A Tomato Myth is Born – More About Tomato Ripening

A few weeks ago I published the post called Ripening Tomato Myths – Both on the Vine and in the Home which explained why tomatoes ripen completely when picked at the breaker stage. Some people pointed out a discrepancy in that post. How could the fruit be sealed off from the plant at breaker stage and still show fruit cracking due to excessive water in the plant? This was a valid point that needed some more investigation.

In this post I will clarify the situation and explain how this tomato myth got started. This will lead us to a better understanding of the tomato ripening process.

The Truth About the Seal at Breaker Stage

In my previous post I wrote:

The breaker stage is reached when the fruit has a definite pink coloration on 10 – 30% of the fruit.

An important change takes place in the stem of the fruit when it reaches the breaker stage. “A layer of cells form across the stem of the tomato, sealing it off from the main vine. When this occurs, there is nothing that can move from the plant into the fruit. The tomato can be harvested and ripened off the vine with no loss of flavor, quality or nutrition.”

Let me clarify this statement. The change does take place, but it is not as dramatic or as sudden as suggested. It is also not 100% complete until the fruit is fully ripe and is ready to fall off the vine.

I will explain this in more detail below, but in short, the development of this layer starts before the flower opens and continues until fruit drop. At the Green Mature Stage much of the flow of water and nutrients has been reduced, but they are not shut off completely. Even when the fruit is ripe, excess water can still enter the fruit and if too much is absorbed it can cause cracking.

Once the breaker stage is reached the fruit no longer relies on receiving nutrients or sugars from the plant because it now contains all the ingredients it needs to fully ripen on its own.

This whole process is very complex and scientists are still piecing the puzzle together, but the statement that a seal prevents anything from entering the fruit at breaker stage is a myth.

A Myth is Born

How did this myth get started?

When I wrote the original article I found the above quote on quite a few sites including the one I quoted, which was Kansas State University. None of the sites using the quote reported a source and all of them repeated it exactly word for word. This usually indicates that the details were not investigated and should have been a red flag.

I should have checked the quote more thoroughly. My mistake.

I’ve now done some more digging and found that the original information came from Dr. Charles Marr, Horticulture and Natural Resources, Kansas State University, who is now retired. I contacted him and confirmed the source. He was also kind enough to provide some clarifications and point me to more research on the AZ.

The cause of this myth is a common one. As gardeners, we want simple answers that are not very technical. To complete this post I read a dozen scientific reports on this matter and there is no way to present all that detail in a format suitable for gardeners. A garden writer job is to simplify things, and invariably in the process, some important details are lost. In this case it was easier for Kansas State to make the brief statement they did, instead of providing a much longer explanation.

This myth is a case of over simplification combined with writers, including myself, accepting a simple explanation without digging deeper.

Understanding the Abscission Zone Better

The Abscission Zone or AZ, is really interesting and explains a lot about how plants lose their leaves and fruit development.

Role of Xylem and Phloem

Think of the xylem and phloem as subway tunnels running through the plant, moving stuff from one place to another. The xylem moves water and nutrients from the roots up into the plant. The phloem moves water, sugars and many other plant metabolites (plant-made chemicals) from their origin – usually the leaves, to other parts of the plant, including fruit and roots.

I could never remember which was which, until I associated the “X” in xylem with the end of the alphabet where you also find “W” for water.  It’s how my brain works.

As the tomato develops, up to 90% of its water needs comes from the phloem and 25 days after flowering, the water import from the xylem almost ceases. Once the fruit reaches the Mature Green Stage it no longer needs the phloem since it now has everything necessary for ripening. However, water is still a requirement. The fruit does lose water through its skin and this needs to be replenished.

The ratio of water received from the xylem and phloem is affected by environmental conditions and by the amount of light. As light levels drop, more water enters the fruit from the xylem, decreasing the influx of sugars and other metabolites through the phloem. This will have an impact on flavor, and may explain why some greenhouse tomatoes, grown out of season, have less flavor.

Formation of the Abscission Zone (the AZ)

While the flower and peduncle is developing a special set of cells forms known as the abscission zone (AZ). If pollination fails this AZ seals off the flower from the main plant and the flower falls off. If pollination is successful, the AZ grows and becomes stronger so that it can support the weight of the developing fruit. You can easily see this swelling, commonly referred to as the “knuckle”, on more mature fruit. Once the fruit is fully ripe, the plant no longer needs the fruit and the AZ seals off completely allowing the fruit to fall.

Internal changes are also taking place. You can think of the xylem and phloem as a bunch of little tubes running from the plant to the fruit. As the AZ develops, both the number and diameter of these tubes in the AZ get smaller. This decreases the flow of water into the fruit and it is believed that this also prevents the fruit from cracking even more than it does. Leaves have pores (stomata) that allow excess water to escape so a higher flow rate does not harm them. Fruit on the other hand does not have these same pores and would just expand if too much water is pushed into them. The constricted AZ prevents this to some extent.

These changes are matched to development in the fruit. As the fruit reaches full size and no longer needs resources from the plant, the AZ reduces movement of water and nutrients in and out of the fruit. The plant can now divert these resources to other less developed fruit.

A key point in all this is that the xylem and phloem tubes are not shut off completely until the fruit is ripe and ready to fall.

Why Do Tomatoes Crack?

For us gardeners this seems so simple. We water more , we see cracks. But scientists have been working to understand this process for quite some time and it is very complex. We still don’t have all the answers.

The xylem and phloem are not fully turned off until the fruit is ready to drop. This means that a high influx of water into the plant due to either rain or an ambitious gardener watering too much, still results in too much water entering the fruit.

It is believed that the fruit itself has another mechanism for stopping this excess water, so even though the xylem is partially open, the fruit can overcome this problem when there is a lot of water pressure. Genetics controls this process, as well as the chemistry of the skin itself. This is why some varieties crack easier than others. I grow Sweet 100 tomatoes and they crack all the time. I also grow Garden Gem, another cherry tomato, and they never crack until I pull them off the vine.