3D modelling is set to play an ever greater role in research. It’s an important tool in the quest to design better LED modules, better greenhouses and better cultivation systems. With the calculations produced in this way, researchers can pre-test the designs and then try out the best options in greenhouse trials.
“Just as an engineer designs a new industrial product and tests it using CAD/CAM, we too can model the ideal plant and ‘grow’ it in a simulated greenhouse under various conditions,” says Pieter de Visser of Wageningen University & Research in the Netherlands. The simulated greenhouse can then be optimised, bringing the ideal crop ever closer.
New research questions
De Visser and his colleague Gert-Jan Swinkels receive requests from both colleagues in the Kas als Energiebron research project and suppliers of greenhouse roof materials to calculate which aspects of a greenhouse design can be improved in order to achieve a more productive crop. Issues they look at include how diffuse the light should be to achieve the best result in both summer and winter. De Visser can’t comment on the outcomes for businesses since this type of research is strictly secret.
3D modelling has been rapidly gaining ground at Wageningen over the past two or three years. The models have steadily improved over time, which is just as well: new greenhouse and agricultural technologies raise so many questions that it would take vast numbers of practical trials to answer them, at a time when budgets are being slashed. Understanding of plant processes has also improved greatly. This benefits the model calculations on the one hand, but it also raises a whole set of new research questions.
Temperature distribution in the crop
In recent years, for example, it has become increasingly clear that the shape of the plant is a very important factor, particularly in the early stages of the crop. So what is the ideal shape? Does a cucumber crop with smaller leaves perform better in winter? Are leaves in a horizontal position better? What root system is best? What aspects of plant shape can you steer using light colours? And is there any point in steering in this way if the LAI (leaf area index, or the total leaf area per ground surface area) is already at the optimum level?
With so many questions, the 3D calculations are an effective tool for separating the meaningful from the less relevant ones and sorting them by their potential outcomes.
“Utilisation of light is still very much at the top of the list,” de Visser says. “We are doing a lot of modelling in that area, both with natural and artificial light. But we have gradually started to focus more on temperature distribution in the crop. After all, changing the light often changes the way the temperature is distributed. And if you do a lot of tinkering with a greenhouse, you get other temperature gradients and the crop really does start growing differently. You invariably get places where the crop grows more slowly or more rapidly. We can identify those places.”
Another area being given increasing attention is the shape of the root system, both in open-field systems and in greenhouse crops grown in the ground.
The researcher is involved in a lot of light-related research at WUR’s Bleiswijk site, but commercial companies also call on his services. One example is Philips, who have developed a new type of higher-performance LED based on his 3D calculations. The greenhouse trial with the new LEDs in Bleiswijk was screened off on all sides to prevent people from looking inside.
But de Visser is happy to share some general principles for improving LED lighting: “With LED interlighting, it’s easy to work out the optimum height of the modules. We now know that the light incidence on the upper and lower surfaces of the leaf impacts differently on photosynthesis and growth. That is something we never used to take into account. Another important factor is the LEDs’ emission patterns. With a Lambertian distribution (evenly decreasing light output to the side), the modules are positioned quite low down. With an optimised emission pattern, they are hung half a metre higher and the plant makes better use of the light. There is also less light loss.”
Better models lead to new insights
Light loss is an important criterion, particularly in the Winterlight greenhouse. In winter it may make sense to grow a more open crop. According to de Visser’s calculations, the aisles should then be as narrow as possible. It also makes sense to space the plants slightly further apart in the row so that they are evenly distributed over the surface.
“Another important issue is light penetration,” he says. “When plenty of light reaches the lower leaves, they photosynthesise better. But the more light there is at the bottom of the crop, the more falls on the aisles between the rows and is therefore wasted. So increasing light penetration is not always better.”
The improved model calculations are leading to new insights. Three years ago, the researcher was able to demonstrate that interlighting was more effective than top lighting because the light from top lighting reflects off the crop and therefore can’t be used for assimilation. But the picture has since become even more nuanced. “The model shows that lighting from above delivers slightly more photosynthesis than interlighting, provided you can limit the light loss,” de Visser says. That loss can consist of reflection but also of light that falls unused on the ground.
