' ....... u..--.~uon...u.- .V ..-. ..- _ , A. ‘ ~ _ V—_ ‘J ' . .‘ . . -_, ‘ 1‘ . . " _ . . ' ‘ .. . _ .. . . « _‘ . . a . - . ‘ ‘ ‘ . . w . ' . - ' ' ' ‘ . r‘ 7 . ’ > - ‘ ‘_.. . ‘. . ~ r . . . . . ‘ ' _ ‘ . . \ z ‘ ‘ . - . _ _ r: ‘ . ’ . ‘. _ ‘ . > . v _ . v _ ‘ , . v . .|‘ . ‘ ‘ _ , , R . ~ . . 0‘ ' _ u ' ' 0 ‘ .. , . , . - . ‘ ' .' _ _ . . 1 . . ' R ‘ . _. . . PHYSICAL PROPERTI-ES- 0? FRUIT RELATED 1:0 _ - CRACKING * : Thesis f0r_th:e= Degree of Ph. D. ' . - MICHIGAN STATE, UNIVERSITY ~.' . BERNARD ROBERT TENNES _ ' 1973 83".? ! yr. ‘1 ’. '. '4 . v . an“; ' :27" ' '1 L . ’ . £1,237.44 1" .' ’7 hi? . I. . MI ’5’ ,3 "v J a . ml. 2 RRRRRRRRRRR ‘ 00816 9421 ABSTRACT PHYSICAL PROPERTIES OF FRUIT RELATED TO CRACKING BY Bernard Robert Tennes Sweet cherry and tomato growers have experienced problems with fruit cracking under various climatic con- ditions late in the season. Under certain conditions, rupturing of the fruit skin results in the loss of the entire crop. Much research has been done selecting varieties that are crack resistant. However, a few of the more crack susceptible varieties exhibit outstanding fruit quality with high consumer demand. Thus, growers continue to produce these high quality fruit and accept the risk of crOp loss. The objectives of this investigation were the following: (1) to construct a representative model of the fruit related to past and present research; and (2) to contribute to knowledge of the cause and effect relation- ships related to fruit cracking by (a) making field observations to determine orientation and location of fruit cracks, (b) analyzing osmotic behavior of the fruit, Bernard Robert Tennes (c) studying the water potential characteristic of the sweet cherry tree, and (d) making a stress analysis of the skin of the fruits under various conditions. From field observation and soak tests the orienta- tion and location of fruit cracking were determined. The effect of bruising on fruit cracking was analyzed. For sweet cherries, puncture determinations were made at nine locations to determine possible differences in the skin's resistance to puncture. Tomato skins were analyzed for both tensile and puncture testing. A significant linear relationship between the puncture and tensile testing was established using a 1/8 inch cylindrical probe. Determinations of water potential for fruit and leaves of sweet cherry trees were made during a 24 hour period and replicated the following year. Significant variations occurred among the fruit, the leaves, and over the 24-hour period of recording. The relationship between tensile and puncture tests for tomatoes was established. With this prediction, equation values can be established based on the rapid puncture test that will give an indication of the tensile stress in a fruit skin. Based on the experimental values obtained for skin stress and the possible internal pressure that can be present in a fruit, the fruit skin cannot be the major factor that prevents fruit cracking. Localized cell Bernard Robert Tennes dimensional changes at constant cell and skin volume can be as great as 1.38 times the initial dimension. PHYSICAL PROPERTIES OF FRUIT RELATED TO CRACKING BY Bernard Robert Tennes A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Agricultural Engineering 1973 TO ANN ii ACKNOWLEDGMENTS The author wishes to express his sincere thanks and appreciation to the following persons who contributed to this investigation: Dr. Bill A. Stout, my major professor, whose inspira- tion, guidance and advice helped make the completion of my academic and research program possible; Dr. George E. Merva, my major professor, for his guidance and assistance in the water potential analysis; Dr. Donald H. Dewey, Horticulture Department, for serving on the guidance committee and his review and sug- gestions on the statistical and horticultural considerations; Dr. Rolland T. Hinkle, Mechanical Engineering Department, my minor professor, for serving on the guidance committee and advice on academic courses in my minor field; Mr. Jordan H. Levin, AERD, USDA, my project leader, for his advice, suggestions, and help during the investiga- tions and writing of this paper; Dr. Chester J. Mackson, Agricultural Engineering Department, for serving on the guidance committee and his suggestions and advice on my academic program; iii Dr. Larry J. Segerlind, Agricultural Engineering Department, for serving on the guidance committee and for his advice on the stress analysis; Dr. Charles E. Cress, CrOp and Soil Science Department, for his advice on the statistical design and analysis; Mr. Richard J. Wolthuis, AERD, USDA, for his assistance in constructing experimental equipment; My wife, Ann, and sons, Michael and Christopher, who inspired and assisted me throughout this project. iv TABLE OF CONTENTS Page DEDICATION . . . . . . . . . . . . . . . ii ACKNOWLEDGMENTS . . . . . . . . . . . . . iii LIST OF TABLES. . . . . . . . . . . . . . vii LIST OF FIGURES . . . . . . . . . . . . . ix LIST OF SYMBOLS . . . . . . . . . . . . . xii Chapter 1. INTRODUCTION. . . . . . . . . . . . l 1.1 The Problem . . . . . . . . . . 1 1.1.1 Sweet Cherries . . . . . . . 1 1.1.2 Tomatoes . . . . . . . . . 2 1.1.3 Possible Solutions. . . . . . 3 1.2 Objectives. . . . . . . . . . . 4 2. LITERATURE REVIEW . . . . . . . . . . 6 2.1 Sweet Cherry Cracking . . . . . . . 6 2.2 Tomato Cracking . . . . . . . . . 8 2.3 Puncture Studies. . . . . . . . . 12 2.4 Physical Description and Development of Cherries and Tomatoes . . . . . . 14 2.5 Stress Analysis of the Cuticle and Attached Layers of Cells . . . . . 17 2.6 Description of Tomato Cuticle . . . . l9 3. STRESS AND WATER RELATIONSHIPS. . . . . . 22 3.1 Stress Analysis of Thin Shell . . . . 22 3.2 Volumetric Analysis of Cell Element . . 25 3.3 Fruit Model . . . . . . . . . . 29 3.4 Stress as a Function of Fruit Diameter . 30 3.5 Major Components of the Fruit Water Potential Gradients . . . . 34 3.6 Water Potential Gradient Caused by Humidity Fluctuations . . . . . . 40 3.7 Fruit Water Potential . . . . . . 41 3.8 Change in Fruit Diameter Caused by a Four (4) Percent Change in the Soluble Solid Content . . . . . . 50 Chapter 3.8.1 Changes in Diameter for Sweet Cherries . . . . . . . . 3.8.2 Changes in Diameter for Tomatoes . 4 0 EXPERIMENTAL O O O O O O O O C O O O 4.1 4.2 General Field Observations . . . . . 4.1.1 Procedure for Sweet Cherries . . 4.1.2 Discussion of Results. . . . . Osmotic Tests for Sweet Cherries . . . 4.2.1 Procedure for Sweet Cherries . . 4.2.2 Discussion of Results. . . . . Water Potential Tests for Sweet Cherries. 4.3.1 Procedure for Water Potential Determinations . . . . . . 4.3.2 Discussion of Results. . . . . Puncture Tests for Sweet Cherries . . . 4.4.1 Procedure for Puncture Determinations . . . . . . 4.4.2 Discussion of Results. . . . . Puncture and Tensile Tests for Tomatoes . 4.5.1 Procedure Stress Determinations . 4.5.2 Discussion of Results. . . . . 4.5.3 Prediction Equations for Tensil Stress from Puncture Determina- tions on Tomato Skins . . . . 5 0 CONCLUSIONS 0 O O O O O O O O O O O 6. SUGGESTIONS FOR FURTHER STUDY . . . . . . REFERENCES APPENDIX. vi Page 51 52 53 53 53 54 56 56 59 77 77 82 86 86 91 102 102 102 106 108 110 111 116 Table 2.1 LIST OF TABLES Physical properties of tomato skins reported Voisey (1965) . . . . . . . . . . . Location of crack on the fruit surface and the average soluble solid content of the cracked fruit. I O O O O O O O I O O O 0 Average fruit diameter next to foliage sub- jected to 100 percent relative humidity . . Changes of fruit diameter and soluble solid content with time when foliage and fruit were subjected to 100 percent relative humidity. . . . . . . . . . . . . Percent of fruit cracking during a 22-hour soaking period in various solutions. . . . Percent of fruit cracking during a 24-hour soak period in various potassium nitrate solutions for SADH treated Schmidt sweet cherries . . Percent of fruit cracking during a 24-hour soaking period in various potassium nitrate solutions for ethephon treated Schmidt sweet cherries. . . . . . . . . . . Analysis of variance table for soluble solid content of sweet cherries (1970). . . . . Analysis of variance table for soluble solid content of sweet cherries (1971). . . . . Analysis of variance table for diameter of sweet cherries for the 1970 season . . . . Analysis of variance table for the diameter of sweet cherries bruised and soaked in water for 29-hours . . . . . . . . . Analysis of variance table for the diameter of sweet cherries bruised, punctured, and then soaked in water for 29-hours . . . . . . vii Page 13 55 55 56 59 61 61 66 67 69 70 74 Analysis of variance table for water potential (atm) of leaves and fruit . . . . . . Analysis of variance table for water potential (atm) of sweet cherry fruit (1971) . . . table sweet Analysis of variance (atm) of leaves of cherries (1971) . Analysis of variance table for puncture of sweet cherries. . . . . . . . . . Analysis of variance table for puncture force on fruit in two locations (l970)--cheek and opposite cheek. . . . . . . . . . Analysis of variance table for puncture force on cheek of tomatoes (1971) . . . . . Analysis of variance table for puncture force on an 1/8 inch diameter probe for tomato fruit held for 24 hours at the various temperatures . . . . . . . . . . Values obtained for tensile and shear stress calculated by Equation 25 . . . . . . viii for water potential Page 84 88 89 93 97 100 104 106 LIST OF FIGURES Figure Page 2.1 Cross section of sweet cherry fruit (Prunus avium) showing the basic dimensions, structural and cell arrangements. . . . . 15 2.2 Cross section of tomato fruit (Lycopersicon esculentum) from the side showing the basic dimensions, structures, and cell arrange- ments. . . . . . . . . . . . . . 18 3.1 Element on a surface of revolution. . . . . 23 3.2 Diagram of a mature collenchyma cell based on the description by Esau (1965) demonstrating the relatively thick walls and the charac- teristic angular shape . . . . . . . . 26 3.3 Model of collenchyma cell under low turgor pressure. . . . . . . . . . . . . 27 3.4 Model of collenchyma cell under high turgor pressure. . . . . . . . . . . . . 28 3.5 Diagram of a mature parenchyma cell by Ray (1963) o o o o o o a o o o o o o 31 3.6 A theoretical model for cherry and tomato fruit vessel . . . . . . . . . . . 32 3.7 Diagram of the relative activity of water (ratio of water potential in the system to that of pure free water) along the path of supply from soil to air. . . . . . . . 35 3.8 The relationship of water potential (V) to relative humidity (go) at different temperatures . . . . . . . . . . . 43 3.9 The relationship of soluble solid content to osmotic potential (molecular weight-180) at different temperatures . . . . . . . . 44 ix Figure Page 3.10 The relationship of soluble solid content to osmotic potential (molecular weight-200) at different temperatures . . . . . . . 45 3.11 The relationship of soluble solid content to relative humidity (molecular weight-180) . . 47 4.1 The percent of fruit cracking that occurred after the Napoleon cherries were held for 24 hours in a sugar solution at different temperatures . . . . . . . . . . . 63 4.2 The percent of fruit cracking that occurred after the Schmidt cherries were held for 24 hours in a sugar solution at different temperatures . . . . . . . . . . . 64 4.3 The relationship between time of day and soluble solid content of sweet cherries at various ethephon treatments during the 1970 season. . . . . . . . . . . . 65 4.4 The effect of time on sweet cherry size under the influence of ethephon for the 1970 season 68 4.5 The relationship between bruise levels and fruit diameter of Napoleon cherries when soaked in water for various lengths of time . 72 4.6 The relationship between bruise levels and fruit diameter of Schmidt cherries when soaked in water for various lengths of time . 73 4.7 The relationship among bruise levels and fruit diameter when Napoleon cherries are punctured and soaked in water for various lengths of time . . . . . . . . . . 75 4.8 The relationship between fruise levels and fruit diameter when Schmidt cherries are punctured and soaked in water for various lengths of time . . . . . . . . . . 76 4.9 Schematic of PMS Instrument Company's pressure chamber used in water potential determina- tions of plants . . . . . . . . . . 78 4.10 Pressure chamber calibration determinations made for various turn settings on pressure load rate needle valve for the PMS Instru- ment Company's chamber . . . . . . . . 