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WSU-TFREC/Postharvest Information Network/Predicting Physiological Disorders: Bitter Pit



Predicting Physiological Disorders: Bitter Pit


Introduction

A number of disorders in fruit are known as physiological because they are not the result of damage by microorganisms or insects, nor do they appear as symptoms of physical or chemical injury. Physiological disorders often begin at the cellular level, become visible more slowly, and can be influenced by environmental, horticultural, or biological factors. Such disorders include russetting, scald, lenticel blotch (LB), and bitter pit (BP).

BP was first described over 100 years ago. Although advances have been made in the control of BP and other associated disorders (corking disorders), information regarding the physiological basis of the disorder is lacking. Symptoms are mainly found in the cortical tissue and appear first as soft, brown areas, which eventually become desiccated due to the collapse of surrounding cells, forming a dry cavity or 'pit'. Factors that have been associated with BP are: 1) genetic predisposition, 2) fruit mineral status, 3) fruit size and cropping status, 4) canopy attributes, 5) rootstocks, 6) irrigation and water status, 7) fruit developmental rate, 8) fruit maturity/energy status, 9) storage temperature and humidity, and 10) climate. Wow! There have been a number of reviews on BP describing these factors (Faust and Shear 1968; Perring 1968a, b; Ferguson and Watkins 1989).


Theory

The theory of BP development that seems to account for a number of observations links calcium (Ca) deficiency with vascular tissue. During periods of transpirational stress (e.g., hot weather + low relative humidity + inadequate soil moisture, or desiccating conditions in storage less than 32 °F), it is conceivable that Ca could be withdrawn from sites within fruit vascular tissue with a reversed xylem flow. This withdrawal could result in localized Ca deficiency, with subsequent effects on cellular membranes, cell wall structure, or signal transduction. An inadequate pool of Ca outside the cell, with access to the plasma membrane, could result in a reduced capacity of the cell to undergo normal responses when triggered by stimuli or signals such as phytohormones. Because of the requirement of cell walls for high Ca, any reduction in extracellular Ca concentration (reversed xylem flow) could decrease the concentration of Ca within the cell and, therefore, decrease the ability of the cell to respond in a metabolically normal manner—sick cells. With adequate Ca, Ca-dependent metabolic events do not result in breakdown. However, when the level of Ca drops below a critical threshold, cell breakdown occurs and symptoms become visible.

Beyond this abbreviated hypothesis, the physiology of BP or LB (which I believe are similar symptoms, the initiation of which may occur at different seasonal times and environmental conditions) will not be discussed.


Weather

From one orchard, I generated the number of growing degree hours (GDH) from the average daily air temperature from three growing seasons in which there were contrasting amounts of BP. Figure 1 indicates that in 1995 and 1999, years in which BP was particularly severe, spring temperatures were significantly lower than those in 1998 when BP was practically absent. Other researchers around the world have noted relationships of BP with weather as well.

Figure 1. Growing degree hours from April 1 to July 15 in 1995, 1998, and 1999 extracted from a PAWS weather site near Pasco, Washington.


Experiments

In July 1999 when spring temperatures were examined, it was clear to me this season would be one in which BP would be a serious problem. I selected six Braeburn orchards from Tonasket to Pasco and used a treatment to induce BP in an effort to try and predict occurrence out of storage. Fruit were harvested the day of commercial harvest in each orchard and taken to the lab. At the lab, 300 fruit were immediately dipped for 2 minutes in a solution of ethrel (2000 ppm) and magnesium chloride (MgCl2, 1%), allowed to air dry, and held for 0, 2, and 4 weeks at 68 °F (ripening). The remaining 300 apples were untreated, and placed in regular storage at 33 °F for 8, 16, and 24 weeks. At 0, 2, and 4 weeks, fruit held at 68 °F were removed and the BP was assessed both by examining the fruit surface and cutting the fruit into 6 to 8 transverse slices. At 8, 16, and 24 weeks, fruit held at 33 °F was also removed and assessed for BP in the same manner. In this way, we hoped we could determine whether the fruit treated at harvest could predict the amount of BP seen in untreated fruit after storage. Figure 2 shows how BP develops in Braeburn held at room temperature for a month after treatment with ethrel and MgCl2. Clearly, as fruit ripen, BP becomes manifest. It is likely the BP "initials" were present, and the ripening events rendered them visible.

