Photosynthesis is responsible for the vast majority of plant growth. Chlorophyll fluorescence measurement can be very helpful in giving growers more control over this process. Wooning Orchids in Bleiswijk has several years of experience with this technique. ‘It gives us more insight into the plant’s needs and helps us control the amount of light in the greenhouse.’
At the beginning of May, Wooning Orchids, a subsidiary of Green’05, sees a daily stream of lorries coming and going to load phalaenopsis. In the busiest weeks, up to 350,000 plants leave the premises. ‘We produce year-round, but weeks 4 to 22 are the peak weeks,’ says managing director Walter Spek. The company supplies around 6.5 million plants annually, mainly to retailers in Europe. These are phalaenopsis in pot size 12 with a branch length of 60 to 75 centimetres. ‘We distinguish ourselves mainly in terms of flower size. At 9.5 centimetres, they are slightly larger than average.’
Only efficient varieties
Cultivation takes place under 9.2 hectares of glass, where around fifty varieties can currently be found. ‘We are going to significantly reduce that number,’ says Spek. ‘We only want to keep the best-performing varieties, so that we can plan and manage cultivation more effectively.’
Sendot’s fluorescence meters play an important role in determining which varieties these are. The company first started using them over three years ago. ‘We started with two meters and now we use eight in different places in the greenhouse,’ says Tom Tielemans, who is responsible for cultivation and purchasing at Wooning Orchids.
Phalaenopsis has nice stiff leaves to which the fluorescence meters can be easily attached. Tielemans: ‘You attach them to the youngest mature leaf using a clip, choosing the leaf of a plant that is as horizontal as possible. The meter then emits a light pulse every five minutes and measures how much light is reflected. The plant uses the light that is not reflected for photosynthesis.’
Light utilisation and stress
According to Delphy consultant Aad van Holsteijn, fluorescence measurement is an interesting tool. ‘It closely resembles photosynthesis measurement. And photosynthesis is responsible for the vast majority of plant growth. There is also a photosynthesis meter on the market, but it is still prohibitively expensive.’
What a fluorescence meter does not take into account is the CO2 uptake by the plant. It only measures the percentage of light used by the plant for growth. ‘This also makes it a stress meter,’ says Van Holsteijn. ‘After all, at higher light levels, the percentage of light used by the plant decreases. The unused light is converted into heat, which causes plant stress.’
‘We want to prevent that stress as much as possible,’ adds Tielemans. ‘Stress prevents the plant from recovering optimally during the night.’
Good insight is important
Phalaenopsis is a so-called CAM plant. This type of plant has closed stomata during the day. At night, the plant absorbs CO2 and produces malate. ‘The plant breaks down this malate during the day, under the influence of light,’ explains Tielemans. ‘This happens in the first ten hours of the day. Once the malate is used up, light efficiency is significantly reduced. Now that we understand this, we can control the amount of light in the greenhouse.’
‘We also see that phalaenopsis experiences light stress more quickly in direct sunlight,’ notes Spek. ‘Diffuse light works much better.’ And because it is not an easy crop to grow, where screens and chalking are crucial and growers have to deal with many different varieties, it is particularly important to gain a good understanding of what is happening to the plant. Van Holsteijn: ‘It is no coincidence that we are now seeing the widespread use of fluorescence measurement in this crop.’
Determining light levels
The team at Wooning Orchids has noticed that using fluorometers has given them much better insight into the plants’ needs. ‘We can now adjust accordingly,’ says Tielemans. “For example, by starting the lighting later or switching it off earlier. The type of light also has an influence. It can be beneficial to switch on the SON-T lighting first and only switch on the LED lighting later, for example.”
Spek: ‘In our greenhouses, we have to deal with many different conditions due to the wide range of products, the different crop stages and the different structures or years of construction. Sensors help to make many variables measurable. With that insight, you can make targeted adjustments and determine the right light levels.’
Limits in focus
Have they started doing things differently since using the fluorescence meters? ‘Definitely,’ says Tielemans with conviction. ‘We have started providing more or less light at certain times. The same applies to screening. We are looking for light optimisation for each variety in every department. Thanks to the meters, we can see the limits of the crop much earlier.’
Growers try to give their plants as much light as possible, with the highest light sum being reached at the end of the day and the plant still having beautiful green leaves. Van Holsteijn: ‘A higher light sum means a higher malate level, more root growth and ultimately better plant quality. However, the light sum must be such that light stress is prevented as much as possible and the plant retains the correct leaf colour.’
Phalaenopsis can utilise more light at the start of the day than later in the day when the malate in the plant has been used up. ‘In this cultivation, you have companies with up to three hundred varieties and multiple cultivation departments. So there is a lot to be gained in how best to deal with light.’
Details must be accurate
There are various fluorescence meters on the market. According to the men, Sendot’s meters have proven themselves and provide reliable data. What’s more, they are wireless, which is not the case with all suppliers, and that is essential in a greenhouse where the products are placed on moving tables.
According to Spek, the meters are a good example of tools that help to make the necessary sustainability improvements. ‘I am convinced that we in greenhouse horticulture must move towards climate-neutral cultivation. This is only possible if all the details are correct. And to measure is to know. We have to make choices about whether varieties that are difficult to root or do not respond well to light are necessary for our customers.’
Source: Onder Glas – June 2024: https://www.onderglas.nl/dankzij-fluorescentiemeters-zo-efficient-mogelijk-belichten/
Text: Jeannet Pennings, image: Michel Heerkens
