnormalities or irregularities in 

 the water relations of plants are often associated with the abscission 

 of various plant parts. Balls 2 was able to cause complete shedding 

 of leaves, flower buds, and bolls of the cotton plant Gossypium hcr- 

 baceum within four days by pruning the roots and so limiting the 

 ability of the plant to take up water. Lloyd 3 in his investigation of 

 the cause of abscission in the same plant came to the conclusion that 

 the causative factor lay in a steady decrease in the moisture content 

 of the soil in contact with the roots of the plant. This reduction 

 causes a severe tax on the power of the plant to maintain normal 

 water relations and results in fluctuations in the water content of 

 the aerial parts which, in turn, leads to abscission. 



Although the work of Lloyd was performed in the humid southern 

 states, he makes the statement that "there seldom occurs a day on 

 which there is no minus water fluctuation in the plant." He based 

 this conclusion not only on data derived from shedding records but 

 also on a study of transpiration rates, and water deficit in the leaves. 

 In connection with his observations on the effect of temperature in 

 causing acceleration of abscission, he came to the conclusion that ' ' the 

 water deficit is the cause of the rise of temperature in the tissues and 

 that this constitutes the stimulus which directly leads to abscission." 



Other evidence of the occurrence of marked deficits in the water 

 content of plant organs is not lacking. Livingston and Brown, 4 work- 

 ing with a number of plants growing near Tuscon, Arizona, found 

 that (with the exception of the true xerophytes as Covillea and 

 Prosopis) during the afternoon the leaves suffered a marked decrease 

 in water content which was made up during the night. This periodic 



2 Cairo Sci. Jour., vol. 5, p. 221, 1911. 



3 Trans. Eoyal Soc. Can., ser. 3, vol. 10, p. 55, 1916, see also Bull. Torr. Bot. 

 Club, vol. 40, p. 1-26, Jan., 1913. 



4 Bot. Gaz. vol. 53, p. 319, April, 1912. 



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