Figure 2. Development of BP from Braeburn apples held at room temperature for 4 weeks after harvest from an orchard showing symptoms at harvest.


Predictions

Now, let's consider our six orchards and see how the prediction model fared. Table 1 shows for each orchard, the amount of BP induced at 0, 2, and 4 weeks compared with the amount found in fruit after 8, 16, or 24 weeks storage at 33 °F.

Table 1. Mean starch and ethylene values at harvest, and BP development in Braeburn apples from six orchards in Washington state. Attempt was made to correlate BP after 4 weeks at room temperature with that of fruit held for 6 months.

 
Induced
Post-Storage
Orchard
Starch
(1 - 6)
Ethylene
(ppm)
BP (%)
0 wks
BP (%)
2 wks
BP (%)
4 wks
BP (%)
8 wks
BP (%)
16 wks
BP (%)
24 wks
CV
2.6 0.22 5 60 70 10 10 25
CO 2.6 0.15 10 10 20 15 15 20
OB 2.3 0.73 0 10 30 0 0 0
RR 2.5 0.25 5 10 20 15 15 20
SE 2.7 1.24 10 10 10 0 0 0
TW 2.5 0.45 0 35 45 5 5 5

Highlighted are those predictions which, at 4 weeks after the induction treatment, came within 10% of the actual observed value after 24 weeks regular storage. Predictions for the other three orchards were all over-estimated. It is interesting to note, and may or may not be relevant, that fruit harvested with the highest mean ethylene values, (orchards OB and SE) had no BP out of storage, whereas those harvested with the lowest ethylene values (orchards CV, CO, and RR) had the most. The amount of BP is graphed against the value of SXE (starch index) X (ethylene) in Figure 3.

Figure 3. Data showing possible correlation between BP development for Braeburn apples in regular storage and the SXE values (starch index X ethylene) for 1999-2000.

With regard to data generated in this preliminary report, this figure might indicate that if fruit is harvested when the starch conversion is advancing and the ethylene generation is lagging, the potential for BP out of storage is high. When the value of SXE for this particular cultivar (using the parameters stated above) is greater than about 1.0, the potential for BP drops and remains low.


Research Needs

In the most current review of bitter pit, Ferguson and Watkins (1989) suggest four major areas in which research is inadequate: 1) how mineral deficiencies and imbalances develop within the fruit, 2) localization of symptoms in the fruit flesh, 3) biochemistry of Ca (uptake, transport, distribution, and metabolism within the cell) in plant tissue, and 4) possibilities for genetic control. As evidenced by the persistence of BP, uncovering the science behind this disorder is not an easy task. Nonetheless, in light of recent technological developments in biochemistry, cytology, molecular biology, and microscopy, concerted research efforts would likely bear good fruit.


References

Perring, M.A. 1968a. Mineral composition of apples. VII. The relationship between fruit composition and some storage disorders. J. Sci. Food Agr. 19:186-192.

Perring, M.A. 1968b. Mineral composition of apples. VIII. Further investigations between the relationship between composition and disorders of the fruit. J. Sci. Food Agr. 19:640-645.

Faust, M. and C.B. Shear. 1968. Corking disorders of apples: A physiological and biochemical review. Bot. Rev. 34:441-469.

Ferguson, I.B. and C.B. Watkins. 1989. BP in apple fruit. Hort. Rev. 11:289-355.

Dr. Eric A. Curry, Plant Physiologist

USDA, ARS Tree Fruit Research Laboratory
1104 N. Western Ave.
Wenatchee, WA 98801
CURRY@tfrl.ars.gov

16th Annual Postharvest Conference, Yakima, WA
March 14-15,  2000

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