79 X The effects of rates of loading pressure chamber on water potential determinations of sweet cherry fruits and leaves . . . . The relationships among temperature, humidity, water potential (fruit and leaves), and soluble solid content for sweet cherries as a function of time throughout a 24 hour period taken during the 1970 season. . . . The relationships among temperature, humidity, water potential (fruit and leaves), and soluble solid content for sweet cherries as a function of time throughout a 24 hour period taken during the 1971 season. . . . Mean value for puncture force obtained from a 0.125 inch diameter probe after the Schmidt cherries were held for 24-hours in sugar solutions at different temperatures. . . . Mean value for puncture force obtained from a 0.125 inch diameter probe after the Napoleon cherries were held for 24 hours in a sugar solution at different temperatures The effect of time on puncture force for sweet cherries under the influence of ethephon for the 1970 season . . . . . . . . . . The effects of time on puncture force for sweet cherries under the influence of ethephon for the 1971 season . . . . . . xi Page 81 83 87 94 95 99 101 W 3’~ P 01 DP FP Vw ds ds LIST OF SYMBOLS Area Area of circle Cross-sectional area of a shell 4C.V.) P A constant ( Mole fraction of solvent Diameter of a collenchyma cell under high turgor pressure Diameter of probe Puncture force Molecular weight Number of moles Bursting pressure for the tomato fruit Gas constant Specific gravity Skin volume of a fruit Temperature in degrees Kelvin Molar volume of water Dimension of element in the meridional direction Dimension of an element in the direction of the parallel circle Partial vapor pressure of water at temperature T Saturated vapor pressure of water at temperature T xii Relative humidity Subscript for the final value Subscript for the initial value Natural logarithm Mass of material Radius of curvature of a section Meridional radius of curvature Radius of curvature of the section perpendicular to the meridian Subscript for solute Soluble solid content, percent Subscript for solvent Statistical probability of a type 1 error Side dimensions of a collenchyma cell under turgor pressure Angstroms 3.14159 Water potential Tensile stress of fruit skin Shear stress of fruit skin xiii INTRODUCTION 1.1 The Problem 1.1.1 SweetCherries Sweet cherry production for 1970 in the United States was about 121,650 tons (Crop Reporting Board, 1971), valued at about $43.55 million. California produced 25,400 tons, Oregon 40,000 tons, Washington 25,800 tons, and Michigan 21,000 tons (17.3 percent of the national crop). During some seasons, cracking or splitting of sweet cherries occurs and can cause up to 90 percent crop loss. The estimated annual loss from cracking is $1.3 million nationally and about $252 thousand in Michigan. This problem has been investigated by many researchers and to date no practical and reliably effective solution has been found. In 1970 eighty growers located in three areas of Michigan were surveyed to define this problem further. These growers estimated that every year 0 to 10 percent of their sweet cherries crack. They reported that cracking occurred after rains which were followed by sunshine, when trees had stayed wet 3 to 4 hours or longer. Cracking occurred from the time cherries started to turn color until 1 maturity. More mature fruit resulted in a much higher percentage of cracking. A heavier fruit set produced a higher incidence of fruit splitting. 1.1.2 Tomatoes Tomato production in the United States was 14.07 thousand tons during 1970 (Crop Reporting Board, 1971). California is first in tomato production, Ohio is second, and Michigan ranks seventh. Tomato production has shifted from the eastern states to the western area primarily because of the longer growing season, higher yields, and predictable supply of water. During some years large losses from cracking occur due to unfavorable climatic conditions that prevail late in the harvest season. This loss has amounted to $18.79 million on a national level and $409.2 thousand in Michigan. Tomatoes crack similarly to cherries. In the past, cracking of tomatoes was not as important economically as it is now. Previously, handpickers simply did not pick cracked fruit. With the utilization of mechanical harvesters, the cracked fruit is picked, mixed with sand and dirt, and tranSported with other tomatoes to the processing plant. Furthermore, the methodology of mechani- cal harvesting tends to enlarge and increase the severity of the crack during the harvest operation. Nationally, the combined losses for cracking of sweet cherries and tomatoes were $20.09 million. The loss to Michigan's growers resulted in $661.2 thousand annually. These losses for sweet cherries and tomatoes represent only the fruit unsuitable at the field level. Loss to the industry because of lower quality and spoilage in transit and storage is difficult to determine. 1.1.3 Possible Solutions Horticulturalists have attempted to produce tomato and sweet cherry varieties that have a high level of crack resistance, but, to date, the problem reigns. Climatic conditions in certain geographic areas can be less conducive to cracking than in other areas. Tomatoes are grown in California where the watering of the commercial field is controlled through irrigation. However, unpredictable rainfalls do occur and result in major crOp losses. In an attempt to alleviate sweet cherry cracking, Michigan fruit growers have utilized such practices as the following: (1) an air blast to remove moisture after rains; (2) SADH [(2,2-dimethylhydrazide) succinic acid] applications; (3) applications of lime, sulphur, or salad oils; (4) reduction of tree vigor by decreasing fertilizer and the application of captan (fungicide); and (5) clean cultivation of the orchard surface instead of a sod. All these methods may or may not alleviate sweet cherry cracking, but each has proved helpful to some degree for a particular grower. As of now, little that is truly significant can be done to alleviate the problem of fruit and vegetable cracking. Partially effective means of controlling fruit rupture include harvest of mature fruit as soon as possible, maintenance of constant, plentiful soil moisture supply, avoidance of heavy irrigation just prior to harvest, a good fertility program, and selection of varieties resistant to cracking. To alleviate losses from cracking of cherries and tomatoes preventative measures are essential. Prevention can be prescribed only when the phenomena of cracking is better understood. Therefore, determination of the cause and effect relationships is basic. 1.2 Objectives The ultimate goal of this study was to enhance knowledge of the cause and effect relationships concerning cracking by studying the physical prOperties of the fruit structure of tomatoes and cherries. The objectives of this investigation were the following: (1) to construct a representative model of the fruit related to past and present research; and (2) to contribute to knowledge of the cause and effect relation- ships related to fruit cracking by (a) making field observations to determine orientation and location of fruit cracks, (b) analyzing osmotic behavior of the fruit, (c) studying the water potential characteristic of the sweet cherry tree, and (d) making a stress analysis of the skin of the fruit under various conditions. LITERATURE REVIEW 2.1 Sweet Cherry Cracking One of the early researchers, Verner (1937), advocated cultural and handling practices to help reduce the heavy losses in cherry fruit cracking. Verner's plan was to systematically harvest the trees in the order in which the fruit ripened. Contemporary fruit pickers refuse to harvest orchards having a light crop during normal years. Thus, spot picking of cherries is not feasible. Furthermore, with the prevalence of mechanical harvesting, selective picking is not economical. Burtner (1942) and Webb (1947) suggested that growers shake the water from the trees immediately follow- ing a rain to reduce the length of time that water sur- rounds the fruit. From the 1970 survey mentioned, some growers use speed Sprayers and helicopters to remove water from the fruit and leaves. There was general consensus that some benefit is obtained. During the 1942 Oregon season, Burtner (1942) reported that an application of borax to the soil at the rate of thirty pounds to the acre reduced cherry cracking to a negligible amount even during a rainy period near harvest time. A similar effect was observed in a sweet cherry tree row adjacent to an alfalfa field that had an 6 application of boron. Boron was believed to give elasticity to the plant cell membranes. Verner and Blodgett (1931) reported that excess soil moisture and faulty irrigation practices have no influence on fruit cracking. Cracking was observed during periods of rain and believed to be caused by direct absorp- tion of the moisture through the skin of the fruit. By using covers to exclude rain from tree branches, they found that cracking was prevented on ripe fruit while severe cracking occurred on the exposed parts of the same tree. Levin, gt_al. (1959) reported that with fruit soaked in water, the higher the soluble solids, the water is absorbed more quickly into the fruit and extensive cracking will occur. The rate of increase in fruit weight was about 1.8 times greater at 85°F than at 40°F. Levin, §E_3£. (1959) stated that when the sun comes out after a heavy rain there is more cracking in the humid environment. The modulus of elasticity was found to increase as the moisture content decreased. At higher moisture contents the longitudinal section has a higher modulus of elasticity than the transverse section. At lower moisture content the reverse relationship is true. Wann (1949) found that three factors contribute to fruit cracking: (l) variety; (2) stage of maturity; and (3) the length of rainy periods and local climatic conditions. Variety differences influence mainly the skin's permeability, skin modulus of elasticity, and soluble solids content at maturity. The water supply can only be controlled in certain arid regions where irrigation is used as the primary moisture supply. In early experiments, Wann (1949) found cracking to occur primarily because of absorption of water through the fruit skin. Thus, tests were designed to reduce the rate of absorption through the skin by control of external moisture. Various calcium solutions were sprayed on sweet cherry foliage by Verner (1938); these reduced remarkably the susceptibility of fruit to cracking. Because of plant residue and foliage burn, calcium sprays have not been used on commercial trials. Oil sprays have been tried, but have not been successful because of the off flavor the oils reportedly give the fruit. Copper sulfate was reported by Verner (1938) to have good success when applied at 0.1 to 0.25 percent in conjunction with the cherry fruit fly spray. COpper was believed to have a toughening effect on the fruit skin. 2.2 Tomato Cracking Tomato cracking usually is in the form of a con- centric and radial cracks in the skin at the stem end of the fruit. These cracks vary in depth and seem to develop regardless of rainfall as reported by Frazier and Bowers (1947). When new cracks develop, they are about 0.2 to 0.5 centimeters in length. As a rule these cracks increase linearly from day to day with the development of new cracks. Frazier and Bowers (1947) found no close agreement between volume increase and cracking indices;l cracking was the highest near the pink stage of maturity of the fruit. They concluded that cracking results from pressure of locular contents on the ovary wall which tends to make cracking of the skin a localized phenomenon. Another type of skin cracking also occurs in fruit that ripens during a period of dry weather. Under such conditions high temperature often causes a sudden splitting of the skin either radially or longitudinally about the fruit. Soil moisture was not found to be in itself a limiting factor. Frazier and Bower (1947) found the main factors that cause tomato cracking to be temperature, wind, humidity, shading, maturity, foliage, size of root system and nutrient availability, particularly calcium. Severity of cracking was found to vary with the year, the soil, the variety, among vines and among fruit on the same vine. Horticulturists have explored different varieties to determine any that are possibly resistant to radial and concentric cracking. Armstrong and Thompson (1967) con- cluded that resistance to cracking is a quantitative l I o o . The linear cracking per fruit was measured in centimeters. 