INTRODUCTIONWatercore is a disorder of certain apple cultivars that appears as glassiness of fruit flesh late in the growing season. It is usually associated with advanced maturity, and recent tests indicate there may be low calcium levels in watercored fruit. Red Delicious is particularly prone to watercore, especially when grown in certain locations. The intensity of the disorder varies from year to year. Watercore does not develop in fruit after harvest. Where watercored fruit may be particularly sweet and attractive at harvest, such fruit will deteriorate rapidly in storage. It is, therefore, not suitable for long term storage, even in CA, since internal breakdown can result.
DESCRIPTIONApple fruit has a large amount of intercellular air space, (20-35% of total tissue volume). The one watercore symptom that all observers agree upon is that a liquid fills this air space. This liquid reduces the light scattering ability of the tissue, causing it to appear more translucent than normal. Liquid in the air spaces also increases the specific gravity. The exact nature of the liquid filling the intercellular spaces is not known.Watercored tissue is usually associated with vascular bundles of the core line, although other tissues may be affected. In extreme cases this may include the pith adjacent to the core and the cortex (flesh) all the way to the skin. Watercore that appears around the core line of vascular bundles should be distinguished from that which appears around the perimeter of the cortex, because the causal factor(s) may differ. In severe watercore the fluid may fill the seed cavities, stems become sticky and transparent, and a sticky exudate may appear on the surface.Just after harvest, watercored fruit may be at a premium in certain markets. In fact, as one USDA horticulturist stated:"Watercore in itself is a sign of good maturity and an indication that the fruit has more sugar than most apples of the same variety that are free of watercore. It is a wonder to me that evidence of small-to-moderate amounts of watercore has not been used to advantage in merchandising apples. It usually indicates that apples were picked when fully mature and have a high sugar content."
Watercored apples are used extensively in processing. If stored for any length of time, however, anaerobiosis and, subsequently, breakdown become problems. Watercored pears are also acceptable in Japan.Analytical comparisons of affected fruit have shown an elevated water content, decreased reducing sugars, increased anaerobic products, and a higher sorbitol content in Watercored apples.One anatomical study found senescent degradation in the vascular tissue of watercored apples. Vascular elements would occasionally become nonconductive, leading to development of new vascular tissue from adjacent parenchyma to bypass the nonconducting tissue. Whether the senescence was caused by, or resulted from, watercore could not be determined.A characteristic of watercore is that the condition develops only while the fruit is still on the tree.
DETECTIONUnless the Watercored tissue is immediately under the epidermis, it is not visually detectable in intact fruit. Cutting fruit, flotation and light transmission are needed for detection.Flotation. Nonwatercored apples have a specific gravity from 0.699 to 0.850. As watercore develops, the specific gravity begins to approach that of cytoplasm, approximately 1.10. Floating apples in solutions of appropriate density will separate watercored from nonwatercored apples. The drawbacks of this method arise from the fact that smaller apples tend to be more dense, so a different flotation solution is really needed for each size. Temperature fluctuations also change the density of the flotation bath. An alcohol-water solution will produce a slight surface sterilization. Even if only 78% efficient in detecting watercored apples, the method may account for considerable savings in a commercial operation.Light Transmission. Watercored tissue transmits light more readily than normal tissue. Use of light to detect watercore does not damage fruit. This method has proven quite accurate when compared with visual inspection. The accuracy falls off when the temperature of the apple fluctuates, when the affected tissue is near the surface or flecked throughout the cortex, or if internal browning has started to develop. A hand-held, battery-operated watercore detector developed by private industry (Trebor Industries, PO Box 2159, Gaithersburg, MD 20879: telephone (301) 948-7650) can detect the severity of watercore in single fruits. While not suitable for use on a packingline, the instrument points to the possibility of future instrumentation for determining internal quality.Occurrence Geographically, watercore has been recorded in all the important apple growing regions of the world. Several writers have mentioned that watercore occurs more often in the arid parts of the world, especially the western United States and Australia. Watercore is more prevalent at higher elevations, but no attempts have been made to pinpoint the relationship.The sporadic occurrence of watercore has been well documented. With few exceptions, all sources seem to agree that watercore occurs only late in the season in mature apples. Watercore has been the focus of scientific studies since the early 1900's. A text written in 1886 described the disorder and indicated that it had long been known even at that time.