A particular criterion to consider is the colour of the light, such as the ratio between red and far-red light. In a trial with far-red light in tomatoes, the model calculations supported the conclusion that the increase in production under far-red light was to a very small extent due to slightly higher photosynthesis, to a slightly greater extent due to the changed plant shape, and largely due to dry matter being distributed differently, probably as a result of hormonal changes. Hormones can’t yet be modelled, by the way.
In addition, the trials in which red light was alternated with pure green or blue light for a few hours were also analysed with model calculations. “The colour changes the shape of the plant,” de Visser says. “This has a dramatic effect on light interception to begin with, but there is almost no difference at all above a LAI of 3. What’s more, green is absorbed less than blue but green delivers more photosynthesis, so there is virtually no difference between green and blue at the crop level. Red scores much better, both in terms of absorption and photosynthesis, so red light is the best choice for assimilation lighting.”
The model constantly needs to be updated with new knowledge, so measurements have to be taken on plants on an ongoing basis. Not enough is known about green light as yet to enable everything to be predicted on the basis of a model, for example. De Visser again: “We calculate the results before the greenhouse trials, and this enables our colleagues to structure the trials better. An important question is what plant shape is best for intercepting light from LEDs. We are currently studying this in a project in collaboration with Bayer CropScience. You need to know what the ideal plant is and whether a new technique could help achieve it. So the model informs the trials and the trials inform the model.”
New greenhouse and agricultural technologies are producing so many options that it is impractical to investigate them all in greenhouse trials. Calculations with 3D computer models act as a filter so that only the most promising options are studied in the greenhouse setting. The main focal points are the shape of the plant and its roots, utilisation of light and temperature distribution in the greenhouse and the crop. One example being studied is the optimum position and radiation of LED modules.
Text: Tijs Kierkels.
Images: Wilma Slegers and WUR.
A trial with hybrid lighting (SON-T + LED) at Dutch tomato nursery Gebroeders Koot has yielded good results. The LED lamp used in the trial, which was developed on British soil with Dutch input, offers several advantages. One stand-out benefit is its clever design which makes it easy to integrate into existing SON-T installations.
Yields up by more than nine percent after seven months (weeks 48-26). That was the auspicious outcome of a greenhouse trial at Prominent growers Gebroeders Koot in Poeldijk, the Netherlands, where a tomato crop grown under 150 μmol/m2/sec SON-T grow light was compared with an identical crop supplemented with 58 μmol deep red with a little blue LED light. Geert Koot, who had had no previous experience in growing under grow light, was very impressed. “I hadn’t expected the higher light level to make such a difference,” he says. “That will appeal to a lot of growers. The same goes for the lamp itself, which has a surprisingly simple design. It’s fully interchangeable with SON-T, so it fits seamlessly into an existing system.”
“A lot of thought has gone into the functional design,” cultivation specialist Maarten Klein adds. He and his assistant, Tim Valstar, oversaw the trial, which was run on behalf of the British LED manufacturer Plessey. Klein, who has had a lot of experience with grow light, developed this lamp in collaboration with the technology company.
“Most LED systems are difficult if not impossible to integrate into existing lighting installations,” Klein continues. “Growers looking to switch to hybrid lighting currently have to install a whole new system alongside their existing one, often with extra C profiles. That pushes up the cost and results in more light interception, which causes problems all year round. Plessey Semiconductors in Plymouth wanted to eliminate these problems.”
To test the practical value of the lamp in the greenhouse setting, Klein approached several Dutch nurseries. In addition to Gebroeders Koot, trial setups were installed at nearby alstroemeria and gerbera growers and a pot plant nursery.
Although Gebroeders Koot were not growing tomatoes under artificial lighting, they did have a SON-T system in place in a section that had previously been let to another grower. These 1000W lamps supplied 151 μmol/m2/s extra grow light and, of course, the usual radiated heat. LED lamps were added in one bay, ramping up the artificial light level to 209 μmol.
Tim Valstar assisted with the trial and, together with Geert Koot, took measurements in the trial and reference sections. All the relevant crop and fruit features of the variety grown, Brioso, were recorded, varying from growth rate and stem thickness to leaf size, leaf colour, fruit weight and Brix value.