10 characteristic controlled by several to many additive genetic factors. Dominance plays a minor role. They found higher resistance to cracking can be obtained by imposing heavy selection pressure in advanced generations. Thompson (1965) demonstrated that it is possible to select lines with higher levels of crack resistance than found in either of the resistant parents. Carolus, §E_al. (1965) found that cracking, pre- dominately radial, is erratic in its occurrence having no trends consistently associated with tomatoes grown under different ASM (Available Soil Moisture) conditions. Cracking is most pronounced during warm years in the fruit from plants receiving natural rainfall. Carolus, gt_§l. (1965) found that low atmospheric stress or low DPD (Diffusion Pressure Deficit) in the plant promotes vegetative growth and fruit enlargement. As ASM falls, the rate of top growth decreases and the fruit contains increasingly higher concentrations of soluble solids. At low ASM, fruit size and rate of enlargement decrease. Carolus, g£_§l. (1965) found high atmospheric stress is associated with high radiation and temperature which results in a high DPD in the plant. Under these conditions, fruit enlargement is rapid at first, but as the DPD increases rapidly with decreasing ASM, the trend is reversed and water may be withdrawn from the fruit to such 11 an extent that blossom-end-rot develops. Carolus, 23.31' (1965) found that the tomato fruit is a very sensitive indicator of plant water stress. Frazier and Bowers (1947) found that it is not necessary for water to be absorbed through the fruit skin to cause cracking, but the mere decrease in transpiration of water from the plant is sufficient to cause expansion and rupture. He also found that fruit near the base of a fruit cluster cracks in a shorter period of time than fruit located further from the main stem. Frazier also reported that plots heavily irrigated throughout the season have more cracking of fruit than plots left continuously dry. Dry treatment followed by continued heavy irrigation pro- duces significantly more cracked fruit and larger cracking indices than heavy irrigation throughout the growing season. Freeman and Kretchman (1968) reported that as leaf and soil nitrogen increase, cracking also increases. They also found that as the potassium content of the leaves increases, the incidence of cracking decreases. The precise total nitrogen content at which fruit cracking becomes a serious concern was not determined. However, the amount of nitrogen required for maximum yield of fruit would most likely result in considerable fruit cracking. 12 2.3 Puncture Studies Frazier (1934) found that most radial cracks occur in the suture of the fruit. Cracking resistance of the suture by puncture is much less than in the cheek region. He concluded that the tissue beneath the skin must play an important role as well as the skin itself. Microsc0pic examination reveal a fission beneath the crease of the fruit. Hood and Webb (1968) made puncture tests on tomato fruit at different stages of maturity, which they classi- fied as mature green, starbreaker, pink and ripe. With a 1/4 inch diameter probe and a cross head speed of 10 centimeters per minute, they found that as the percent of reflectance increases the puncture force increases linearly. Voisey and MacDonald (1964) found that the major resistance of the puncture test occurred when the probe penetrated the skin. They used a puncture probe of 0.062 inches in diameter and a head speed of 0.0455 inches/second for their evaluations. Puncture force was about 1.00 pound plus the weight of the fruit. Johannessen (1949) found that the stem end has less resistance to puncture than the middle, and the blossom end resistance is greater than the middle. Fruit from different plants and fruits from the same plant were found to vary greatly in their resistance to puncture. 13 Most all the puncture tests were made to determine if there is a difference in resistance to fruit cracking and the magnitude of puncture value. Some attempt was made to determine the strength of the skin. Voisey (1965) indicated that the skin strength at blossom end is the strongest and the weakest at the stem end creases. TABLE 2.l.--Physical properties of tomato skin reported by Voisey (1965). Bursting Pressure Tomato Fruit Cracking (p31) Tensile MOdU1PS.°f Classification —— Loading Elast1c1ty Stem End Blossom End (psi) (psi) Resistant 31 44 1750 11,700 Moderately Susceptible 28 30 1770 10,940 Susceptible 23 31 1750 12,050 Very Susceptible 22 32 1515 11,280 It was found that very susceptible varieties have the highest proportion of linear load extension curves, indicating that time dependent mechanical properties are a major factor governing cracking resistance. Creep pr0perties of the skin were found to be one of the main factors in cracking. An analogy was made with two balloons, one of polyethylene film (resistant variety) and the other of thin steel (crack-susceptible variety). As also reported by others, Voisey (1965) found the effects of firmness are negligible since the load drOps to zero 14 upon skin rupture. Creep tests for three hours with a tensile load lowered the ultimate strength, which resulted in failure. 2.4 Physical Description and Development of Cherries and Tomatoes Both sweet cherries and tomatoes have a spherical shape with a complex internal structure. About the spherical shape is a viscoelastic membrane called the skin. Esau (1965) described the cherry as being composed of an exocarp or skin, the fleshy mesocarp, and the stony endocarp (Figure 2.1). The exocarp includes the epidermis and several layers of collenchyma cells. The fleshy mesocarp consists of loosely packed parenchyma cells that increase in size from the periphery toward the interior. In growing, the cells change in shape from ovid (with the largest diameter parallel to the surface of the fruit) to cylindrical (with the longest diameter in the radial direction). Voisey (1965) described skin tensile tests as being a portion of a pressurized diaphram. In his Opinion skin strength is one of the important components in resistance to cracking. According to Tukey (1934), there are three stages of pericarp develOpment. A rapid increase in size follows fertilization (stage I). There is a delayed increase during mid-season in which the stony endocarp enlarges (stage II). A second increase in size occurs from 15 \ STEM CHEEK SIDE EXOCARP (CUTICLE AND COLLENCHYMA I I CELLS) PIT FRUIT MESOCARPIershy) HEIGHT HEIGHT _ RARENCHYMA CELLS I I CROSS SECTIONAL METER FRONT VIEW STONY MESOCARP 1 STEM END MIDDLE (CHEEK) 1....— BLOSSOM END w E—szfia’gm I CROSS-SECTIONAL MINOR SI DE VIEW Figure 2.1.--Cross section diagram of sweet cherry fruit (Prunus avium) showing the basic dimensions, structural and celI arrangements. 16 mid-season until fruit ripening (stage III). The change to the period of delayed growth is abrupt and the duration of this period (stage II) is independent of the rate of growth and the size attained. The increase in size during stage I is primarily because of cell division, whereas the increase during stage III is primarily because of cell enlargement. Nitsch (1953) stated that a fruit consists of cells with walls, protoplasm, and vacuoles, with the protOplasm constituting the bulk of fruit tissue up to anthesis. As cell division ceases and cell enlargement begins, the relative volume of the protoplasmic fraction tends to decrease, while the cell wall and the vacuole gain in importance. As cell enlargement proceeds, individual cells tend to become spherical and they loosen from each other. Thus, intercellular spaces are formed and lined with relatively thick pectin layers. The cell walls con- sist of cellulose, hemicellulose, and pectins. Nitsch (1953) described fruit maturation as follows: When maturation commences, the protopectin content of the fruit decreases and pectin is formed. The continuous phase of the young primary wall consists of protopectin in which cellulose strands form only an open lace pattern. As fruit cells enlarge, the volume of the vacuoles increase steadily, being correlated with a large uptake of water. In addition to water, the vacuoles of fruits contain many other compounds such as tannins and pigments. 17 Esau (1965) stated that tannins frequently accumulate in the epidermis and vascular bundles of fruits. Tomatoes have an exocarp skin consisting of the cuticle and three to four layers of collenchyma cells for support (Figure 2.2). The fleshy mesocarp consists of the cuticle and three or four layers of collenchyma cells for support. The mesocarp is made up of parenchyma cells (storage) and 2 to 18 locules (gelatinous substance and seeds) which make up 15-32 percent of the fruit's weight. The endocarp consists of parenchyma cells with vascular bundles coming from the corky stem and passing down the main part of the fruit and out into the mesocarp. The locules are described as cavities filled with placenta between the ovary walls and cross walls perpendicular to the ovary. 2.5 Stress Analysis of the Cuticle and Attached Layers of Cells Miles, gt_al. (1968) analyzed tomatoes as an engineering material by considering contributions of peel, ovary walls, and placenta to the total strength of the fruit. They used a flat-plate compression test to cause failure of the skin and ovary wall. Comparisons were made between tomato fruit and an elastic sphere. Tomatoes were found to have viscoelastic pr0perties. Their test results indicated failure occurs by skin rupture extending from excessive tensile hoop stresses. PERICARPXSkin) CUTICLE ND COLLENCHYMA CELLS (support calls) PARENCHYMA CELLS (storage Dells) GELATINOUS SUBSTANCE I— LOCULES (2 to l8) L. NMUOR 1 DIAMETER CROSS SECTLONAL SIDE VIEW Figure'2.2.--Cross section.diagram of tomato fruit (Lycopersicon esculentum) from the side showing the basic dimensions, structures, and cell arrangements. 19 The peel was found to be the single most important component of the tomato related to mechanical strength. The peel acts as a membrane surrounding a mass of more easily deformable material. Removing the peel alters the fruit's mechanical properties more than any other changes made to the fruit. Extracting and injecting fluid changes the fruit's mechanical prOperties in a predictable fashion. A plot of the log force (flat plate loading) versus log deformation gives a lepe of l for a natural tomato and 0.95 for an elastic balloon. A tomato behaves similarly to an elastic balloon under flat plate loading. These fruit tend to behave much like a balloon filled with fluid. To analyze this spherical pressure vessel, a description of the skin (shell membrane) and prOperties is necessary. 2.6 Description of Tomato Cuticle Bukovac (l970b) stated that the tomato cuticle is between 6 to 10 microns thick. However, he indicated that many surface imperfections of small holes and cracks can be seen in the skin. In general, the cuticle appears as a thin non- cellular membranous covering with projections extending between the anticlinal walls of epidermal cells. This cutin substance possesses both hydrophilic and hydrophobic properties owing to the presence of polar and non-polar groups. 20 The composition and the nature of the chemical linkage of the cuticular waxes in the curtin matrix is not fully understood. Bukovac (l970b) described the cuticular wax on the tomato cuticle as a very soft or viscous wax. When viewed on an electronmicrograph, smearing of the epicuticular wax can be done by pressing with a glass slide. Bukovac and Norris (1966) reported the half dis- sociation value for tomato fruit cuticle to have a pH of approximately 3.2. Thus, there is a possibility that the surface may be negatively charged.. It was found that 2 and 804-2) penetrate the inorganic nutrient ions (Ca+ isolated tomato fruit cuticle, but to a lesser degree than urea. Yamada, 22.21' (1965) reported that the rate of penetration of urea through tomato cuticle increases with time. Yamada (1962) hypothesized that the enhanced cuticular permeability with prolonged exposure to urea may be caused by extraction of some cuticular constituent resulting in a cuticle of greater permeability. Possingham, §E_al. (1967) discussed the influence of wax levels and wax structure on cuticular transpiration. The waxy components of the cuticle provide the prime barrier to water loss from the plants. Bukovac (l970a) reported that the coefficient of osymmetry2 for several compounds penetrating the tomato 2The moisture flux through the skin from the morphological outer to inner surface divided by the moisture flux from morphological inner to outer surface. 