ECONOMIC CONSEQUENCESWatercored apples may be accepted or even preferred in some areas of the world, but in general such fruit are difficult or impossible to store for any length of time. Watercored fruit often must be diverted to the less profitable processing industry.Watercore poses a serious problem for extended marketing seasons--particularly for 'Delicious.' This has placed additional emphasis on the necessity for controlling watercore, as well as storage scald and internal breakdown.A number of more recent cultural practices lessen the impact of watercore on the fruit industry, especially in the Pacific Northwest. Controlled atmosphere storage greatly extends the fresh market season of apples, but requires that apples be harvested at a more immature stage than those used in conventional marketing operations.A number of other disorders and diseases occur following watercore. The following associations have been reported: breakdown, watercore breakdown, 'Delicious' breakdown, 'Jonathan' breakdown, internal breakdown, internal browning, crinkle and mealiness. In addition, watercored apples are reported to be more susceptible to fungal rots.
THEORIES OF CAUSEResearch and observation have identified 18 factors that either cause or correlate with watercore. The disorder is probably due to an interaction of several factors discussed below.
Theories of Cause: PredispositionCultivars show a markedly different incidence of this Genetic disorder. Table 1 lists some common cultivars and their susceptibility.Morphological differences were found between the watercore susceptible 'Delicious' and the resistant 'Golden Delicious.' Susceptible types showed cellular breakdown and proliferation near the vascular bundles while the resistant type did not.
Theories of Cause: Water RegimeMost of the early ideas about watercore Environmental Factors development centered around water relationships. A series of experiments at Wenatchee in the early 1920s demonstrated not only that heavy irrigation did not lead to watercore, but that heavily irrigated plots showed less watercore than lightly irrigated ones. Watercored apples have less sugar on a dry weight basis and have been reported to show diminished cell wall pectins. Fruit grown at higher humidities were shown to have less carbohydrate on a dry weight basis and thinner cell walls.
TemperatureA few writers have associated watercore with low temperatures and frost. It was found that in three different seasons watercore started to appear when the minimum temperatures approached 39°F. Observations associating watercore with low temperatures must take into account that cold temperatures occur late in the season when maturity is well advanced. The watercore observed may be as easily associated with maturity as with cold temperatures. Some observers have seen a relationship between unusual temperatures and watercore. Possibly fruits are more susceptible to temperature extremes at various stages of growth.Low temperatures may affect fruit and watercore development by accelerating leaf senescence. As this occurs, leaf storage sugars (primarily sorbitol) could then be moved to the fruits to initiate watercore symptoms. Max Williams found a 4% decrease in leaf sorbitol content late in the season that correlated fairly well with watercore development. The decrease was slower in a watercore resistant 'Golden Delicious' than in a susceptible 'Delicious' which developed watercore in fruits on branches with yellowing foliage. Low temperatures might also lead to membrane damage in fruit tissues. Once the translocated sorbitol has arrived at the fruit, the leaky membranes may allow it and other liquids to accumulate and induce watercore symptoms.Watercore has also been associated with high temperatures. Fruit tissues also become leakier at high temperatures, which could allow symptoms characteristic of watercore. Exposed fruit tends to advance in maturity more than nonexposed fruit. Elevated temperatures could hasten maturation and ripening or result in loss of calcium, either of which might induce watercore.Observations of watercore prevalence on the side of the fruit exposed to direct sunlight have not been further verified, but even normal apples are known to exhibit a number of differences between the exposed and shaded sides of the fruit. In addition to the obvious differences in the amount of red pigment (anthocyanin) near maturity, larger intercellular volume, fewer cells per gram fresh weight and lower respiration occur on the shaded side. The exposed side exhibits greater sugar concentrations, higher pressure tests, and a thinner cuticle before maturity. On a hot, sunny day, temperature differentials of 12° to 23°F may be set up between the exposed and shaded side of an apple, with the exposed side reaching temperatures of 104°F. If membrane organization is at all sensitive to high temperature, it would not be surprising to find watercore more prevalent on the exposed side of the fruit or on the exposed side of the tree.
