The Ripe Stuff: Green Tomatoes, Bananas & Ethylene

green_tomatoes.jpgFried Green Tomatoes

About this time of year in the North Temperate Zone people may be getting very tired of fried green tomatoes and green tomato pie.

As a plant physiologist, I was often asked how to ripen green tomatoes. (Rather than go into this here, I’ll refer you to this blogpost.)

You probably already know that tomato ripening is promoted by the gaseous plant hormone ethylene.

Regarding ethylene, one of the best online resources I’ve found provides comprehensive information about this gas, including a table listing the relative production of, and sensitivity to, ethylene in a large number of fruits and vegetables.

And if you want to inhibit fruit ripening, there’s even a commercial website for reducing ethylene.

Going Green to Red (or Yellow)

But what’s happening to the tomato when it ripens from green to red? And to the banana when it goes green to yellow (and then brown)?

The change in color is primarily the result in the active conversion of chloroplasts to chromoplasts. The chlorophyll is actively (enzymatically) broken down, and the red and yellow colors are due mainly to carotenoid pigments in the chromoplasts.

Since the fruits are there primarily to promote seed dispersal by animals, red colors apparently tend to attract birds, for example. Interestingly, as reported here, at tropical temperatures (about 90o F), the Cavendish banana skin tends to stay green, though the fruit ripens.

“Over-ripe” fruits (the brown banana, for example) tend to be primarily the result of senescence.

Fruit Softening

The “softening” of the fruit is the result of the breakdown of plant cell walls by cell wall-digesting enzymes such as cellulases, pectinases, and expansins.

Indeed, scientists were able to extend the shelf-life of the Flavr Savr tomato by effectively turning off the gene coding for a pectinase.

Getting Sweeter

Another characteristic of fruits that tends to attract some animals for seed dispersal is the presence of sugars. Increased sugars in the process of fruit ripening is due mainly to the production of enzymes that break down starch.

Interestingly, the process of fruit ripening involves many enzymes that digest polymers of sugar molecules (i.e., cell walls and starch).

How does ethylene promote ripening?

The expression of many of the above enzymes is promoted by ethylene in plant species that have so-called climacteric fruits, such as tomatoes and bananas. (Please see here for a list of climacteric and non-climacteric fruits.)

Climacteric fruits are characterized by a burst of cellular respiration that is often immediately preceded by, or happens simultaneously with, a sharp increase in ethylene. (Ethylene also may promote its own biosynthesis.)

Ethylene is an essential component of climacteric fruit ripening. Blocking ethylene biosynthesis or action prevents ripening.

Ethylene apparently regulates the expression of many ripening-associated genes, including those coding for enzymes involved in color change, fruit softening, and starch breakdown. (How ethylene activates such genes is a topic for another day.)

How “One Bad Apple Spoils the Bunch” (and ripens your green tomatoes).

As mentioned above, ethylene may promote its own biosynthesis in climacteric fruits.

So an over-ripe apple producing a lot of ethylene gas – which diffuses throughout the bunch – triggers the biosynthesis of ethylene in the rest of the apples (and also in your green tomatoes), thus promoting the ripening process.

Bottom line: For climacteric fruits, the gaseous hormone ethylene is indeed “the ripe stuff’.

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One Comment

  1. Very informative and eye catching. Thanks, really helped me with and exam question I needed to know on climacteric fruit.

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