21 fruit cuticle is greater than one favoring penetration toward the inner surface. However, Bukovac (1970a) stated that the data presently available is not suffi- ciently conclusive to distinguish whether cuticles from different plants behave differently as suggested by reported data or if these differences are related to the experimental technique utilized. Hall and Jones (1961) showed that brushing the leaf surface disturbs the epicuticular wax and increases the permeability of the cuticle. Levin, gt_al. (1959) reported that the incidence of sweet cherry cracking is much greater among cherries which have been soaked in water than in unsoaked fruit when the cherries are drOpped a fixed distance on to a hard surface. No evaluation was made on the wax structure on the fruit cuticle. STRESS AND WATER RELATIONSHIPS 3.1 Stress Analysis of Thin Shell To better understand the stress that contibutes to the ultimate failure and cracking of the fruit skin, stress concepts must be evaluated. Many assumptions must be made regarding the homogeneity of the material inside the fruit and the homogeneity of the external membrane (skin surrounding the internal material). A simple model is assumed to make the initial analysis meaningful and work- able. This model consists of a spherical elastic membrane with internal pressure and an opening at the top of the spherical vessel. The membrane (skin) is considered as the main factor responsible for the elasticity of the fruit and impermeable to moisture movement. The internal mass is considered as an incompressible fluid subjected to pressure in excess of atmospheric pressure. For a membrane vessel with internal pressure, Timoshenko (1948) developed the general equations for a surface of revolution, which is subject to a continuous internal pressure of intensity p. If one assumes that the wall thickness is small compared to the radius of curvature and there are no discontinuities in the meridional curves, stress can be calculated with sufficient accuracy by 22 23 neglecting the bending of the wall of the vessel and assuming uniform distribution of the stress throughout the wall thickness. If one defines the terms for the element on the surface of revolution (Figure 3.1) as the following, equations for l the skin stress can be derived. Where; Tensile stress in the meridional direction (meridional stress), Tensile stress along the parallel circle (hoop stress), Uniform thickness of membrane, Dimension of element in meridional direction, Dimension of the element in the direction of the parallel circle, Meridional radius of curvature, Radius of curvature of the section perpendicular to the meridian. - tdszo1 I,/ r2 /l 1,. ~ "tdSO' -2 uUZ / 1 2 ‘ t 4/ ” tdsloz ~ tdszol I Figure 3.1.--Element on a surface of revolution. 24 The total forces acting on the sides of the element are tolds2 and tOzdsl and have a component acting normal to the element equal to; _ 2 l 2 _ tdszoldel — r (l) 1 to ds ds and tdslozde = 2 l 2. (2) 3‘2 The sum of these normal components are in equilibrium with internal normal pressure of the vessel element. Thus, tO ds ds tO ds ds 1 2 2 l 2 _ r1 + r2 - pdsldsz, (3) 1 . (4) or .qup + 51L? u rfit From this development of stress related to wall thickness, radius of curvature, and membrane thickness, the spherical vessel can be treated as a special case where r1 = r2 = r and O1 = 02 = O. Thus, Equation 4 reduces to Equation 5, (5) 0 II :06 .III 25 From Equation 5, the stress in the skin can be calculated by using the values reported by Bukovac (l970b) for the tomato skin as 6D minimum and 100 maximum thickness, the value of p as 30 psi, reported by Voisey (1965) and assuming r = 1.5 inches as an average radius. For minimum thickness (6n = 2.383 x 10-4 inches), :25. 0 2t’ _ 30 (1.5) O‘- _4, 2(2.38 x 10 ) o = 9.4537 x 104, O = 94,537 psi. For maximum thickness (lOu = 3.937 x 10-4 inches), O = 57,150 psi. From this calculation of the skin stress under internal pressure loading, the fruit skin would have to withstand stress up to approximately 47 times greater than the known stress carrying capacity of 1515 psi reported by Voisey (1965). 3.2 Volumetric Analysis of Cell Element Collenchyma cells are angular in shape as observed under a microscope (Figure 3.2). Upon the addition of water to the cells, the cells take on a round or spherical 26 "I; s CYTOPLASM . OPLAST INTERCELLULAR SPACE VACUOLE MIDDLE LAMELLA STARCH GRAIN PRIMARY WALL PLASTID PLASMMLEMMA Figure 3.2.--Diagram of a mature collenchyma cell based on a description by Esau (1965), demonstrating the relatively thick walls and the characteristic angular shape. 27 shape. Assuming the cell is initially a cube and as the cells absorb moisture they change from cubical to spherical shape, the relationship between the initial and final volume of the cell can be determined. This change of volume will most likely occur without an increase in the surface of the cell. For this development the assumption was made that the surface area remains constant during a delta change in volume of the cell. I I | I I l I I )__.__..__.__ Figure 3.3.--Model of collenchyma cell under low turgor pressure. where; volume of cell = 63, (6) surface area of cell = 6 6 . (7) 28 Membrane of Cell (Elastic) Figure 3.4.-—Model of collenchyma cell under high turgor pressure. where; volume of sphere = % D3, (8) surface area of sphere = RDZ. If one equates the surface area of the spherical cell to the same value of surface area of the cubical cell (Equation 9) which will maintain the minimum free energy of the cell system, one obtains Equation 10. where; 6 6 = H D , (9) 29 or D = 1.38 6. (10) D equals the diameter of the sphere that has the same sur- face area as the cube. Therefore, the increase in the dimension of each unit cell can be 1.38 times the dimensions of the cube if the surface area of the cell is not to be increased. The ratio of g was found to equal 1.38. With this ratio in mind, the ratio of the volume of a cube to the volume of a sphere was calculated in Equation 11. where; Volume of sphere _ 6 (11) Volume of cube 53 ' Volume of sphere % (1.38)3 Volume of cube, Volume of sphere = 1.38 Volume of cube. Thus, the change in volume of the cube to the sphere can be represented directly by the relationship D = 1.38 6. 3.3 Fruit Model How do the cells of a fruit behave? Falk, et a1. (1958) measured potato tuber parenchyma and found that Young's modulus was linearly dependent on the turgor pressure of these cells. Thus, they assumed these cells to be liquid-filled with thin elastic membranes. Nilsson, 30 gt_al. (1958) constructed a model of the parenchyma cells assuming that each was filled with an incompressible fluid contained by a thin elastic membrane (Figure 3.5). The cell membrane is assumed to be impermeable (for short periods of time) to the internal fluid which exerts pressure out on the cell wall. Thus, the model for a cherry and a tomato fruit becomes an arrangement of cubical or spherical shape cells depending on the turgidity of the cells (Figure 3.6). 3.4 Stress as a Function of Fruit Diameter The relationship between stress and fruit diameter for a thin wall vessel concept is given in Equation 12; _ 4 tO P- D , (12) where; P = pressure inside vessel, t = thickness of vessel wall, O = tensile stress in wall, D = vessel diameter. If one transposes and solves for D, one obtains Equation 13; _ 4 tO D - P . (13) If one assumes that the fruit skin maintains a constant cell volume after the fruit has grown to k of its mature size,Equation 14 gives the skin volume; 31 INTERCELLULAR MIDDLE SPACE LAMELLA ’\.\ VACUOI. 7 \J TONOPLAST PLASMALEMMA ’ PRIMARY WALL PLASTID Figure 3.5.--Diagram of a mature parenchyma cell by Ray (1963). 32 ‘57 EM ( Input and output from vessel) A T—CUTICLE _ \—COLLENCHYMA CELLS ________PARENCHYMA CELLS CROSS SECTIONAL SIDE VIEW CUTICLE- WAXY SURFACE, IMPERMEABLE T0 GAS AND LIQUID. BEHAVES LIKE A VISCOELASTIC MEMBRANE PARENCHYMA CELLS - SPHERICAL IN SHAPE WITH AN ELASTIC MEMBRANE FILLED WITH AN INCOMPRESSIBLE FLUID AT A PRESSURE (P2) IN EXCESS OF ATMOSPHERIC PRESSURE. FUNCTION OF CELLS IS STORAGE. COLLENCHYMA CELLS-ANGULAR TO CUBICAL IN SHAPE WITH THICKER PRIMARY WALL FILLED wITH AN INCOMPRESSIBLE FLUID ATA PRESSURE IF.) IN EXCESS OF ATMOSPHERIC PRESSURE. FUNCTION OF CELL Is SUPPORT. Figure 3.6.--A theoretical model for cherry and tomato fruit vessel. 33 2 SV (skin volume) = (H D (t)) = constant. (14) If one substitutes into Equation 13 for the value t from Equation 14, one obtains Equation 15; 4SVO I n DZP D: 3 4sv5 (15’ D = n P ' where 4, SV, H and P collectively are assumed to be a constant "B". Thus, D3 = B O. (16) As can be seen in Equation 16, the stress in a fruit skin increases as the cube of the fruit diameter, assuming that the skin maintains a constant volume during the fruit enlargement, and the internal fruit pressure remains constant. Under these assumptions, there is little hope in growing a fruit variety that will have a skin with stress characteristics that will not fail under a sudden fruit enlargement period. Emphasis should be placed on the selection of fruit varieties that have a highly elastic outer fruit protective surface and skin. Thus, substantial fruit enlargement can occur without excessive stress buildup within the skin area. In addition to high elasticity, the skin should have a good moisture barrier to avoid excessive moisture flow occurring from the outside environ- ment to the internal fruit cell complex. 34 3.5 Major Components of the Fruit Water Potential Gradients The loss of water by plant tissue results in a water potential gradient being established within the plant system. If the resistance is uniform, the rate the water moves is directly proportional to the affinity (difference or gradient of water potential). This water loss is largely a result of transpiration from the plant during the day. The uptake of this lost water may occur through several different mechanisms within the plant and its environment. Taylor (1963) constructed a diagram (Figure 3.7) which shows the relative activity of water (ratio of water potential in the system to that of pure free water). Figure 3.7 illustrates the following: a. The resistance to water flow is less at the root-soil interface. The water potential at the root-soil interface is nearly the same as that within the xylem or leaf tissue. This resistance is accompanied by a moder- ately high energy barrier that results from the interaction of root colloids with the water. b. The resistance to moisture flow across the leaf- air interface is normally much higher that in all other portions of the plant environment. This interface has a high energy barrier caused by the vaporization that occurs. 35 I l i 1 I o 99 I ' I I ‘IMOIST - T TURGID PLAIIT | - a: 0.98 - I J t". I DRY .. N wILTED PLAN‘I’ I 50.97— I I l | A El50.9 I I I - oaJ-lUMlDDAYIg I '3' _I F): . I E I I IE I 5°” ATMOSPHERfi 5 LEAFI STEM] ROOT E I SOIL ()ID.6 - 6; I I 35 __ “ I 22 I I3 I UJO'5 " I LI. I m " 2 :5 I '1 I 20.4 "" I .JI l I o '— d o 3 _ I 92 I I 2 I a: ’ .I 02; I 7 o 2 __HOT DRY DA? 3 I I I I _ ° 5: l“ I I I l I Figure 3.7.--Diagram of the relative activity of water (ratio of water potential in the system to that of pure free water) along the path of supply from soil to air. The potential drop is most rapid where the resistance is highest, at the leaf-air interface. Taylor (1963). 