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Measuring fast, saving big: photosynthesis sensor indispensable in energy crisis?

The question of what the crop does when there is a change in lighting cannot be answered precisely by growers without sensors. The photosynthesis sensor is an important sensor for visualising this. It measures the photosynthesis efficiency of a crop, in real time. With the shift from HPS to LED or hybrid lighting, the sensor is already an increasingly important tool, but the current high energy prices make accurate measuring even more important, especially real-time measuring.

"Growers who are currently reconsidering their strategy want to be able to adjust their lighting quickly, while maintaining quality and production," says technical manager and senior researcher Arie Draaijer of sensor manufacturer Sendot. "Anyone who starts measuring will see that many savings are possible. Saving even tenths of a percent on energy is already saving serious money at the moment."

The fact that it is possible to measure photosynthesis efficiency is something that has been known to growers for a while. Measuring photosynthesis efficiency is no longer just for researchers. Sensors have become increasingly affordable. "With today's energy prices, a grower will easily pay off the investment in some of our affordable sensors this winter." More and more growers were already investing in photosynthesis sensors, but the energy crisis is undoubtedly going to be a huge additional incentive, Arie expects. "We are currently seeing a lively interest."

CAM plants
How crops deal with lighting differs from crop to crop. Growers of so-called CAM plants, an abbreviation for Crassulacean Acid Metabolism, were working on photosynthetic efficiency even without high energy prices. Arie gives the example of phalaenopsis growers, who are among the 'early adapters' of measurement technology. "They are already very advanced in measuring and calculating changes in lighting on growth and production.

CAM plants, including orchids and many other ornamental plants, close their stomata during the day to grow without drying out. With closed stomata they cannot absorb CO2. With the CO2 they absorb at night, they also grow during the day. But when the stored CO2 supply is depleted, it stops. Extra lighting is therefore pointless. "That is why these growers started to pay close attention to what the plant can still absorb in terms of lighting. Our sensors, which measure in real time, help growers to understand this."

The situation is different for production crops such as tomatoes, cucumbers, lettuce and strawberries, and currently also for greenhouse vegetables that are exposed to winter light. These crops convert CO2 directly into growth. Yet here too, growers benefit from insight into photosynthesis efficiency, according to Arie. "In principle, every grower is always busy optimising his energy consumption. With today's high energy prices, this is even more the case. Growers are reconsidering their lighting strategy, but want to do so without making too many concessions in terms of production and quality."

He points out that the effects of changes in lighting on a crop in, say, tomato or cucumber described above are 'more nuanced'. "The game of lighting, photosynthesis efficiency, production and greenhouse climate is more complicated here. That's why it took about thirty years to arrive at the current optimised strategies for lighting with HPS, while another ten years of research with LED is already underway in order to arrive at an optimum schedule here too. Growers play with temperature in the head, among other things. Due to the change in fittings, this changes too and so they are now looking for the optimum again."

Measuring and responding immediately
Thanks to the possibility of real-time measurement, growers can (out of necessity) take quick steps in the current energy crisis. According to Arie, very few growers still have a plan for the current energy crisis that states exactly what less lighting will do to production. "And even in science there is still a lot unknown about how crops exactly react. Nobody has taken this into account to this extent."

He gives an example. "If you continuously monitor energy prices and immediately want to anticipate this by turning off the lights at very high price peaks, it could be that in the current dynamic energy market, the lights are turned off or dimmed several times a day. However, there is still very little insight into what this exactly means for production and quality. And I don't mean discovering after a few weeks that fewer bunches are being produced, but seeing in real time the effect of turning the knobs on the crop. That effect is namely very different when you turn off the lights when there is more light from outside, as already happens, than when you turn off the lights when there is no extra light from outside. That's what you see happening now, out of necessity, and so there is still much to discover and learn."

All crops
Sendot's photosynthesis sensors can be used in all possible greenhouse crops. Erwin Grafe, Commercial Manager at the sensor developer: "The grower places the sensor on the leaf. We have various solutions for this. For most leaves there is a standard clip for this. You place it high up in the crop on a leaf that contributes a lot to the production of the crop. With crops that do not grow very high, you choose the right leaf for this, depending on its age.

If that leaf, for instance with some herbs or leafy crops, is not sturdy enough, you can place the sensor on a stick and the clip on the leaf. Only in the multilayer cultivation of certain young leafy crops and herbs is this still a challenge. Young leaves in particular are vulnerable here. It therefore requires a little more attention from the grower, but here too the energy savings that growers can achieve soon pay for themselves."

A few sensors are often sufficient. "Our advice is often: start small with data collection and scale up as soon as you get a better grip on the data. We also help with that. In a greenhouse with one crop and similar conditions, a few sensors can make a big difference. Where growers have several crops that react differently to lighting, it may be worth investing in more sensors. In all cases, sometimes big but sometimes only small savings add up, especially now, with the current energy prices."

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