82 FUNDAMENTALS OF FRUIT PRODUCTION 



To what extent a very high transpiration may lead to the formation 

 of abscission layers and the dropping of fruit in other varieties and in 

 other species is not known, but presumably the phenomenon is not 

 limited to the Washington Navel orange. On the other hand there is a 

 limited amount of experimental evidence showing that very high atmos- 

 pheric humidity tends to cause the abscission of partly developed 

 apples from the spur.''^ 



Summary. — Evidence from both tillage and irrigation experiments 

 shows increased vegetative growth, as measured by length of new shoots, 

 leaf area and increment in trunk circumference, with increasing moisture 

 supply up to a certain limit (the optimum for growth). The amount of 

 soil moisture available during the short period of rapid growth in early 

 spring is particularly important. When the optimum moisture supply 

 is exceeded the correlation becomes negative. Second growth of mid- 

 summer and the late summer months is generally associated with an 

 irregular moisture supply. An increased moisture supply late in the sea- 

 son results in an increase in size of fruit and in larger yields. Regularity 

 of bearing is encouraged by an adequate and continuous moisture supply. 

 There is a limit, however, to what can be accomplished in this direction 

 through increasing soil moisture. In certain species, as the apple, dry 

 soil conditions tend to promote an oblate form of fruit. There is no 

 very direct relation between moisture supply and fruit color, though 

 good moisture conditions tend to yield fruits with brighter colors than 

 are obtained from soils that are too dry for best growth and development 

 of tree and fruit. The higher colors of fruit from irrigated sections may 

 be attributed to more nearly cloudless skies, in comparison with those of 

 more humid regions. Fruits that develop where the soil moisture is 

 either deficient or in excess are inferior in quality to those developing 

 where soil moisture conditions are more nearly normal. Disease suscepti- 

 bility is often modified materially by the rate of growth, as influenced 

 by soil moisture conditions. The injurious effects of deficient moisture 

 supply may be more evident the season following the drought than during 

 its occurrence, taking the form of decreased vegetative growth and lowered 

 yields. The effects of variations in atmospheric humidity are hardly less 

 pronounced than those in soil moisture supply. Russeting of fruit is 

 common in many species when the humidity is high. Water deficiencies 

 at the time of fruit setting are likely to result in an undue amount of 

 dropping. 



