In the application of chemicals to modify or regulate maturity, as with any growth regulator, there are several general considerations that, if understood, will help the grower and the warehouse in optimizing the quality of treated fruit.

1. What is the desired action? Simply stated, is the desired outcome an advancement or a delay in maturity?
   
   
2. What are the properties of the chemical being applied? The chemical properties are important to know because they determine the proper pH of the aqueous solution, and conditions that favor proper action.
   
   
3. What are some of the environmental factors that mitigate the effects of the applied compound?
   
   
4. What is the developmental stage of the fruit at the time the compound is being applied?
   
   
5. Based on the likely effects of the compound, will the cost of application be recovered?

Ethephon

As seen in Figure 1, ethephon is a simple molecule that undergoes a physicochemical (non-biological) reaction at a pH > 3.5 to produce ethylene and phosphoric acid. Generally this means that at pH < 3.5 ethephon is stable. The higher the pH the faster the reaction takes place. This is important for growers who want to maximize efficacy because it indicates use of a neutralizing buffer would be inappropriate. In addition, it is important to note that this is not a biochemical reaction. That is, it will occur slowly over time regardless of whether or not the material penetrates the cell. If it does enter the cell, which is normally buffered at higher pH, ethephon will be reduced to ethylene at a higher rate of reaction.

Figure 1. Reaction of ethephon with water at pH > 3.5.

There is also published research in this field that illustrates several important points (interpreted from Beaudry and Kays 1987):

1. For approximately every increase in temperature of 20 °F, the rate of ethylene evolution increases by a factor of two. For example, at 50 °F, if the rate of ethylene evolution from ethephon is 1 ppm per minute, then at 70 °F, that rate increases by a factor of 2, making the final rate 3 ppm per minute.
   
   
2. The higher the temperature during and following the application, the shorter the time that the material is present. For example, at 50 °F there is sufficient degradation so that only 50% of the original material remains after about 12 hours. At 83 °F, it only takes about three hours for 50% of the material to degrade.

ReTain

At the opposite side of the chemical spectrum is the compound known as ReTain or AVG (aminoethoxyvinylglycine). With a mechanism different from that of ethephon, ReTain functions via a biochemical reaction (Figure 2).

Figure 2. ReTain (shown as AVG) blocks the formation of ACC (precursor to ethylene) in a biochemical reaction.

Therefore, it is necessary for AVG to enter the cell in order to function correctly. Because ReTain functions as an inhibitor, modest changes in temperature are not like to influence its activity. On the other hand, temperature, relative humidity, solar radiation, and wind, are all factors that will affect how long the material will wet the tissue, and therefore affect the absorption. For example, if ReTain is applied four weeks before harvest during high August temperatures together with a mild breeze under conditions of low relative humidity, drying time could probably be measured with a stopwatch. In contrast, if the same material were applied during the cool, windless, early morning hours of mid-September, leaves could remain wet for hours.

Fruit Surfaces

Temperature, rate, and coverage are the three most important considerations in optimum efficacy of foliarly applied chemicals. When considering the target of chemical application of ethephon or ReTain, namely the fruit surface, it must be remembered that the basic function of the cuticle/wax structure is to prevent diffusion of cellular moisture and entry of pathogens. The application of aqueous chemicals to the fruit surface is not a 'user friendly' operation. Consider the micrograph of a mature 'Delicious' apple shown in Figure 3.

Figure 3. Micrograph of a mature 'Delicious' apple peel.

Clearly, this surface is covered with an abundance of wax, which is hygroscopic in nature. Therefore, to increase penetration and eventual contact with the cell, the addition of a surfactant would enhance this process (Figure 4).

Figure 4. Effect of a surfactant on the contact angle between a droplet and an imaginary leaf surface.

Obviously, we cannot remove the wax from the leaf, or in this case, the fruit surface, therefore, the addition of a surfactant seems appropriate for maximizing coverage.

Reference

Beaudry, R.M. and S.J. Kays. 1988. Application of ethylene-releasing compounds in agriculture. Plant Growth and Leaf-Applied Chemicals, ed. Peter M. Neumann, CRC Press, Inc., Boca Raton, Florida.