36 c. The resistance to water flow in the xylem is negligible when the stem is attached to the roots and/or leaves. Flow in the xylem invokes the same molecular mechanism as viscous flow or diffusion. There are changes in the plant structure that influence the rate of water loss by controlling the resistance to flow at the point where resistance is highest, the plant-air interface. Changes in temperature will affect the water lost from the plant. One of the components of the water potential within the plant system is due to the soluble solid content. By assuming that the resistance to water flow in the xylem is negligible when the stem is attached to the roots and/or leaves, it is implied that resistance to water flow to and from the attached fruit is also negligible. Under conditions of isothermal equilibrium Kramer (1969) stated the water potential equations to model the free energy status of water in a plant cell as follows; w cell = Us + up + mm, where; w cell = water potential within the fruit cell, Us = osmotic potential of the solutes, Up = internal pressure of the cell, Um = matrix potential of the cell constitutents 37 Kramer (1969) further stated that under most conditions the value of Um is very small compared to values of Us and Up and can be considered negligible. Depending on the water stress of the plant and on climatic conditions, the water potential relationships for a plant system could be as follows; Uairwleaf>wxylem>¢fruit, during period of low water stress or high humidity of the air. Depending on climatic conditions and/or soluble solid content of the fruit, the fluctuations in UP, the (pressure component of the fruit cell water potential will vary since, wfruit = Us + wp' where; II) = Bir- ln N , (17) S {I w w WP = p = internal pressure of the cell N = 1 . (18) w M w (m ) I e €< ll Nw = 38 82.052‘1‘3m3) (atm) gas constant, 0 K (mole) I molecular weight, mass, temperature in degrees Kelvin, molar volume of water = l8(grams)(cm3) Hole) (1) (gram ’ n n +n , mole fraction of solvent, w 5 number of moles (%). Subscripts; S W solute, solvent. (19) If one calculates the value of Us for a fruit having a soluble solid content of 16 percent at 200 centigrade, a value of approximately -26 atmospheres is obtained. Kramer (1969) lists values of leaf water potential for dogwood trees and tomatoes ranging from zero to -40 atmospheres. If one assumes negligible resistance to water flow between the leaf and fruit these values are representative of the fruit cell water potential and one can calculate the value of internal pressure in a fruit cell as; -20 atm, —26 atm, 6 atm. 39 Under normal climatic conditions (high water stress or low humidity of the air), the internal pressure of the fruit or fruit cells would be approximately 6 atm pressure. If one assumes that a period of prolong precipitation (low water stress or high humidity of the air) can result in a water potential value of approximately -8 atmospheres for the plant and the fruit and, for a short period of time, the value for the osmotic component remains constant at —26 atmospheres, then the value of the pressure com- ponent of the water potential must change to maintain equilibrium giving; wfruit = -8 atm, = 18 atm. lI)? Thus during a prolonged period of rain, the internal pressure component of the fruit cell water potential might increase up to 3 times. Therefore, climatic conditions may cause considerable fluctuation in the pressure component of the fruit water potential. Levin, et_al. (1969) reported that values of soluble solid content of the fruit can change from 16 to 18 percent in 4 days. If one assumes the fruit water potential to remain constant at -20 atmospheres for favor- able climatic conditions and further assumes that the weather remains stable producing an accelerated ripening 40 process, the value of the pressure component of the water potential must change to maintain equilibrium giving; wfruit = -20 atm, Us = -29 atm, = 9 atm. WP During periods of rapid growth and increasing soluble solid content of the fruit, the internal pressure component of the fruit cell water potential might increase up to 50 percent in a 4 day period under these assumptions. Such fluctuations in water potentials could result in fruit cell rupture and/or cracking of the fruit skin. 3.6 Water Potential Gradient Caused by_Humidity Fluctuations The water potential changes in the plant canopy with changes in the relative humidity can be calculated. From Kramer (1969), the equation for calculating the water potential in the atmosphere is given as Equation 20: ln 2— , (atmospheres), (20) 0 air *6 ll é'la where; R, T, and MW are given in Equation 17, and, e = partial vapor pressure of water at temperature T, eO = saturated vapor pressure of water at temperature T, §—’= relative humidity. 41 From a plot of negative water potential versus the relative humidity (Figure 3.8), temperature has an effect on the water potential as the humidity drops from 100 to a lower value. At a relative humidity of 97 percent, which can occur during rainfall, the difference in water potential from 10 to 40 degrees centigrade temperature results in about 4.166 atmospheres negative water potential difference. 3.7 Fruit Water Potential Soluble solid content of sweet cherries varies from about 14 to 20 percent, Levin, gt_gl. (1959), which is the range of most interest to Michigan cherry growers. Molecular weight of fruit sugars has been used by Bedford (1972) as having a molecular weight of 180 grams per mole. Percent soluble solid content has commonly been used by the fruit industry and can be calculated by Equation 21; Percent soluble solids = SS = mass Of sugar x 100% . mass of sugar + mass of solvent (21) The osmotic potential is listed as in Equation 17; —— 1n Nw, (atmospheres), (22) If one substitutes the values for molecular weight quoted above into Equation 18 for Mw and MS; 42 50F a) 40— n] a: h] I: Q_ _ U) C) 2 '5 30- E _J S _ I... 2: n] F- C) I1 20 - a: :2 <1 _ .3 T .C V .0 ICE .C . C —°—°— 40 DEG. C " _ —o——o— 50 DEG. c I o .11 I I l l l 0’95 96 97 98 99 I00 RELATIVE HUMIDITY IN PERCENT Figure 3.8.--The rslationship of water potential (V) to relative humidity (3°) at different temperatures. 43 where; TE.— 18 gram/mole = 0 1 ms — l 0 gram/mole ° ' N = l w m8 1 + 0.1(fi—I W From Equation 21, one obtains the relationship between SS and mass of the fruit as; If one solves for mw, one finds; SS 1 - SS ' m ___m (1-ss> w s SS or Saki! If one substitutes the value of m.W into Equation 18, one obtains Equation 23; Nw = 1 . (23) l + 0.1(I—§§§§) N = 1 - SS w l - 0.9 (SS) ° If one plots the water potential versus the soluble solid content for a molecular weight of 180 and 200, the result illustrates that at a high soluble solid content (18), the effects of molecular weight of 180 versus 200 on the osmotic component of the water potential is approxi- mately 2 atmospheres (Figures 3.9 and 3.10). 4O 38 36 34 32 30 28 26 24 22 20 l8 l6 I4 I2 I0 (“I OSMOTIC POTENTIAL IN ATMOSPHERES C) “3 4b 0’ a: 0 44 . O DEG. C -----—-- IO DEC. C __.__ 2O DEG. c I 1 30 DEG.C 1 c 40 DEG.C -°----»- 50 DEG.C // / I l l I l I 2 4 s 5 IO l2 M It IS 2'0 SOLUBLE SOLID CONTENT IN PERCENT Figure 3.9.——The relationship of soluble solid content to osmotic potential (molecular weight 180) at different temperatures. 45 4OI— O DEG. C 33~ ——--- -—— IO DEG. C 36_ 2O DEG. C L ,2 so DEG. C. 34- -° 2 4O DEG. C. —-o——o— 50 DEG.C 0232— N) I“ I0 I“ (N h) a» O! a: C) I V’ I I I III—I OSMOTIC POTENTIAL IN ATMOSPHERE ‘6 5 z a a 8 I I I I I I (D I who I I l l I l L l l 4 6 8 IO l2 I4 l6 l8 20 SOLUBLE SOLID CONTENT IN PERCENT 1 Cf: “3 Figure 3.10.--The relationship of soluble solid content to osmotic potential (molecular weight 200) at different temperatures. 46 The osmotic potential for fruit within the 14 to 20 percent soluble solid content range can go from apprOximately 22 to 34 atmospheres, or a difference of 11.80 atmospheres at a temperature of 300 centigrade and a molecular weight of 180. At a higher sugar content in the fruit, the temperature effect on the osmotic water potential is very significant, i.e., at 20 percent soluble solid content, the water potential difference between 10 to 40 degrees centigrade is about 3.5 atmospheres. If all other parameters are held constant, the change in temperature from night to day can cause a moisture move- ment from the fruit environment to the fruit with a potential gradient of 3.5 atm per distance to an available water source. A change of molecular weight from 180 to 200 grams per mole effectively lowers the lepe of the lines for osmotic potential at the various temperatures (Figures 3.9 and 3.10). As the relative humidity drops from 100 to 96, the water potential of the air rises from O to about 50 atmospheres (Figure 3.8). If one plots the relative humidity equations versus the osmotic potential, one can observe that the osmotic component of the water potential at high soluble solid content (18%) is equivalent to a relative humidity of 97.7 percent (Figure 3.11). Relative humidities of this magnitude or higher occur only when precipitation occurs or when the ambient temperature falls below the dew point. RELATIVE HUMIDITY IN PERCENT (f0) 47 IOO.O 21 In £0 .- 21 In Nw vw ° vW _e__ g N : I—SS 0° W I-O.9(88) 99.5- 99.0- 98.5-- 98.0- 97.5“- J». 1L I L I 1 I l I L I J I 1 I 1 I I_l 1 L 2 4 6 8 IO I2 I4 l6 l8 20 SOLUBLE SOLID CONTENT (88) IN PERCENT content Figure 3.ll.—-The relationship of soluble solid to relative humidity (molecular weight 180). 48 Thus, the possibility of a water potential gradient caused by high humidity that might produce a sufficient moisture gradient to move into the plant-fruit complex from the atmosphere seems remote. If one calculates the volume of water taken on by a fruit if the soluble solid content changes 4 percent, an approximate value for the fruit diameter at the lower soluble solid content can be obtained. Taking data from Watt and Merrill (1963), the percent water content for sweet cherries and tomatoes is 80.4 and 93.5 percent, respectively. Using these percentages for water content, the percent of mass of water taken into the fruit can be obtained. If one takes Equation 21 and substitutes the values of sweet cherry soluble solid content of 18 percent and 80.4 percent water content initially, the decimal fraction of fruit sugars content can be calculated for a unit mass; m S 0'18 = 0.804 + ms ’ mS = 0.1765. One can substitute this value of ms in Equation 21 for a soluble solid content of 0.14 and ms value of 0.1765 units of mass since the fruit sugar content will not be lost in the uptake of moisture by the fruit; 49 l4 _ 0.1765 0' ‘ m + 0.1765 W m = 1.0842. w Thus the increase in mass of water should be the difference between the two values of 0.2802 units of mass, indicating that the water content can increase approxi- mately 34.8 percent for a change of 4 percent in the soluble solid content of a sweet cherry. If one makes the same calculation for a unit mass of tomato fruit initially at a soluble solid content of 6 percent and 93.5 percent water content, the increase in the mass of water for a 4 percent decrease in soluble solid content can be calculated by Equation 21; ms 0'06 = 0.935 + m ' S mS = 0.0596. J If one assumes that the soluble sugar content will remain constant, the new mass of water can be calculated for a 2 percent soluble solid content. The result is; _ 0.0596 0'02 ' m + 0.0596 ' W m = 2.8910. W The percent increase in the mass of water would be the difference between the initial and final values of mw 50 divided by the initial value or 209.1 percent. Therefore, for the soluble solid to decrease from 6 to 2 percent, the mass of water must increase 209.1 percent over the initial amount. As observed in Figure 3.10, the osmotic potential for a change in 4 percent soluble Solid for cherries and tomatoes is approximately 8 atm and 4 atm respectively. The osmotic potential change is much less for tomatoes than cherries. As observed from the calculations of per- cent change in water mass, the cherry changes only 34.8 percent, where the tomato changes 209.1 percent. Thus, it would appear that the tomato can be affected more by osmotic potential than the cherry; this assumes that the water potential of the fruit environment is approximately zero. 3.8 Change in Fruit Diameter Caused by_a 4 Percent Change in the Soluble Solid Content To determine the change in the fruit diameter that resulted in the change in the fruit mass, several assump- tions had to be made. The fruit was assumed to be spherical and any change in the fruit would maintain the spherical relation- ship. A further assumption was made that the fruit diameter increased prOportionally to the required change in the fruit volume that was necessary for the increased fruit mass. 51 3.8.1 Changes in Diameter for Sweet Cherries From the calculation in Section 3.7, the increase in initial and final mass of the fruit can be calculated. Tennes, §E_al. (1969) reported the average Schmidt fresh sweet cherry is equivalent to a sphere having a diameter of 0.883 inches and a specific gravity of 1.094. By further assuming that the fluid is incompressible and the relationship between mass and volume is one to one, then Equation 8 can be written as Equation 24; D3i (24) :«mp 8 m w < m w H} D f ¢| where; m = Mass of fruit, V = Volume of fruit, D = Diameter of fruit, SP = Specific gravity. subscripts; i initial value, f = final value. Values for Equation 24 are; m = 0.804 + 0.177 mf = 1.084 + 0.177 Di = 0.883 inches, SPi = SPf, m. 3. D _£ = __£ (1), mf 03f 52 m m. 1 3 (1.084 + 0.177) 3 D f (0.804 + 0.177) (0°883) ' Df = 0.965 inches. or a 34.8 percent change in the fruit mass of water should cause an 13.2 percent increase in the fruit diameter. 