The plants arrived in the greenhouse in week 46. “That’s later than the usual for an artificially lit Brioso crop – they would usually go in in mid-October – but the lighting period was long enough to get a reliable impression of any differences,” Koot says. “The plants developed well in both light environments. But the plants under the higher light level were that little bit stronger with slightly thicker stems and more dark green leaves.”
Due to the extra vigour, the plants under the hybrid lighting regime held the first trusses for longer and they were harvested a few days later than those in the reference sections. The higher yield potential quickly expressed itself in a higher average fruit weight. To maintain the desired fineness, one fruit more was kept on the truss (11 instead of 10) from the tenth truss onwards, without the plants forfeiting vigour.
Valstar: “After week 26 we stopped taking measurements and were able to take stock.” The harvest under the hybrid lighting regime was 38.32 kg per m2 compared with 35.04 kg under SON-T. That represents an increase in yield of 9.35%. The average fruit weight was also slightly higher than under SON-T, at 39.2 grams compared with 38.8 grams.
The attractive increase in yield can’t be ascribed solely to the higher light levels in the periods when both systems were in use. The SON-T system was switched off and the CHP unit shut down for maintenance at the beginning of week 19, whereas the LED system was used from 4 am to 7 am for a further three weeks.
“The option to only use the LED lamps either end of the lighting season would be an extra benefit,” Klein says. “Those are often the times when you don’t need the radiated heat produced by the SON-T lamps. LEDs have virtually no impact on the climate. You can always switch them on if you need more grow light. And because they are much more energy-efficient than SON-T lamps, you also have more flexibility when it comes to deciding whether to generate the energy yourself with CHP.”
375 and 600W
Klein is keen to point out that the prototype trialled at Gebroeders Koot was developed exclusively for research purposes. But the lamp has since undergone further development and a commercial 375W version was launched at IPM 2017. All the LEDs are now in one bay and the fitting, which has integrated cooling ribs, can be attached directly to the trellis.
The lamp is called Hyperion 1000 because it has a photon flux of 1000 μmol/s. “Because of the higher uptake of deep red light, it’s the equivalent of a 600W SON-T lamp but it uses 40 percent less electricity,” the cultivation specialist says. “The producer has also recently brought out a more powerful 600W version which is the equivalent of a 1000W SON-T lamp.”
Ten years ago
There is a lot of added value in the new lamp, Koot believes. “It’s efficient, it has a broad spectrum, and its clever design makes it easy to incorporate into an existing system. That will appeal to a lot of growers. I’m also quite impressed. But because of my age and the fact that I have no successor in place, I have decided not to invest in any more grow lights now. If this trial had taken place ten years ago, I would almost certainly have gone for them. But we very much enjoyed taking part in the trial.”
A new type of LED lamp produced in the UK is achieving interesting results. The clever design makes the lamp particularly attractive. It can be attached to the trellis without the use of C profiles and can be integrated into existing 600W SON-T systems with standard connectors. A more powerful version equivalent to a 1000W SON-T lamp was brought out earlier this year.
Text and images: Jan van Staalduinen.
The shift towards closed growing systems is forcing growers to take a critical look at their sodium figures. Would higher sodium levels affect the crop? And could sodium be “harvested” and removed from the system that way? The answer to both questions is “yes”, initial Dutch research results indicate.
When the sodium concentration in drain water rises above 5 mmol/l, almost every grower will discharge the water. Ideas on acceptable concentrations are based loosely on the results of research carried out in the past, in which a very generous safety margin was applied. But nowadays we live in different times: zero-emission growing is getting ever closer and discharging drain water costs money. So now is a good time to take another look at the margins within which you can work safely. This was the background to the sodium study carried out by Wageningen University & Research in the Netherlands as part of the “Prevention and Control of Leaching from Greenhouses” research programme.
Sodium is not an essential element for the plant and can be toxic in high concentrations. It also competes with the uptake of potassium and calcium. Too much sodium in the irrigation water can cause blossom-end rot in fruiting vegetables by inhibiting calcium uptake.
“There are three input streams,” project manager Wim Voogt explains. “Sodium can enter in the water, with fertilisers and in some organic substrates such as coco. The crop absorbs some of it and the rest leaches out into the drain water. Some crops, such as cucumber, tomato, aster, carnation and gerbera, absorb a lot of sodium. Others absorb barely any, such as rose, orchid and sweet pepper.”
