During my experiments with long term storage in ultra-low oxygen conditions one of the findings was that ethylene concentrations sometimes reached extremely high levels. As can be seen from the figures below, in 2019 the concentration of ethylene in the boxes went above 1,000 ppb. In 2020 we decreased both temperature and oxygen levels faster and this resulted in somewhat lower concentrations of up to 400. Box 1 was opened after 5-6 weeks, box 2 was only opened after 9 weeks. The ‘doos’ is the cardboard box with no lowered oxygen and thus ‘normal’ cold storage where ethylene levels remained low as the ethylene produced by the peonies themselves could ‘escape’ here contrary to the situation in the ULO-boxes.
When discussing this at one of the ‘working group meetings’ a professor in plant biology remarked that these numbers were extremely high as it is known that damage can already occur at levels as low as only 20 ppb. We ourselves are obviously not used to working with ethylene concentrations but “according to Dr. George Staby, founder at Chain of Life Network and president at Perishables Research Organization in Phoenix, less than 40 parts per billion is the level to maintain in order to avoid damage to sensitive varieties of cut flowers and plants.”(1) If you search for it in on the internet, you may find 10 ppb, 20 ppb and others as levels that can give damage. Whatever it may be, we clearly went far above this threshold.
It is well-known that different flowers have different thresholds for damage from ethylene. Nearly all cut flowers are sensitive to some level of ethylene. But then within each flower group, there are still differences between cultivars, one being very sensitive to it, whilst another nearly immune. You may thus find on some websites the argument that peonies are insensitive to ethylene whilst other warn strongly against it. Generally the damage is not done from the ethylene that the flowers themselves produce, but rather from fruit that is ripening in close proximity to it. It is thus recommended to place your cut flowers away from that fruit basket. Sometimes cut flowers are treated with some chemicals that can block the sensitivity of the cut flowers to ethylene. 1-MethylCycloPropene (1-MCP, sold as EthylBloc in the USA) can help during storage and SilverThioSulphate (STS) is another. Unfortunately where I live (Belgium), 1-MCP is ridiculously expensive and STS is forbidden, whilst in our flower-friendly neighbor country The Netherlands it is still allowed and even obligated to use (the European Union is not really a ‘union’ in all circumstances). But in peonies 1-MCP may not be the best chemical to use as it has been reported to even decrease vase life of peonies.(2) And STS is known to be environmentally hazardous. Nano-silver may be a solution for the future, but we’ll keep our results with still ongoing testing that for another post.
The enormous levels of ethylene in our storage experiments did however not seem to induce any adverse effects that we could see. So at the research centre (PCS Ornamental Plant Research) they set up some small experiments to see what ethylene can do to cut peony flowers. They had two cultivars available, Sarah Bernhardt and The Fawn, both are lactiflora varieties and the former is the most widely grown cut flower variety. To reproduce high levels of ethylene, they placed some fruit together with the peonies in cold storage for 24 hours. Then they were placed in a vase and left to flower and compare with some control flowers that had not been placed with some ripening fruit. As you can see from the following images: higher ethylene concentrations during cold storage do not result in lower quality flowers.
Now that is was shown that the high ethylene levels are not detrimental during cold storage, another experiment was set up. Here some flowers were placed with (or ‘without’ as control of course) some fruit (1 banana, 1 pear) for 24 hours at normal ‘house’ temperatures. They were placed inside a sealed container so that the ethylene couldn’t escape and would reach rather high levels, albeit only for that one day of course. Here the results clearly showed peony cut flowers to be sensitive to ethylene. The ethylene treated peonies wilted far earlier, didn’t open as well and had far more botrytis damage in the end. The images below are quite convincing I would think:
So, what do we learn from all of this? As a cut flower grower it is not really so much of a problem to have peonies in conditions of higher ethylene concentrations when they are stored at low temperatures. The damage does however occur at higher temperatures, so in a retail store where temperatures are not as low and where they are placed together with fruits, you may expect a disappointed customer. For the end customer clearly it is advisable to keep them away from ripening fruits. One small remark of course is at what combination of temperature and ethylene concentration the damage starts to occur. Our experiments were either at very low temperatures (0-2°C or 32-35°F) but with sometimes extremely high concentrations of ethylene, or rather high ‘house’ temperatures of some 20°C (68°F). Intermediate temperatures or ethylene levels will probably give less conspicuous damage, but as we all want to enjoy our peony flowers as long as possible it’s at the very least a good idea to inform the end customers. Peonies and fruit together won’t take peony cut flowers to full fruition.Footnotes:
- G. Smith. “Ethylene — silent but deadly.” https://theproducenews.com/ethylene-silent-deadly , Oct 2017.
- Hoffman, G.D., Mattinson, D.S. and Fellman, J.K. (2010). 1-MCP SHORTENS PEONY VASE LIFE. Acta Hortic. 857, 169-178.