photosynthesis

Sweet peppers can manage with 800 ppm CO₂ in the winter months. Supplying more than that doesn’t boost photosynthesis: in fact, the sweet pepper plant simply gets used to a higher dose, resulting in ‘lazy’ leaves. Luckily, this is easy to reverse when spring arrives. The lazy leaves are back in action after six days.

These are the conclusions of Dutch research into sweet peppers carried out in 2014 and 2015 by Plant Lighting, Inno-Agro and Plant Dynamics. The research was conducted under the guidance of the Horticultural Technology Development growers association (TTO).

Greenhouse air quality

Many growers have a free supply of CO₂ from their CHP units. But finding out what the ideal dose is doesn’t feature high on their list of priorities. “It may not help, but it can’t do any harm” is often the rule of thumb. Nevertheless, Stefan Persoon, innovation specialist at Inno-Agro, has noticed that this is changing.
“First of all, we are starting to use less fossil fuel as a sector. Of the 1,650 hectares of sweet peppers in the Netherlands, 300 hectares are grown using geothermal heat or another alternative energy source. Those growers pay for their CO₂, for example via OCAP (CO₂ from the port city of Rotterdam), and this encourages them to do more with less. A second reason for finding out the ideal dosage is the quality of the air in the greenhouse. On the face of it, this doesn’t seem to be a problem, but measurements taken inside greenhouses reveal that the air quality can easily come under pressure. Oddly enough, this mainly happens in spring when the vents are opened. Growers will then provide additional CO₂ to compensate for the loss. This pollutes the air in the greenhouse, for example with NOx.”
Sander Hogewoning, a researcher at Plant Lighting, adds: “There is a third reason why growers should be taking a critical look at the dose. International research on arable crops indicates that prolonged exposure to higher CO₂ levels can lead to ‘lazy’ leaves. When that happens, the key enzyme RuBisCO binds CO₂ less effectively. Does this also happen in greenhouse horticulture? That’s precisely what this research was about.”

Airtight cabins

The trials took place at the Westland Demo Nursery (Demokwekerij Westland). They built six 1.4 m² glass cabins in which CO₂, RH and temperature can be precisely controlled. In the first phase, both sweet pepper and tomato plants were studied. The researchers put young plants from a breeder in cabins with 400, 700 and 1,000 ppm CO₂. Using a photosynthesis meter, Sander Pot of Plant Dynamics recorded in detail how the leaves use the different concentrations for photosynthesis.
CO₂ saturation in tomato plants was found to be 600 to 700 ppm, while in sweet peppers the figure was 700 to 800 ppm. “The step up from 400 to 600 ppm provides far more additional photosynthesis than the step from 600 to 800 ppm,” the researcher says. The amount a grower has to dose for that second step is also much more than for the first step, especially when the vents are open a crack. So the rule “the more, the better” doesn’t hold water. “And yet there can be downsides to high CO₂ concentrations. Not all growers take that on board,” Hogewoning explains.

Lazy leaves in sweet pepper

The next important issue the study looked at was whether this high dose would yield “lazy” leaves. The answer? Not in the short term. Leaves that had formed in the breeder’s nursery did not turn lazy. But the same was not true of leaves that had developed entirely in the trial cabins. A CO₂ dosage of 1,000 ppm did not produce lazy leaves in tomatoes, but the outcome was different in sweet peppers: photosynthesis was just as high in plants grown at 1,000 ppm and dosed with 900 ppm as it was in plants grown at 400 ppm and dosed at 600 ppm. This is clearly illustrated in Figure 1. In other words, plants that were “pampered” with 1,000 ppm needed a sustained high dose to keep their productivity up.
Hogewoning: “This is caused by the enzyme RuBisCO, which is the key to photosynthesis. The capacity of this enzyme drops. In this case the ‘laziness’ has nothing to do with the stomata, as some growers believe.”

Reactivating quickly

The follow-on research focused exclusively on sweet pepper plants and looked at whether the lazy leaves could be reactivated. “We call that ‘reversible’. We wanted to see whether and how quickly the leaves could get used to a lower CO₂ dosage. That is actually what happens in practice. After a winter with a high dosage, the vents are opened a crack in spring. The question is how long the leaves stay less productive then,” Hogewoning explains.
To test that, in 2015 they carried out a trial with sweet pepper plants with doses of 400 and 1,000 ppm. Once the researchers had identified the lazy leaves, they switched two cabins of 1,000 ppm to a varying regime of between 500 and 1,000 ppm, similar to spring in the greenhouse. Using the photosynthesis meter, they determined CO₂ uptake in the leaves after six and fourteen days.
Good news for sweet pepper growers: the laziness turned out to be reversible after just six days. Therefore, lower CO₂ levels only cause the crop to be less productive for a very short time. “We were quite surprised by that. It means that it isn’t necessary to adjust the dosing strategy in winter. But what is important to remember is that growers who opt for a high concentration need to keep it up right through the winter. The plant gets used to it. And dosing above 800 ppm has very little added value,” Hogewoning concludes.

Eye opener

Whether or not sweet pepper growers can make use of the findings in practice depends on their situation. We ran this past Bart van der Valk of Zwingrow, who grows orange peppers on three sites in the Westland area of the Netherlands. He is positive about the outcome of the research. “We use geothermal heat and we pay quite a lot per square metre for the CO₂ we source from OCAP. So we are keen to use it more efficiently.” For him it was an eye opener to discover that the level could be lower in winter.
“We are now dosing 600 to 700 ppm in winter. It’s just as effective as 1,000 ppm. I prefer to keep the CO₂ for the spring. What the research also revealed is that lazy leaves can recover again quickly. That’s good to know. Of course, there are still some unanswered questions. For example, I would like to find out what time of day is best for dosing CO₂. More research is needed in that area.” But a survey among growers using geothermal heat reveals that there isn’t enough money for practical research yet.

Summary

If sweet pepper plants receive a high dose of CO₂ over a long period in the winter, they get used to the high level. That produces “lazy” leaves which use the CO₂ less efficiently. But this is reversible: when the dose is reduced in spring, the plants adapt within just six days.

Text: Karin van Hoogstraten. Images: Studio G.J. Vlekke.