The mid-year results from the low-energy tomato trial initiated by Grodan and Philips Horticulture LED Solutions indicate that not only cherry tomatoes, but also larger truss tomatoes can be grown successfully with reduced heat input in a full-LED setup.
Importantly, another major goal of this trial is to demonstrate to growers of Macxize XR (Axia seeds) how blotchy ripening (green spots), a troublesome potential quality issue in this high-yielding variety, can be reduced by adjusting the nutrient balance – specifically the potassium and nitrogen ratios.
Summary of mid-year results based on full LEDs, precision irrigation and active air dehumidification
- Philips toplighting plus interlighting is on track to deliver 5-7% yield increase
- Low-nitrate strategy supports generative plant steering
- Adjusted potassium/nitrate ratios potentially lead to a reduction in blotchy ripening (green spots)
- Fine-tuning of screening, heating and lighting strategies delivers additional energy savings
Interlighting improves light use efficiency resulting in higher yield
The greenhouse trial introduced a new approach in lighting application. Both halves of the greenhouse received an equivalent total light input, but the method of light delivery differed between the two sections. One half utilised Philips GreenPower toplighting force at 350 µmol/m2/s, while the other half combined toplighting at 247 µmol/m2/s with supplementary Philips GreenPower LED interlighting, providing an additional 103 µmol/m2/s. The purpose of this configuration was to enhance light penetration directly into the plant canopy, thereby increasing light use efficiency (LUE).
The interlighting was activated in Week 46. Harvest data collected from Week 52 onwards shows that plants grown under the toplighting plus interlighting setup have achieved an average of 4% greater fruit weight compared to those under toplighting alone (Figure 1). These findings are encouraging and support the hypothesis that improved LUE will result in a projected yield increase of 5–7% throughout the season. In addition to increased fruit weight, the interlighting treatment has also produced plants with more robust growth and accelerated flowering rates, further contributing to the anticipated yield improvements.
Adjusted nutrient strategy for generative plant steering
Randomised across the two lighting treatments are two nutrient strategies: ‘Reference’ and ‘Adjusted’. “The reference nutrient strategy is supplying a nutrient composition at a drip EC of 3.8 mS/cm as per commercial practice,” states Andrew Lee, Knowledge Manager at Grodan. “The adjusted strategy is an optimised nutrient strategy delivered at a lower EC of 3.1 mS/cm, designed to achieve fruits with less blotchy ripening (green spots),” he adds. To date, the nitrate gifts have varied between 9 and 16 µmol/l for the standard treatment, and between 8 and 13 µmol/l in the adjusted strategy.
As in the previous two trials conducted by the partners, the lower nitrate concentrations at the start of the crop were designed to control leaf area index (Figure 4) and to achieve generative plant balance. This was realised. However, the nitrate in the drip water proved to be too low temporarily, resulting in nitrate uptake initially being less than required (see the solid blue line in Figures 2 & 3). “This is a trial and we had to find the pain point,” states, Lee. “Unfortunately, this cost us some development speed. Thankfully, increasing nitrate concentrations instantly corrected the situation, demonstrating the steerable nature of the GT Master slabs, not only for water content and EC but also individual nutrient elements,” he adds.
Besides a favourable K/N uptake ratio to limit blotchy ripening, the researchers also state that a minimum nitrate uptake is important to maintain sufficient vegetative growth and support potassium uptake at the same time. The increased potassium concentration in the adjusted nutrient composition has resulted in lower concentrations of magnesium and calcium in the drip water. However, despite 60% lower magnesium content in the drip water of the adjusted nutrient composition, the uptake is 60% higher compared to standard. In the adjusted strategy, the calcium content (µmol/kg dry weight) of the fruits is also higher than the defined minimum of 30 µmol/kg dry weight which would trigger blossom-end rot.
Looking ahead to the rest of the trial, the partners intend to continue to test their hypothesis and prove their theories about the adjusted nutrient strategy, with modifications based on uptake analysis and the predicted incidence of blotchy ripening. “Indeed, such is the interest in this trial that we are now sharing our data on a weekly basis with four commercial growers cultivating the same variety under full-LED systems in Netherlands,” states Marcel Raats, plant specialist for Philips Horticulture LED Solutions at Signify.
Extra energy savings
Over the course of the previous two full-LED trials, the research team has successfully demonstrated that heat input in the greenhouse environment can be reduced by more than 50%. In the current year, by adhering to established principles such as active dehumidification, intensive use of climate screens and minimised reliance on the pipe rail heating system, energy consumption remains effectively controlled.
A key element in this energy-saving approach involves the prolonged closure of screens from Ridder. The energy screens remain closed for nearly 24 hours each day, while blackout screens are opened for five to six hours daily. This schedule is designed to closely replicate the practices of commercial growers, and it also stems from observations that extended blackout periods can negatively affect flower quality.
Further refinement of the screening strategy has been achieved using net radiation data from Wireless Value, together with other Wireless Value sensor data that is integrated into the Ridder climate computer. The combined data provides real-time insight into outgoing radiation from the greenhouse. When excessive radiation losses are detected, the screens are closed earlier than usual. This proactive approach prevents the crop heads from becoming too cold, helping to maintain production levels while also contributing to additional energy savings.
In sections of the greenhouse equipped with interlighting, toplighting is applied at a rate of 247 µmol/m2/s rather than the standard 350 µmol/m2/s. This adjustment delivers a further 1.5% reduction in energy consumption (Figure 5). While this percentage may seem modest, it can have a significant impact on the overall cost structure of cultivation, especially considering that LED lighting accounts for approximately 65% of total energy inputs.
Stay tuned
Both Raats and Lee consider the mid-year results to be very promising. These initial findings imply that a combined setup of toplighting plus interlighting not only makes it possible to maintain energy efficiency at the higher light level of 350 µmol/m2/s in low-heat conditions, but also improves the total yield.
By the end of this trial, it is expected that the researchers will also have an even greater understanding of the role of the nutrient balance in the overall nutrient uptake to resolve the blotchy ripening issues.
