Tomato Red Face: Ripening = Hormones, Not Energy

When I started this blog, I figured I was a better-than-average tomato grower. I’d picked up a lot of tricks over the years, I said. I also said that tomatoes are super resilient and hard to ruin – which is kind of true. You can do a lot of things wrong and still get some tomatoes, but they may not be very good tomatoes, or the plants may not produce to their full potential – and it’s pretty clear that this is what happened to me this summer.

As I mentioned I’ve been taking some classes through the U of S’s Prairie Horticulture Certificate program and having multiple “eureka” moments about what I may have screwed up this summer, usually based on common but bad advice. While this is embarrassing for me, someone who thought they were so good at growing tomatoes that I could have a whole blog about it, I owe it to you, my 11 or so readers, to talk about the practices I used that were not based in sound science. These posts focus on four specific topics I didn’t understand very well (photosynthesis, plant hormones, nutrient uptake, root development) and why my well-intentioned but misguided practices may have led to my 2020 gardening woes.

Regarding sources – unless otherwise specified, the majority of general plant science information in these posts is based on what I learned in the University of Saskatchewan Prairie Horticulture Certificate courses HORT 13 Applied Botany and SLSC 14 Soils for Horticulture.

The thing that I didn’t understand about ripening, but now seems obvious, is that it is a hormonal reaction, not an issue of “energy imbalance” as is common thinking. Apologies for this slightly horrifying comparison, but cutting leaves off a plant to try and get it to ripen its fruit faster is kind of like assuming one could induce human labour by cutting off the expectant mother’s pinkie toe (because now the body can put the energy that the toe was previously using into powering the labour). So, is it possible to influence the plant’s ripening hormones late in the season before our tomatoes freeze?

It’s time to go back to (you guessed it) Grade 7 science, and talk about ethylene – the gas that makes fruit ripen, also known as the ripening hormone. We’ve all heard the tip that to make avocados ripen faster, put them in a paper bag with a banana. This is because bananas give off a lot of ethylene which will trigger ripening in other fruits that are susceptible to it. If you pick two green tomatoes off a plant and put one next to a banana and one farther away, it is likely that the one closest to the banana will turn red quicker. However, this is not the entire story, and doesn’t do us any good for the fruits on the vine, unless you are planning to hang bananas off your tomato cages.

Tomatoes are a type of fruit that will not ripen until their seeds are mature[1]. If you’ve ever had a green tomato that never ripened indoors, this is why – its seeds didn’t “finish,” and thus it never got the internal hormonal trigger to start ripening. However, if an immature green tomato is still attached to a live plant, exposure to ethylene can hasten its maturation[2]. Is there a way to get the plant to start producing ethylene before it’s ready?

Ethylene is a stress hormone, i.e. its production can be induced by stress to the plant. So while it’s possible that my cutting all the leaves off my plants may have induced maturity/ripening via stress, it also had the undesirable effect of cutting off sugar production. After reading and researching for quite a while, my conclusion is that it’s probably not a good idea to do anything that will drastically stress your plants if you don’t know exactly what you’re doing (I’ll classify myself into this category). However, based on the science that I do understand, it sounds like there are a couple of safe things one could try.

Inducing mild drought stress by cutting back on watering is acceptable and may even give you better flavoured (but smaller) fruits, according to Dr. Linda Chalker-Scott[3], a trustworthy voice in horticulture (one of her books was our textbook for Applied Botany, and now if I’m Googling plant questions I append her name to my query). Gently pulling up on the plant to disturb the roots will also stress the plant without harming the fruit. By late season, annual plants do not need a ton of nutrients anymore and thus disturbing the roots won’t be as detrimental as if you did it in the spring. (I know I’d mentioned this tip in August but didn’t understand why it was recommended, and funnily enough it’s the one thing I didn’t try in 2020.)

Note – when she was marking this assignment, my instructor also commented that you can pull out the entire tomato plant and just hang it up and let the tomatoes finish ripening on the plant this way (provided they have reached full size), but cautioned that tomatoes ripened in this manner should not be canned.

The cooling temperatures of late August should also naturally create some stress for the plants, and ideally a home gardener shouldn’t have to manipulate their plants’ hormones, but it’s definitely tempting for us prairie dwellers as we play chicken with the unpredictable fall weather. In an ideal world we all start with quality plants and take good care of them throughout the growing season by giving them optimal amounts of sunlight, water, and nutrients, which should promote on-time maturity as much as possible.

While I’m not proud of any of this, I’m excited to be able to learn from these mistakes. I’m writing this down for myself a) (of course) to fulfill the requirements of my Applied Botany class but also b) so I can identify and remember the bad practices I used so I don’t repeat them in the future. And I am sharing it with you, even though it is a bit humiliating to do so, in case it is helpful for anyone. If you followed any of my bad advice this summer and similarly screwed up your tomatoes, I hope that the hours of effort that I’ve put into these posts might also serve as a sufficient apology.

Let’s grow some better tomatoes in 2021!

References

[1]      J. R. Ecker, “Epigenetic trigger for tomato ripening,” Nat. Biotechnol., vol. 31, no. 2, pp. 119–120, 2013.

[2]      H. J. Klee and J. J. Giovannoni, “Genetics and Control of Tomato Fruit Ripening and Quality Attributes,” Annu. Rev. Genet., vol. 45, pp. 41–59, 2011.

[3]      L. Chalker-Scott, “A salt bath for your tomatoes?,” The Garden Professors, 2011. [Online]. Available: http://gardenprofessors.com/a-salt-bath-for-your-tomatoes/. [Accessed: 01-Nov-2020].

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