Dr. Julio Retamales, Universidad de Chile and Valent BioSciences Corporation, Santiago, Chile, (julio.retamales@gmail.com), February 2009.
Ethephon is a known plant growth regulator that has been applied for many years and for which a myriad of possible uses on various agricultural crops have been developed. In horticultural crops it is applied, among other uses, as a harvest agent and ripening promoter. Its mode of action is based in the splitting of its molecule allowing to free the ethylene gas, the known endogenous plant hormone that participates in the regulation of processes in plants as diverse as abscission (e.g. shedding of leaves and fruits), plant growth and senescence and most particularly fruit ripening (especially in the case of so-called climacteric fruits). Further, ethylene can be used as a degreening agent in citrus and to enhance color development on table grapes (both known examples of non-climacteric fruits). In the latter case it is frequently used as derived from ethephon applications on colored grapes when natural conditions (e.g. warm temperatures at night) and cultural practices (e.g. high yield and berry growth as induced by other plant growth regulators) are precluding the obtention of enough berry skin color for marketing purposes.
Recent information has been released by the EFSA (European Food Safety Authority) indicating that ethephon (available under different commercial names like Ethrel®) could face restrictions to its usage in various horticultural produces being sold at the European market, e.g. in cherries, tomatoes, pineapples and peppers (EFSA, 2008). For the case of table grapes it is proposed that the current MRL (Maximum Residue Limit) of 1.0 mg ethephon per kg of produce should be reduced to 0.05 mg/kg. With such reduction it is considered that the present usage of ethephon to induce appropriate color in table grapes is no longer feasible. Therefore, not only the European production, but also all the colored table grapes imports to the European market from foreign countries would be restricted to such limits and, in the practice, ethephon could no longer be applied as presently done. This would affect colored table grapes of countries as diverse as India, South Africa, Mexico, Chile, Peru, Brazil and United States, just to mention a few examples.
Attaining appropriate and homogenous berry color is a paramount quality factor in colored table grapes, as it can easily be appreciated by the consumer that relates this factor with a fully ripe and tasty fruit. Certainly, regarding handling of the crop for increased berry color, there is still room for improvement in cultural practices ensuring appropriate light exposure, plant nutrition and regulation of fruit load that can lead to increased berry coloration. However, market requirements and economic conditions should be also satisfied that are based in high yield and berry size precluding in many instances best color to be obtained. Therefore, not being able to use a tool like ethephon can constitute a major limitation for countries with rather warm conditions that make obtention of appropriate berry coloration more difficult.
Interestingly enough, as it has been shown already some years ago, abscisic acid (ABA), an endogenous phytohormone, can also upregulate the synthesis of anthocyanins, the natural plant compounds that are mostly responsible for berry coloration in grapes. A relevant recent development allowing ABA to be produced in a scale compatible with its usage as plant growth regulator is enabling a partial or full replacement for ethephon on table grapes. For such purpose, a full research program has been carried out in the last 5 years, including field trials in countries like USA, Australia, South Africa, Israel, Mexico, Brazil and Chile. Further, being a natural plant product and owing to its favorable biochemical and toxicological profile, registration procedure for ABA for such usage is favorably prospected and already under way in some countries. Therefore, it is considered that the introduction of ABA as a commercially-available plant growth regulator will eventually allow that the table grape industry of relevant countries can continue producing colored grapes even without ethephon being used as before.
EFSA (European Food Safety Authority), 2008. Reasoned opinion of EFSA prepared by PRAPeR on MRLs of concern for the active substance ethephon. EFSA Scientific Report (2008) 159, 1-31. Issued on September 15, 2008.