3.8.2 Changes in Diameter for Tomatoes For a 4 percent change in soluble solid content, it was calculated in Section 3.6 that the increase in fruit mass due to water was 209.1 percent. Using Equation 23 with the values obtained in this study for tomatoes; mi = 0.935 + 0.059, mf = 2.891 + 0.059, D. = 2.713 inches. Thus, Equation 23, D = (2.891 + 0.059) f 10.935 + 0.059) U I — 3.920 inches. A 209.1 percent increase in the fruit mass should cause a 44.2 percent increase in the tomato's diameter. EXPERIMENTAL 4.1 General Field Observations 4.1.1 Procedure for Sweet Cherries First, field tests were conducted during the 1968 sweet cherry season to observe conditions which possibly could cause cracking. Observations were made in commercial orchards to orient the crack location with resepct to the fruit tree. These tests were followed by a test to deter- mine the effect on fruit cracking when the fruit was sub- jected to the following three different conditions: (1) submerging the fruit in water while still attached to the tree, (2) subjecting the tree foliage to 100 percent rela- tive humidity, and (3) subjecting both the fruit and foliage to 100 percent relative humidity. These environ- mental conditions were established by placing plastic bags on the fruit trees' limbs to produce the desired treatment effects. In addition to these on-the-tree tests and obser- vations, samples of fruit were handpicked and administered different bruise levels of 0X, 1X, 2X and 3X (X is the number of times the fruit is dropped from a 3-foot height into a hard plastic laboratory tray). One test lot was punctured in addition to being bruised. After the above 53 54 treatments, samples were placed in water at 21.1° centi- grade temperature for different time periods. Changes in diameter of the fruit were recorded for the different soak periods and time when cracking occurred. Soluble solid content of the fruit was recorded at the beginning and end of each test. 4.1.2 Discussion of Results In an attempt to determine the location of the cracked fruit on the tree, observations were made on Schmidt and Napoleon sweet cherry trees in the Grand Traverse, Michigan area. All recorded observations were made on the lower seven feet of the tree canOpy or within reaching height from the ground. The observed trees averaged 4 to 6 percent cracked fruit. In approximately 95 percent of the recorded incidents of cracked fruit, the fruit was in a position with the crack oriented away from the tree canOpy or outward, and on the outer ends of tree branches. A higher percentage (60-70) of the cracks were in conjunction with a wind—whipped or bruised spot. Cracking occurred more frequently (70 percent of observa- tions) on the windward side of the tree. Of the recorded cracked fruit, 82.8 percent of cracks were located at the apex end of the fruit (Table 4.1). Several tests were conducted to determine if a significant change in the fruit diameter could occur 55 TABLE 4.1.--Location of crack on the fruit surface and the average soluble solid content of the cracked fruit. Location of Crack Percent of Total on the Fruit Observations Stem End 0.0% Cheek 15.3% Apex 82.8% Suture 1.9% while the fruit was still attached to the tree. Two of the tests involved having the cherries in water and 100 percent relative humidity. The tree foliage was subjected to similar conditions in an effort to induce cracking. For the test of 100 percent relative humidity on the tree foliage, there was an increase in the fruit diameter over a period of 24 hours (Table 4.2). TABLE 4.2.--Average fruit diameter next to foliage sub- jected to 100 percent relative humidity. Average Fruit Diameter Day (inches) lst 0.740 2nd 0.827 56 Similar results were obtained when both the tree foliage and fruit were subjected to 100 percent relative humidity conditions (Table 4.3). TABLE 4.3.--Changes of fruit diameter and soluble solid content with time when foliage and fruit were subjected to 100 percent relative humidity. Da Average Fruit Average Soluble Solid y Diameter (inches) Content (percent) lst 0.866 17.0 2nd 0.884 18.0 3rd 0.892 18.0 4.2 Osmotic Tests for Sweet Cherries 4.2.1 Procedure for Sweet Cherries A widely used practice by sweet cherry growers is the application of a water solution of sulfur and lime applied with an air blast sprayer in an effort to prevent or reduce cracking. As the fruit matures, a couple of applications are made prior to expected precipitation. Additional applications of 20 pounds of lime per 500 gallons of water are used during and after a rainstorm. These applications of sulfur and lime are made in an attempt to alleviate the cracking of fruit that occurs after rain- storms. Following this idea, spray tests of different concentrations of calcium nitrate, sodium nitrate and 57 potassium nitrate were applied to sweet cherry trees to the various concentrations of each mentioned chemical. Of four nitrate levels (1, 2, 3 and 4 molar solu- tions applied to the tree foliage), it was observed that foliage burning was most severe for the calcium nitrate and sodium nitrate (in some cases, more than 50 percent defoliation). Less damage occurred with potassium nitrate applied at the same levels (less than 10 percent foliage burn). Thus, potassium nitrate was selected and tested. At Fennville, Michigan, eight sweet cherry trees were treated in their entirety with potassium nitrate solution. Of the three application rates used (4, 8 and 12 molar), all treatments produced severe burning of the tree foliage (10 to 80 percent of surface area). However, the same lower rate (4 molar) applied a month earlier in the season to individual limbs did not produce severe - burning. Like other applied chemical sprays, many other climatic factors influence KNO3 behavior after application. However, the defoliation caused by burning reduced the spread of brown-rot in the treated trees. No apparent damage was done to the fruit at the different concentra- tions of these chemicals. Sweet cherries were handpicked and placed in solu- tions of calcium nitrate, potassium nitrate and sodium nitrate for periods of 24 hours. Solutions of each 58 compound were of 0.2, 0.4, 0.6 and 0.8 molar concentra- tions. Water temperature was maintained at approximately 21.1° centigrade for the 24—hour soak period. Additional soak tests using cane sugar were conducted on fruit treated with ethephon [(2 chlorethyl) phosphonic acid]. Each test was repeated three times. Some cherry growers believe that the soluble solid content changes during the night, causing water to enter the cherry fruit early in the morning and swelling the fruit until the skin ruptures. Thus, soluble solid content and diameter of cherries were recorded for a 24-hour period for both the 1970 and 1971 cherry seasons. The chemical abscission compound ethephon was included in this evalu- ation. It was observed that many of the cracks which occurred in sweet cherries after a wind and rain storm had apparently originated from a bruised area on the fruit surface. Thus, 20 cherries for each sample were handpicked with stems attached and bruised by dropping the fruit a distance of 3 feet onto a plastic laboratory tray 0X, 1x, 2x and 3X times (X is the number of times the fruit was drOpped). Each sample of 20 fruit was then placed in water at approximately 21.1° centigrade for 29-hours. The diameters were determined throughout the 29-hour soak period. A second sample of cherries was given the same 59 bruise damage and then the skins were punctured before soaking. 4.2.2 Discussion of Results Various concentrations of potassium nitrate, sodium nitrate and calcium nitrate in a water solution for sort- ing sweet cherries were effective in alleviating cracking. Sweet cherry samples, when placed in water for a 22-hour period, yielded approximately 63 percent cracked fruit. A 0.6 molar solution of potassium nitrate in the storage water reduced the cracking of the fruit to 4 percent (Table 4.4). Thus, a new possibility for field handling of fruit in aqueous solution without serious problems of fruit splitting was found. TABLE 4.4.--Percent of fruit cracking during a 22-hour soaking period in various solutions. Molar Solution of Compound 0.0 0.2 0.4 0.6 0.8 KNO3 63 16 4 4 0 Ca(NO3)2 63 9 l3 l4 0 NaNO3 63 32 21 4 - As seen in Table 4.4, potassium nitrate was more effective in reducing incidence of fruit splitting in a 60 water solution. Of the three tested compounds, potassium nitrate caused the least foliage burn when sprayed on the foliage. For these reasons, potassium nitrate was selected for further evaluations of Spray applications on sweet cherry trees. The incidence of sweet cherry splitting in a water solution was checked for SADH [(2,2-dimethy1hydrazide) succinic acid] treated fruit (Table 4.5). It was found that samples of SADH treated fruit of 1000 and 2000 ppm had higher initial soluble solid content over non-treated fruit taken from the same orchard. This indicated that SADH increased the soluble solid content of Sweet cherries. It would be expected that as the sugar content increases the incidence of fruit Splitting also increases. The inci- dence of splitting went from 72 percent for non-treated fruit to 95 percent for fruit treated with 2000 ppm of SADH (Table 4.5). This increased rate of splitting most likely was caused by the increased soluble solid content rather than the applications of SADH. The same splitting test on fruit treated with a chemical abscission compound (ethephon) resulted in the Opposite effect on fruit Splitting than was found for SADH treated cherries. Both the treated and check samples of Schmidt cherries were taken from the same orchard and soaked in water and different potassium nitrate 61 TABLE 4.5.--Percent of fruit cracking during a 24-hour soak period in various potassium nitrate solutions for SADH- treated Schmidt sweet cherries. Molar Concentration of Solution Treatment SS* 0.0 0.2 0.4 0.6 Non-treated 15.0 72 49 12 5 SADH 1000 ppm 16.5 76 61 40 12 SADH 2000 ppm 18.5 95 75 17 14 *The initial soluble solid content of the samples. solutions (Table 4.6). It was found that the soluble solid content for this sample of 100 fruit was the same for both the treated and non-treated fruit. Therefore, it seemed the sample would behave similarly when subjected to soak- ing tests. However, it was found that ethephon treated fruit had a lower incidence of fruit splitting than the non-treated fruit of the same initial sugar content. TABLE 4.6.--Percent of fruit cracking during a 24-hour soak- ing period in various potassium nitrate solutions for eth- ephon treated Schmidt sweet cherries. Molar Concentration of Solution Treatment SS* 0.0 0.2 0.4 0.6 Non-treated 16.0 65 43 13 16 500 ppm ethephon 16.0 61 35 21 3 *The initial soluble solid content of the samples. 62 A test was conducted to determine the effect of the temperature of a solution in which sweet cherries were held on their percent of cracking. Samples of Napoleon and Schmidt cherries were held at three different temperatures and known sugar (C6H1206) content for a 24-hour soak period. Both varieties of fruit were harvested from the same orchard. The percentage of fruit that cracked decreased as the sugar content of the solution in which the fruit was held increased (Figures 4.1 and 4.2). The Schmidt variety had a lower percentage of fruit cracking than the Napoleon variety when held in the same environment (compare Figure 4.1 with 4.2). The highest percent of cracked fruit occurred at 15.5°C. The lower testing temperature of 4.4°C resulted in fewer fruit cracking. For the Schmidt variety of sweet cherries, the higher soak temperature of 26.6°C did not produce greater fruit cracking than the lower soak temperature of 4.4°C (Figure 4.2). A plot of the soluble solid content of sweet cherries versus the relationship between time of day for various ethephon treatments was highly variable (Figure 4.3). The average value of soluble solid content was higher for non-treated fruit than for either of the two treatments of ethephon (Table 4.7). The lowest average value obtained for soluble solid content was for the highest concentration of ethephon. Therefore, it appeared for the 1970 data that the soluble solid content of the I ‘ II!!!‘ OU§II .l I..I..LIIIM.I. . IQ.\I . V 33‘ 63 .mmusumummsmu ucmummmao um coausHon Human m.cw assoc em How cam: mums mmwuumno comaommz on» Hmumm omuuoooo “may mcwxomuo uflsum mo usmouwm OABII.H.v mucmwm OAm: new? muEmmxo humim m1... 12:3 2. 