WATER-STRESS BEHAVIOR OF PIMA COTTON. 



11 



Table IV shows a striking contrast in the quantity of water used 

 by the plants on plats 1 and 3 during May and June. The table also 

 shows that during the month of June more than twice as much 

 water was lost from plat 1 as from plat 3. The difference is all the 

 more striking in view of the fact that the direct evaporation from 

 the soil surface must have been less on plat 1 because of the shade 

 afforded by the large plants (PI. I). On July 8 the average height 

 of plants in plat 1 was 7.5 inches greater than the average height of 

 the plants in plat 3. Table IV further shows that, when calculated 

 to an acre basis, plat 1 produced 1,413 pounds more of dry matter 

 than plat 3. The water requirement (pounds of water expended 

 per pound of dry matter produced) is shown to be about 5 per cent 

 greater for plat 1 than for plat 3. 



Table V. — Comparison of the height of Pima cotton plants and the percentage 

 of shedding on plats 1 to 4 in 1919. 









Total 

 bolls 







Height, 



shed 





Plat. 



Sept. 1 



prior 







(inches). 



to 



Oct. 1 



(percent). 



No. 1 



63 

 55 

 54 

 50 



26.6 



No. 2 .- 



21.5 



No. 3 



16.7 



No. 4 



17.8 







WATER RELATIONS AND THE SHEDDING OF IMMATURE BOLLS. 



From Table V it will be seen that the greatest percentage of shed- 

 ding occurred on the largest plants growing in soil which constantly 

 contained the greatest quantity of available moisture. Such an 

 occurrence would seem to be at variance with that observed by many 

 investigators who have studied the shedding of the Upland cotton 

 plant. 



Barre (o. p. 23-24) states that "lack of sufficient soil moisture 

 is the principal factor in determining shedding." Ewing (12) con- 

 cludes " that a deficient supply of soil moisture will result in exces- 

 sive shedding." Balls (4) in reporting shedding studies with Egyp- 

 tian cotton states that " shedding becomes abundant toward the end 

 of the interval between irrigations and decreases directly after water- 

 ing, but finally becomes excessive again when the water level is raised 

 and the lower soil becomes saturated." Evidence, however, is not 

 lacking that the water content of the plant itself may be an important 

 factor in furnishing the stimulus f&r the shedding of squares and 

 young bolls, and to the writer it appears significant that most of 

 the investigators have proposed this factor as bearing an important 

 relation to abscission. Harland (14) credits No well with the state- 