Theories of Cause: Mineral NutritionNitrogen. Several scientists indicated that high levels of nitrogen correlate with watercore development. Field experiments have indicated that high but not excessive amounts of nitrogen actually reduce watercore incidence.Boron. Watercore or watercore-like symptoms such as flooded tissues, have appeared as a result of excessive boron treatments to apple trees.Calcium. Watercored apples tested had low calcium content while potassium and magnesium were high. Recent work done at East Malling Research Station, England, indicates that concentrations of calcium were lower in all parts of apples with watercore. Watercored Cox's Orange Pippin apples had a greater proportion of calcium in the outer part of the flesh than in the core region (Table 2).Watercore incidence is reduced both by calcium sprays and by dipping fruits in CaCl solution while still on the tree. Despite adequate calcium nutrition elsewhere in the tree, fruit may show symptoms of calcium deficiency. Soils are rarely limiting in calcium, which is easily taken up by the roots and translocated to growing leaves, flowers, and fruits.While calcium concentration can be fairly high in the shoot or fruiting spur, it is typically half that amount in the stem of the fruit and low within the fruit, thus setting the stage for low calcium disorders.A lack of calcium in apple fruits can affect cell walls, membranes and the functioning of enzymes. Calcium is believed to cross-link cell wall components, a process which affects the rate of softening. Calcium treatments increase cell membrane strength, reducing leakage. Calcium lowers the rate of respiration in apples. Calcium deficient apples could be further advanced in maturity. These factors may set the stage for watercore and other disorders.
Theories of Cause: High Leaf to Fruit RatioWatercore is likely to occur when the leaf to fruit ratio is high. In this case, the fruit receives more photosynthate, allowing it to attain larger size and elevated concentrations of sugars and acids. Young trees coming into bearing characteristically set small crops. A positive relationship exists between watercore and young trees. When a tree sets a light crop, the apples are typically larger in size. This too has been related to watercore. An apple can attain larger size by either laying down more cells than normal or by allowing the usual number of cells to attain larger than normal dimensions. When an apple tree sets a light crop, the resulting large apples tend to have more cells than normal. Large fruit resulting from thinning, however, will have either more cells or larger cells depending on the timing of the thinning and the method used. Hand thinning before anthesis yields fruit with more cells, larger cells, and an increased core diameter, while hand thinning toward the end or after the cell division period results in fruit with oversized cells. Evidence on the role of chemical thinning agents on watercore is contradictory. However, the period during cell division in apple fruits seems to be most critical in determining final calcium concentration. When the leaf to fruit ratio is high, apple fruits have a greater sugar and acid content, whether induced by thinning or naturally produced by light crops. These conditions accelerate the maturation process so heavily implicated in watercore development.
Theories of Cause: Maturation and Ripening"Maturation" and "ripening," when applied to apples are umbrella terms that identify a great many integrated changes in metabolic pathways and anatomical characteristics. A strong relationship exists among watercore development, maturation, and ripening.Ethylene, generally considered to accelerate senescent properties, also increases or accelerates watercore incidence and the watercore-related internal breakdown. Daminozide (Alar) delays maturation and delays or reduces watercore incidence.The following maturation and ripening phenomena may lead to watercore:Cell wall changes. Watercored tissue in apples and pears has a lower pectin content than nonwatercored tissue. Cell wall degrading enzymes increase during the ripening period.Loss of membrane integrity. Mature Watercored tissue leaks at a higher rate than immature nonwatercored tissue. Fruit maturation and ripening are often associated with changes in membrane properties.Rapid starch breakdown. Starch depletion in normal fruit occurs over about a 1-month period. In Watercored fruit, rapid starch breakdown could lead to watercore by causing increased concentration of soluble carbohydrates and a higher osmotic potential within the cell. This would cause water to move into the cell until the pressure forced water into the intercellular space. On the other hand, starch deposition proceeds from the epidermis inward to the core and disappears from the core outwards. In a fully matured apple, starch decreases precisely when and where the watercore is most likely appear.
Theories of Cause: Altered TransportSorbitol is the major translocated carbohydrate in apple trees, comprising 65-80% of the soluble carbohydrate in phloem sap as well as a considerable portion of the xylem sap. Since Watercored apples show an elevated sorbitol content, fruit cells in maturation may lose the ability to take up sorbitol. The carbohydrate may thus accumulate in intercellular space and lead to flooded tissue.
Sorbitol MetabolismWatercored apples are consistently higher in sorbitol content than nonwatercored apples. The capacity of fruit tissue to metabolize sorbitol may be an important factor in the development of watercore, but recent evidence suggests altered transport as a more likely explanation for elevated sorbitol.