20....340m 142.5 to kzuomud 0» ON 0. G Ll I c d A J L .IIILflIIII. ”vomayw .IIIIOIIII. “gamma. IIIIOIIII. Gobi? OI—wI “EU; MNF—KUIO .PMUim MIL. I fund-Pu: 7: za-naJDD L3 Uta-(CULEU. . oo. “08:! OBXOVHO JO 1N3083d 64 .mmusumnmmEOu usmnmmmao um cowusHOm Hmmsm m ca muson vm Mom pawn 0H03 mmwnumno upwsnom mcu nmumm UOHHDOOO umnu mcwxomuo ufisum mo usmoumm OABII.N.v musmwm 64w: mam; mmEmmIo kmmgw mi... 10.13 2. ZOEbJOm «303$ no kzmomma ON 0. |I4l| 0.6.8 In! 0.3. .IIolI 0.6.6 . 3m: mam; $555 536 I06 m1... 10.13 2. 20:3...Om mo mmbbdmmmzmk I llflfld OBNOVHO d0 .LNBOUSd 65 .commmm onma man mGMHSU mucmfiummuu conmmnum m90flum> um mwwunmno ummBm mo ucmucoo oflHOm OHQSHOm can amp mo.wEflu cmm3umn manmcowumamu mnaIl.m.v musmwm AmmDOIV m2: zooz .5929: zooz u. m ¢ N. o c w. J _ l A L _ I l 0. O 29...me 2%. com 4 291%.ka sin. 03 a . I K) D s nu fin n a _I xomro o o 3 62 5 AU ql 6.6. m otm HI 0 a I...» t N i Q C . \l I on. mu. m .Imvm_q: nu u 66 TABLE 4.7.--Analysis of variance table for soluble solid content of sweet cherries (1970). Source of Degrees of . . Variance Freedom F StatIStlc Ethephon 2 4.60* Time 6 1.92 Ethephon x time 12 1.28 Remaining error 84 Total 104 *m = 0.05 Mean soluble solid content (percent) = 16.77 i 1.22 percent Mean non-treated = 17.02 percent Mean 250 ppm ethephon = 16.99 percent Mean 500 ppm ethephon = 16.29 percent sweet cherries was slightly reduced by increasing levels of ethephon concentrations. Differences among the treat- ment means were not significant (a = 0.05) by using Tukey's w-procedure for the non—treated and 250 ppm ethephon treatments. However, the mean value of the high level of ethephon treated fruit for the 1971 tests had a higher soluble solid content than did the check sample (Table 4.8). The type of variation that occurs in the soluble solid determinations indicates other factors not determined responded to ethephon. 67 TABLE 4.8.--Analysis of variance table for soluble solid content of sweet cherries (1971). egggggcgf Dggggggmof F Statistic Ethephon l l9.94** Replication 1 0.01 Ethephon x replication 1 1.73 Time 6 2.10 Ethephon x time 6 2.71 Ethephon x time x replication 6 0.72 Remaining error 118 Total 139 **=°= = 0.01 Mean soluble solid content (percent) = 14.96 i 1.32 Mean non-treated = 14.52 Mean 500 ppm ethephon - 15.41 The effect of time on sweet cherry size under the influence of ethephon was plotted for the 1970 season (Figure 4.4). The effect of ethephon on fruit size was highly significant (0.01 level) (Table 4.9). For the 1970 test, time had no effect upon the diameter of the fruit during this 24-hour period. Ethephon treatments caused a reduction of the fruit diameter. This can partially be explained by the fact that, for the 1970 season, soluble SOlid content for the non-treated fruit was greater than 68 .GOmmmm ohma 0:» ROM cocmmnum mo mucosamcw may Honda Dawn huumno ummzm so mafia mo powwww OssII.v.v whamwm \o Amaze—.3 NEE. zooz P1229: zooz a. o w m. o w m. A) _ _ _ fl _ 2.. 0 ka, 0 4. fi 00. . . nlu c o m o o o I¢Q~ 3 I. v 3 Nu . Ion. S M 13. m zoEmEm 266 com 4 a zozmmxpu 2%. 8.6. 0 Imm. I. xomxo o 166.. 69 the treated fruit. With higher soluble solid content of the fruit, the diameter of the fruit would be expected to be higher as reported by Levin, et a1. (1969). TABLE 4.9.--Ana1ysis of variance table for diameter of sweet cherries for the 1970 season. Source of Degrees of . . Variance Freedom F Statistic Ethephon 2 27.45** Time 6 0.48 Ethephon x time 12 1.00 Remaining error 84 Total 104 ** 9: = 0.01 Mean diameter = 0.815 + 0.041 inches Mean non-treated = 0.848 inches Mean 250 ppm ethephon - 0.790 inches Mean 500 ppm ethephon = 0.808 inches Since there was no significant (a = 0.01) differ- ence in fruit diameter between treatments of 250 and 500 ppm of ethephon during the 1970 tests by Tukey's w- procedure, 1971 tests were made with non-treated fruit and 500 ppm of abscission chemical. The fruit diameter variations during a 24-hour period again were found to be highly significant for the chemical abscission material. Many other variables such as the development of an 70 abscission layer or a higher transpiration rate of the fruit may have an effect on water stress during the day. When cherries were bruised and then soaked in water, the change in fruit diameter was highly significant for both the varieties and the soak periods (Table 4.10). TABLE 4.10.--Ana1ysis of variance table for diameter of sweet cherries bruised and soaked in water for 29 hours. Source of Variance Dgggzgszf F Statistic Variety (Napoleon and Schmidt) 1 32.20** Bruise level (0X, 1X, 2X and 3X) 3 1.75 Variety x bruise level 3 0.75 Time 3 9.59** Bruise level x time 9 0.12 Variety x bruise level x time 9 0.06 Remaining error 611 Total 639 Mean cherry diameter (inches) = 0.850 : 0.059 Mean diameter of Napoleon cherries = 0.863 Mean diameter of Schmidt cherries = 0.837 Mean diameter of various soak times ' Tukey's wéprocedure m = 0.01 I 0.05 Initial = 0.832 5 Hours = 0.848 l 17 Hours = 0.862 I l 29 Hours = 0.859 ' *M = 0.01 Tukey's w-procedure-—Va1ues connected bycrcommon line are not significant at the indicated levels. 71 The diameter increase was more rapid for the Schmidt variety during the first five hours of soak than for the Napoleon variety (Figures 4.5 and 4.6). Part of this more rapid swelling of the Schmidt cherry could be attributed to the higher soluble solid content of 17.0 percent for the Schmidts over the lower value of 14.5 per- cent obtained for the Napoleon fruit. A second sample of cherries having punctured skin, in addition to being bruised, before soaking yielded significance for variety, bruise level and soak periods (Table 4.11). Rapid swelling of the Schmidt cherries occurred during the first five hours of soaking (Figure 4.7 and 4.8). Significance obtained for the different bruise levels can be seen readily by the separation of the average values obtained for the fruit diameter for different bruise levels (Figures 4.7 and 4.8). Part of the decrease in the cherry diameter of higher bruise levels with a punctured skin could be attributed to the loss of cherry juice. Even with a rup- tured cherry skin, the change in the fruit diameter was highly significant with soaking periods. 72 .meu no mnumcma mcownm> mom Hmum3 ca coxOOm cmcz mwfluuono condommz mo Hmumsmwo uwsum pom mHm>wH mmwsun cmmsumn mflsmcoaumaou onBII.m.v whamwm manor 2. NEE. mm m. n o . - 00m. 1 .3. 6;. III¢|I um IIIDII x— ..Iol 3 . .533 323mm . 33 Iowa. (SEHONI) 8313 WVIO .Llan wzomqomdz :0 mm. 73 .meu no mnumcma m50flum> How Hmum3 cw poxMOm cog: mmwuumno uoflacom mo umpmfimwo Danny was mHOPOH masons com3umn mwcmsoflumamu maBII.m.v whomwm manor 2. m2; Jm>m4 ommSmm who—210w L mm 5. m o I+I IA A mus.h um 5mm». IIQII. xm .. . x. Inmh Illoll. x0 0 O Q no ID ID Sr Io N “2 09 “2 . (SEIHONI) HELLBWVIO llnad ID ID 0 immwm. 74 TABLE 4.1l.--Ana1ysis of variance table for the diameter of sweet cherries bruised, punctured and then soaked in water for 29 hours. Degrees of Source of Variance F Statistic Freedom Variety (Napoleon and Schmidt) 1 112.47** Bruise level (0X, 1X, 2X and 3X) 3 15.69** Variety x bruise level 3 4.54 Time 3 7.68** Bruise level x time 9 0.06 Variety x bruise level x time 9 0.10 Remaining error 611 Total 639 **= = 0.01 Mean cherry diameter (inches) = 0.844 t 0.064 Mean diameter of Napoleon cherries Bruise level = 0.868 Tukey's w-procedure, s = 0.01 and 0.05 1X = 0.889 0X = 0.881 I 3X = 0.860 2X = 0.840 Mean diameter of Schmidt cherries = 0.820 ox = 0.864 3X = 0.818 1X = 0.813 2X = 0.803 Mean diameter of various soak times Tukey's w-procedure,<= = 0.01, 0.05. Initial (D) = 0.826 I 5 Hours = 0.845 17 Hours = 0.856 | l 29 Hours = 0.848 Tukey's w-procedure--Values connected by a common line were not significant at the indicated levels. 75 .OEflu mo mnumcma moownm> How Hmumz cw poxDOO can omuouocsm Ohm mowuuoco cooaommz cmg3 Houosmfio uwsum can mHm>OH mmasun macaw mwnmsowuwamu O£BII.h.v musmwh ‘ manor 2. wsE. .mw m. m o q _ com. gm 20;. IIT um x. towm.mw IT .5 .m .53.: 3.223 .63 a . .NW ovm W H «II 6.3. 3 v ua .tooo.MW o H Inuhmw_a: . mw , u . some 0 I 1.0mm. DI. . mzousoama «manna cmo3umn mflnmcowumamu mnaII.m.¢ mucmwm manor 2. mi; N. u IIIIOI xm ILQIII xN '0' x— IIAvIIII x0 Jm>m.._ ommSmm ths—Iom .mhwo. Ammo. .mfivm. .mmwo. .nmwm. E. on». (SBI-IONI) UBlBWVIO llan 77 4.3 Water Potential Tests for Sweet Cherries 4.3.1 Procedure for Water Potential Determination A search of literature did not reveal the use of the pressure chamber for fruit trees. To better under- stand the moisture relationship of the sweet cherry fruit and foliage, a PMS Instrument Company pressure chamber (Figure 4.9) was obtained for these determinations. According to the operator's manual, a pressure loading rate of 15 to 20 pounds per square inch per second had been used satisfactorily with Douglas-fir samples. Tests were conducted on sweet cherry tree limbs at constant temperature of 21.1° centigrade. The pressure chamber calibration curves for various turn settings on the pres- sure load rate control valve were found to behave linearly for the different openings of the needle valves (Figure 4.10). The most accurate setting was for 6 turns on the needle valve, giving a pressure chamber load rate of 0.50 seconds per atmosphere. This load rate was much too high to give realistic results from actual fruit placed in the chamber. To determine the proper setting on the pressure chamber rate control valve, sweet cherry limbs were brought into the laboratory and placed with the cut stems in a water container. Determinations were then performed on both leaves and fruit, removing each from the same 78 .mucmflm mo mcowumcasnmuoo Hmwucmuom Hmum3 cw poms Hmoemno whommmum m.>cmmsoo ucmsfiuumcH win no oaumfimaomlI.m.¢ OHsmwm thm>m 920m mmnmmwmd 6.23 2305.2 3.38 5.8 “:33, .3082 m>._<> 35-5% I / ‘ \ \ Ell. m - J Mw II. n- 'IVI "w _nu n {Ill IIIIII m I I \r .H U m . H— < \_ ~33, 53:50 mmmzsio mmammmma - -qu I- t ..... J .323 958%: [\ I . _ K A Um kl I .II _ J _ «3&2: “ so» 32628 if! zmpm 79 300— LEAST SQUARE EQUATIONS FOR LJNES 270I—TURNS I. -T=r -II.SO+5.24A 2. ---T= - 5.67+3.OI A 3. -T= - 3.43+l.73'A 240— 4.. -T'-' - I.07+I.06 A 5. ~T= ~I23+O.13A Q 6v -T'= OI5IA Q? 7.. -T a - 0.47 4 0.39 A ,3 2IO- 3,.—Ts-O.33+O.3IA \ ISO—- 0) C) 8 .56 (D mu m e Z I20-— 993‘ 4 DH g I» I" 90- “ NO '5‘ TURNS 30- 6 TURNS 1/___ a' O - ""”“ l J, l l L 0 IO 20 30 4O 50 so PRESSURE (ATMOSPHERES) Figure 4.10.--Pressure chamber calibration determi- nations made for various turn settings on pressure load rate needle valve for the PMS Instrument Company's chamber. 80 location on the limbs. These determinations involved setting the instruments at l, 2, 4 and 6 turns on the rate control valve. A good deal of variation did occur within each determination (Figure 4.11). The most important factor in the rate control setting on the pres- sure chamber was to obtain repeatable and dependable results for both the fruit and the leaves of the fruit tree. The plot for water potential versus 1 to 4 turns on the rate control pressure chamber resulted in a linear relationship. There was a rapid increase of water poten- tial values obtained from 4 to 6 turns on the pressure load rate needle value for both the leaves and fruit determinations. A setting of 2 turns of the pressure chamber rate control valve was selected to give what was considered the most reliable and repeatable results. Boyer (1967) compared the pressure chamber method of determining plant water stress with more elaborate techniques and found very favorable agreement. Boyer also stated that this is the best and most convenient field method available at that time. The water potential determinations were made using the pressure chamber method previously described. These determinations were started at 12:00 noon and were taken every 4 hours until 12:00 noon the following day. Water potential determinations were taken on ethephon- treated sweet cherry fruit and leaves over a 24-hour 81 E ML I2— » I g I In I ‘3 Elo— I a m I a a C) 2 I I— I EI < -‘ I 3 I F- 2 I NJ I— 6— I o O. (I MI E 4 3 o O O 24 / o o I/ o o LIMBS !/ :3 FRUIT I/ o I 2 3 4 5 6 TURNS ON RATE CONTROL FOR PRESSURE CHAMBER ‘ Figure 4.11.--The effects of rates of loading pressure chamber on water potential determination of sweet cherry fruits and leaves. 82 period for both the 1970 and 1971 cherry seasons. Each sample consisted of five fruit or leaves individually placed in the pressure chamber and the water potential of each Specimen was determined. The relative humidity and ambient temperature were recorded by a hydrothermograph. 