UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 1018 
Contribution from the Bureau of Plant Industry 
WM. A. TAYLOR, Chief sZEP^Fu 
Washington, D. C. PROFESSIONAL PAPER March 15, 1922 
WATER-STRESS BEHAVIOR OF PIMA COTTON IN 
ARIZONA. 
By C. J. King, Assistant, Office of Biophysical Investigations. 
Page. 
Scope of the investigations 1 
Location and plan of the experi- 
ments 2 
Adaptation and length of the de- 
velopment period 4 
Watei'-stress behavior 6 
Water relations and the shedding of 
immature bolls 11 
CONTENTS. 
Page. 
Shedding studies under Arizona con- 
ditions. 12 
The period between flowering and 
shedding 15 
Variations in periodicity of develop- 
ment ' 15 
Conclusions 21 
Literature cited 23 
SCOPE OF THE INVESTIGATIONS. 
American Egyptian long-staple cotton, which was first grown com- 
mercially in 1912, is now one of the principal crops of Arizona and 
portions of California. In 1920 approximately 250,000 acres were 
planted to this crop in the States mentioned. The acreage planted 
to the Pima variety of American Egyptian cotton in the Salt River 
Valley far exceeded that of any other crop in 1920. 
An investigation of the practicability of using soil-moisture deter- 
minations as an index of the water requirement for growing Pima 
cotton was begun in 1918 and continued through the year 1919, in the 
belief that definite information would be of value in determining the 
best methods of irrigation, which is a very important feature in the 
production of the crop. The plan in 1919 was to apply water so as 
to keep different amounts of moisture available at various depths 
of soil and to study the behavior of the plants in response to the 
different proportions of soil moisture, in order to learn the quantity 
of water that would give the best development of the crop. This 
bulletin relates to the data obtained in 1919, after the experience of 
the previous year had demonstrated that it was possible to control 
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