STORAGE OF WATERCORED APPLESWatercored Red Delicious generally have good dessert quality at harvest, but are highly subject to internal breakdown in storage. This is to be expected, since watercored fruit of advanced maturity and low calcium have begun the senescing process.Although the appearance of watercore varies from season to season and from location to location, each year some fruit with watercore is harvested for storage. The amount of visible watercore can be reduced in storage. However, this fruit remains weak and degrades rapidly within market channels. Once watercore appears, the fruit will never become prime fruit for long distance markets where less than optimum conditions prevail. For example, watercored Gloster apples contained about 2½% less air space between the cells than sound fruit. When watercored Gloster apples were stored at different temperatures, the watercore symptoms rapidly disappeared at the warmer temperatures and lower relative humidity. Fruit in CA storage, with higher relative humidity, still had watercore symptoms at the end of the experiment several months later.It is possible to reduce visible symptoms of watercore by promoting increased respiration under less than optimum relative humidity. The danger in doing this is that the other internal quality parameters will deteriorate as well.Severely watercored apples will show internal breakdown faster than lightly affected fruits. The presence of watercore indicates overmaturity for storage. Industry trials have shown that Red Delicious with very slight watercore harvested during the prime harvest period can be stored successfully for less than 5 months if held under optimum storage regimes and marketed rapidly. Fruit with moderate watercore should be rapidly cooled and marketed soon after harvest. See Figure 1 for renderings of watercored fruit.
FLESH FIRMNESSApples with watercore show higher firmness readings than those without watercore at harvest. This is because the spaces around the cells are filled with liquid. Consequently. harvest maturity should be determined through the use of a number of maturity indicators.
CONCLUSIONSSeveral phenomena are consistently associated with watercore. Flooded tissues, decreased reducing sugars, and elevated sorbitol concentrations are characteristic, as are increased fruit density and light transmittance. The disorder occurs only in susceptible cultivars, and generally the symptoms appear only when the fruit is attached to the tree. Increased anaerobic products often have been observed, but these are probably secondary effects, due to oxygen stress, a consequence of flooding of the intercellular spaces. Also, breakdown that follows watercore is related to the anaerobic products, and results from primary symptoms of the disorder.The disorder has long been associated with maturation and ripening. The cause of the disorder relates primarily to changes in membrane integrity associated with maturation and ripening. Occurrence of watercore symptoms at other times (e.g., heat-induced) appear to be due to membrane changes. These changes, increased permeability in particular, would account for both the accumulation of fluid in the intercellular spaces and the elevated sorbitol concentrations. Both would result in greater fruit cell leakage or in failure to accumulate translocated materials from the free space. That phloem sap is very high in sorbitol and the intercellular fluid is apparently higher in sorbitol than the cellular contents would argue for failure to accumulate translocate as a primary explanation. Occurrence of the symptoms only when apples are attached to the tree is also consistent with this explanation. Until more is known about phloem unloading in apple fruit it will be difficult to provide an alternative or a more definitive explanation for the elevated sorbitol concentrations.Although an inability to metabolize sorbitol has been associated with watercore, no cause and effect relationship exists.Calcium is involved, either by delaying maturation, or by maintaining membrane integrity. The sporadic occurrence of watercore is difficult to explain if the disorder is simply a matter of increased membrane permeability induced by ripening. It could very well be related to fruit calcium level variations caused by a multitude of cultural and environmental factors. In this respect watercore resembles other calcium-related fruit disorders, such as internal breakdown and bitter pit. Although the specific cultivars afflicted are not always the same, bitter pit especially resembles watercore in that it occurs sporadically in susceptible cultivars.From a crop management point of view, considering watercore as a maturity-related disorder provides the grower with ways to avoid occurrence. Take watercore into account when determine proper harvest date. Watercore, like storage breakdown, decreases with early harvest. But such benefits may come at the expense of decreased fruit color and size, major factors in determining economic return to the grower. When storability is a factor, early harvest may bring on other problems, including increased scald and bitter pit and greater moisture loss. Efforts to improve fruit calcium levels may be useful.
Wayne Loescher, WSU Dept. of Horticulture and Dr. Eugene Kupferman, Postharvest Specialist
WSU Tree Fruit Research and Extension Center
1100 N. Western Ave., Wenatchee, WA 98801
Post Harvest Pomology Newsletter, 3(4): 3-13