“Why are my tomatoes not ripening in all this heat?”

This last weekend Woodend hosted the tenth annual Macedon Ranges Sustainable Living Festival. It was an opportunity for visitors to meet sustainability leaders, both commercial and from MRSG’s action groups, who are undertaking activities that they feel will help to make us more in-balance with the natural order, that is, more sustainable. Many visitors came into the Woodend Community Garden precinct to meet with volunteer gardeners and seed savers from Riddell’s Creek, Kyneton, Macedon, and Woodend to discuss gardening techniques and food production. There were presentations and discussions on hugelkulture and wicking beds, which are ways of reducing water use in our drought-like summers, which were well attended and I would like to thank Christine, Jan, and Kyneton Transition Hub again for providing these thought provoking studies.

One question was asked several times: “why are my tomatoes not ripening in all this heat?” Now I have a response to that question as well as a suggested solution: 

There are several elements to tomato ripening. They work together and independently (they include phytochromes, respiration and ethylene). The main activity in climacteric fruit ripening is ethylene biosynthesis. Ethylene causes fruit ripening (softening and colour change). Ethylene (C2H4) is released by the formed fruit in increasing volume from their “breaker” point (whitey green). Many climacteric fruit are inhibited from ripening or exhibit abnormal ripening at high temperatures. Ethylene production peaks at 25C ambient temperature when it starts to taper off; other elements continue to rise until 30C is reached. What does this mean? It means your tomatoes can get too hot to ripen, but it is generally reversible. However, it may cause permanent damage to the individual fruit, if the fruit is overheated for a sustained period (38C). 

I cannot know if your tomatoes are suffering from excess radiation, ethylene, or respiration reduction (site specific climate conditions like the nighttime temp, air movement, shade, sunlight hours, and humidity will all have an effect.) , but I can suggest a response if you think they may be. I keep every old sheet that gets too damaged for use; I keep them for the garden.

Drape an old sheet over your tomato plant when the temperatures start to linger above 30C. This will trap ethylene and gently shade tomatoes, hopefully without impairing the red-light availability for phytochromic activity (increasing Lycopene).
A last word on high heat and tomatoes: too much heat or sunlight may also result in tough skins. The toughness of tomato skins is dependent on the way the crop has been grown: too much heat or sunlight or lack of vigour due to low temperatures or the plants receiving insufficient water and nutrition. These can all lead to toughened tomato skins.

And finally, I would like to thank Joy for starting me on this journey on Saturday; it has been an interesting research topic. Joy, I can now concur with the probability of your diagnosis and my solution remains the same.

Key resources:
Antunes, M.D.C., Sfakiotakis, E.M.,  2000. Effect of high temperature stress on ethylene biosynthesis, respiration and ripening of ‘Hayward’ kiwifruit. Postharvest Biology and Technology 20 (2000) 251–259

Inaba, M., Chachin, K., 1988. Influence of and recovery from high-temperature stress on harvested mature green tomatoes. HortScience 23, 190–192.

Inaba, M., Chachin, K., 1989. High-temperature stress and mitochondrial activity of harvested mature-green tomatoes. J. Am. Soc. Hort. Sci. 114, 809–814.

Yakir, D., Sadovski, A., Rabiniwitch, H.D., Rudich, J., 1984. Effect of high temperature on quality of processing tomatoes of various genotypes ripened off the vine. Sci. Hort. 23, 323–330.

Drape an old sheet over your tomato plant…