4.3.2 Discussion of Results For the 1970 season, the water potential determi- nations were made on both non-treated fruit and ethephon treated fruit of 250 ppm and 500 ppm. Time had a highly Significant effect upon the water potential of the leaves, as well as on the fruit (Figure 4.12). The soluble solid content of the fruit was not affected throughout the 24- hour period of recording. From the analysis of variance, it can be seen that ethephon did not have a significant effect upon the water potential of either the fruit or the leaves (Table 4.12). There was a significant dif- ference (O: = 0.05) between the fruit and the leaves' water potential values as expected. However, an inter- action between ethephon and location was obtained. In other words, the patterns of the values obtained for the water potential for the fruit and the leaves for the different treatments were different. It was found that the water potential for the highest concentration of ethephon was much lower during certain periods of time of day than for the non-treated and lower level of 83 .commom ohma on» mowuso cwxmu common noon v~ m usonmsoncu DEAD mo sowuocsm m an mmwnumno ummzm Mom usmucoo oHHOm manoaom can .AmOPMOH can pagan. Hmaucouom Hmum3 .muwoflson .OHsumummSO» mcosm mmAmQOHumHOH OSBII.~H.¢ whomflm Amy—Dora m: E. zooz «. o .24... z: u. o. . v zooz a. 4444—1dam4_q_.444.4.4.1. .M .7. .0... iv. o cccc I. 'I .0 O. O... 4 o o. N. w. 9.5» m. quaaom \ tact \\ 823.. \ > P2923: .. co o'cu c>¢n cu OIIN Al.- 08 3180108 ONV IWlV) ‘IVIlNBlOd USIVM Q N l l l L I 0 come mom" oo oo. 84 TABLE 4.12.--Analysis of variance table for water potential (atm) of leaves and fruit. Degrees of Source of Variance F Statistic Freedom Ethephon 2 1.98 Location (fruit and leaves) 1 6.09* Ethephon x location 2 4.41* Time 6 74.13** Ethephon x time 12 0.74 Ethephon x location x time 12 0.51 Remaining Error 174 Total 209 Mean water potential for sweet cherry fruits and leaves for different times. Time (hours) Water Potential (atm) 7/15/1970 12:00 Noon 23.12 4:00 PM 20.18 8:00 PM 10.22 12:00 Midnight 8.78 4:00 AM 13.67 7/16/1970 12:00 Noon 29.71 Mean water potential for sweet cherry fruit and leaves for different times listed for Tukey's w-procedure. . Water Potential a Time (hours) (atm) 0.01 and 0.05 12:00 Noon (final) 29.71 12:00 Noon (start) 23.12 I 3:00 AM 21.91 I 4:00 PM 20.18 4:00 AM 13.67 8:00 PM 10.22 i 12:00 Midnight 8.78 **¢ = 0.01 Mean water potential value = 18.23 i 8.55 atm * s = .05 Mean water potential fruit = 17.40 Mean water potential leaves = 19.05 Tukey's w-procedure--Values connected by a common line were not significant at the indicated levels. 85 ethephon treated fruit. Many other variables not measured in these tests will be responsible for these variations. The water potential of the leaves of the trees under the various treatments did not result in similar values. As expected from the theoretical development in Section 3.5, the water potential throughout the 24-hour period was highly significant. There was neither interaction between ethephon and time during the period of the test, nor interactions among ethephon, position, and time of the test. In summary, the most significant effects were the difference between the water potential of the fruit and the leaves, and the different effects of ethephon treat- ment throughout the day on the fruit and the leaves; this verifies the theory in Section 3.5. During the period of the test, the incidence of cracking was approximately 5 to 10 percent within the orchard. Rainfall occurred during the period of the test and is indicated by 100 percent relative humidity on the graph (Figure 4.12). For the 1971 season, the water potential determi- nations were made on non-treated fruit and fruit treated at 500 ppm of ethephon. These determinations were made during a week of relatively stable weather conditions (no rainfall). Under these conditions, the values of water potential obtained for the sweet cherry leaves were con- sistently less during the 24-hour data collection period 86 than the values obtained for the fruit (Figure 4.13). With precipitation occurring during the determination period (1970 season), the values of water potential obtained for fruit and leaves fluctuated, giving higher values for the sweet cherry leaves in the daytime period and lower values during the night time period than the. values obtained for fruit (Figure 4.12). The statistical analysis of the 1971 water poten- tial determinations gave significance (a = 0.01) being obtained for the time of determination for the sweet cherry fruit (Table 4.13). From this analysis, the abscis- sion chemical compound ethephon did not affect the water potential for sweet cherry fruit. The analysis of water potential data for the sweet cherry leaves resulted in a significant (cc = 0.01) effect with time or daily fluctuations (Table 4.14). A less significant (s = 0.05) effect was obtained among sweet cherry trees. Thus, the chemically treated fruit does not vary from the non-treated fruit during a daily cycle. 4.4 Puncture Tests for Sweet Cherries 4.4.1 PrOcedure for Puncture Determinations Field samples were taken from a commercial fruit orchard and placed in a controlled environment chamber at 4.4, 15.5, and 26.6 degrees centigrade temperature and 87 agvg E m hzmhzoo uMmRmESE d 0.40m m...m:..om 52 E31! egg; «Ito Q'IM (3 9.6.4. ONVGNNOWVHNEKHIEHMM 10$ RIB 3190 :;CM ml" 993‘ .cOmmOm Hpma on» moanso cmxmu wowumm Moon «N m pnonmoounu Dawn mo cowuosom n no mOHHHOSO umm3m How ucoucoo owaom mansaom can .Amm>mma can Denna. Hmwusmuom “mums .auwoflssc .musumummfimu mQOEm manmsowumamu OQBII.mH.v whomflm L4 1 L mm mm on m m V. 00. 88 TABLE 4.13.--Analysis of variance table for water potential (atm) of sweet cherry fruit (1971). Degrees of Source of Variance F Statistic Freedom Ethephon 1 0.30 Replication 1 0.78 Ethephon x replication 1 10.63** Time 6 122.99** Ethephon x time 6 1.60 Ethephon x time x replication 6 0.69 Remaining error 118 Total 139 Mean water potential for sweet cherry fruit for different times. Time (hours) Water Potential (atm) 7/12/1971 3:00 PM 29.95 7:00 PM 22.83 11:00 PM 14.83 7/13/1971 3:00 AM 13.40 7:00 AM 16.00 11:00 AM 23.08 3:00 PM 26.20 Mean water potential for sweet cherry fruit for different times listed for Tukey's w-procedure. . Water Potential G Time (hours) (atm) 0.01 and 0.05 3:00 PM (start) 29.95 3:00 PM (final) 26.20 11:00 AM 23.08 7:00 PM 22.82 7:00 AM 16.00 11:00 PM 14.83 ‘ 3:00 AM 13.40 **¢ = 0.01 Mean value of fruit water'potential==20.89-+6.36 Mean value of water potential of check = 21:01 Mean value of water potential of treated = 20.78 Tukey's w-procedure--Va1ues connected by a common line were not significant at the indicated levels. 89 TABLE 4.14.--Analysis of variance table for water potential (atm) of leaves of sweet cherries (1971). Degrees of Source of Variance F Statistic Freedom Ethephon 1 2.09 Replication 1 5.14* Ethephon x replication 1 1.66 Time 6 175.40** Ethephon x time 6 3.58* Ethephon x time x replication 6 0.35 Remaining error 118 Total 139 Mean water potential for sweet cherry leaves for different times. Time (hours) Water Potential (atm) 7/12/1971 3:00 PM 25.70 7:00 PM 13.18 11:00 PM 8.00 7/13/1971 3:00 AM 6.98 7:00 AM 12.60 11:00 AM 22.80 3:00 PM 24.60 Mean water potential for sweet cherry leaves for different times listed for Tukey's w-procedure. . Water Potential a Tlme (hours) (atm) 0.01 , 0.05 3:00 PM (start 25.70 | | 3:00 PM (final) 24.60 | 11:00 AM 22.80 7:00 PM 13.48 I I 7:00 AM 12.60 , 11:00 PM 8.00 I I 3:00 AM 6.98 **<== 0.01 Mean value of leaf water potential== 16.264 : 7.893 * :mmSII.mH.v wusmwm 04m: mam; memmzo .525 wt... 10.13 2. zo.._.:._ow «42.6 ....o hzmommd cm on . o.. oo: AI J . .. o: .uN. on. 95 OS on. .uo_ SWVBO NI 3030! BBOLONfld Ilarll o .68 IIIBIIII 0 n6. { IT 6 .2. - o... 3...... mam; mmammzo :21; 2. 26:38 no mmawqmmasm: - Loo. 96 For ethephon treated fruit, there was no signifi- cant difference between the skin puncture force required for the various locations on the cherries. Similar results occurred for the non-treated fruit. Because of the decrease in incidence of split fruit treated by the ethephon soaking test, one would expect the skin strength to be increased by this treatment. This was not the case. Therefore, the decrease in fruit splitting for ethephon treated cherries must be caused by some mechanism other than soluble solid content and skin strength. For the SADH treated fruit, the overall skin punc- ture force was below the values obtained for the non- treated cherries. The reduction in the average puncture strength for the SADH treated fruit may somewhat explain the increased percent of fruit that Split during soaking. The effect of time on puncture force for ethephon treated sweet cherries varied considerably for some unknown reason throughout the 24-hour period in respect to the ability of fruit to resist puncture (Table 4.16). The position on the fruit in respect to puncture and the cheek on which the puncture force occurred were not significant. Neither was there a significant interaction between the treatment and location of the puncture on the fruit. 97 TABLE 4.16.--Analysis of variance table for puncture force on fruit in two locations (1970)--cheek and opposite cheek. Degrees of Source of Variance F Statistic Freedom Ethephon 2 9.27** Location (cheek) l 0.18 Ethephon x location 2 0.38 Time 6 13.53** Ethephon x time 12 1.33 Ethephon x location x time 12 1.54 Remaining error 174 Total 209 Mean sweet cherry puncture force for different times. Time (hours) Puncture Force (ounce) 7/15/1970 12:00 Noon 4.917 4:00 PM 6.325 8:00 PM 5.808 12:00 Midnight 6.308 4:00 AM 6.325 8:00 AM 6.583 7/16/1970 12:00 Noon 5.825 Mean sweet cherry puncture force for different times listed for Tukey's w-procedure. , Puncture Force “ Time (hours) (ounce) 0,01 , 0.05 8:00 AM 6.583 4:00 AM 6.325 4:00 PM 6.325 12:00 Midnight 6.308 12:00 Noon(fflmfl) .5.825 8:00 PM 5.808 12:00 Noon (start) 4.917 **« = 0.01 Mean puncture force (1/16 inch probe) = 6.01 i 1.02 oz. Mean non-treated = 6.33 oz.* Mean 250 ppm ethephon = 6.00 oz. Mean 500 ppm ethephon = 5.72 oz. *All treatment means were significantly different at the a = 0.01 level as tested by Tukey's w-procedure. Tukey's w-procedure--Values connected by common line were not significant at the indicated levels. 98 Also, the treatment location and time interactions were not significant for this particular test. The mean puncture force value was greater for the non-treated fruit than either level of ethephon (Figure 4.16). The lowest value of puncture force was obtained from the highest level of ethephon treatment. This experiment was repli- cated during another season to determine if seasonal effects influence the fruit resistance to puncture force after being treated with different levels of ethephon. The 1971 test for puncture resistance of the sweet cherry skin resulted in the same order of significance that was obtained the previous season (Table 4.17). Both ethephon treatment and time of day had a significant effect on the puncture resistance of the fruit skin (Figure 4.17). Com- paring the mean values of puncture resistance for the 1970 season, one finds the highest value was obtained for the non-treated fruit and the lowest value for the highest level of treated fruit. The 1971 season results were completely reversed from the previous year's mean puncture force values. Apparently, the effects of puncture values of the sweet cherry skin vary for season, as well as for treatment and time of day. 99 .s0mmmm Ahma may now condocum mo mocOoncH on» Hops: mmwunmno umm3m How mouom wuauossm so mfiwu.wo uommmm DSBII.mH.¢ whomflm .mmnorv NEE. zooz $6.292 zooz m. o v m. a v u. a _ — _ _ _ tow. Ion. zozawEU 2%. com 4 zozamxeu 2.... com a Jo! xomro o Ion. a v I om. o 4 I4 \\& .ION. . \ Ion. .M‘II o 0‘ Ion. Ioou Io.“ (“WP-‘9) 30803 BHOlONfld 100 TABLE 4.17.--Analysis of variance table for puncture force on cheek of tomatoes (1971). Degrees of Source of Variance F Statistic Freedom Ethephon l 4.09* Replication 1 4.09* Ethephon x replication 1 3.27 Time of day 6 5.70** t Ethephon x time 6 0.78 Ethephon x time x replication 6 0.84 { Remaining error 118 Total 139 I Mean sweet cherry puncture force for different times. t Time (hours) Puncture Force (ounce) 7/12/1971 3:00 PM 2.08 7:00 PM 2.42 11:00 PM 2.68 7/13/1971 3:00 AM 2.54 7:00 AM 2.32 11:00 AM 2.42 3:00 PM 2.19 Mean sweet cherry puncture force for different times listed for Tukey's w-procedure. , Puncture Force “ Time (hours) (ounce) 0.01 and 0.05 11:00 AM 2.68 3:00 AM 2.54 7:00 PM 2-42 11:00 AM 2.42 7:00 AM 2-32 3:00 PM (final) 2.19 3:00 PM (start) 2.08 * <== 0.05 Mean puncture force = 2.38 i 0.42 **