Drain water containing sodium can be reused providing the concentration is not too high. Voogt: “That begs the questions: Are the standards from the past still relevant today? And can you ‘teach’ the plant to handle salt?” The EC of the recirculating water partly stems from the nutrient solution and partly from ballast salts. “With tomato and cucumber, you need an EC of at least 1-1.7 mS/cm for the nutrient supply. But growers often work with an EC of 2.5-3, or even more for tomatoes. So there’s leeway for extra salt there,” he says.
To explore the limits, he first carried out a trial with sweet pepper, with tomato following this year. Sodium was added to the basic nutrient solution in increments rising from 2 to 10 mmol/l (10 mmol is extremely high and is regarded as unacceptable in practice).
But the surprising result was that the sweet peppers performed well even at the highest level (see figures 1 and 2). Voogt: “The yield per square metre, the number of fruits and the fruit weight remained the same at all concentrations. Because we anticipated problems with calcium uptake at high Na concentrations, with the associated higher risk of blossom-end rot, we increased the calcium level in the nutrient solution in some of the treatments. But even that turned out to be unnecessary. So our conclusion was that it is possible to grow with higher sodium levels. The Supervisory Committee for Research followed the study with a critical eye but didn’t see any problems. This year we will be looking at tomato with even more extreme values, up to 15 mmol/l.”
The second part of the research project looks at the question of whether you can “harvest” sodium. If you can store the element safely in the crop, it will ultimately end up in the composter and you will be rid of it. The more you can remove this way, the less you will need to discharge.
The plant can take up more Na if you allow the concentration to rise at the roots, but this inhibits nutrient uptake. So you need a workaround: a split-root system (SRS) (see diagram). One half of the roots gets the regular nutrient solution and the other half gets the drain water with rising Na concentrations.
Voogt again: “We know from previous research with these types of systems that water uptake drops as the EC rises, while nutrient uptake increases the more you supply, in other words, the higher the EC. Based on this idea, we want to develop a cultivation system in which half the roots are in a gutter with a normal nutrient solution and low sodium, allowing the plant to take up water and nutrients freely. The other half of the root system is in a gutter with rising sodium concentrations. The proportion of the drain water to be discharged is added to the second gutter.”
When presented with a high supply, the plant will take up high levels of sodium and there will be less remaining in the system. The researcher this year ran trials with tomato and cucumber with Na in a range of 0-15 mmol/l with two EC increments (2.8 and 4.2) in one half of the roots, and with no Na and an EC of 2.8 in the other. The results were surprising (see figures 3 and 4). “This way you can remove sodium from the system with no negative impact on growth. We certainly don’t have answers to all the questions yet, but we now have proof that the principle works,” the researcher says.
There is a catch, however. Despite the fact that the two halves of the root system are separate, sodium was found to have made its way into the other gutter. “It travels up the xylem and passes into the stem. Then it flows down again through the phloem and is secreted by the roots, but not in high enough amounts to reverse the removal effect. In net terms, a lot more sodium still finds its way into the leaves than is secreted,” he says.
Leave more leaves
To begin with, the growers on the supervisory committee were sceptical about how the system could be implemented in practice. But it is feasible, according to Voogt: “You only have to equip a small part of the greenhouse with a split-root system. That’s plenty. The annual costs aren’t too high and it’s a good way of reducing the amount of water that has to be discharged. The project still has a year to go, so we have plenty of time to flesh this out.”
The system should also be ideal for Mediterranean regions where irrigation water is often salty. But you can also harvest more salt without technical adaptations, he adds. “As long as leaves transpire, sodium goes into them. Tomato growers currently aim for 11-15 leaves on the plant to keep the leaf/fruit ratio constant. But if you leave the leaves on the plant for longer, you can get more sodium out of your system. That could be a reason to leave a few more leaves on the plant.”
Growing with higher sodium levels looks possible for sweet pepper. Trials with tomato will follow this year. A split-root system enables sodium to be harvested out of the system and removed with the crop. In this system, one half of the roots gets the regular nutrient solution and the other gets the drain water with rising Na concentrations. Both growing with higher sodium levels and storing sodium in the crop reduce the need to discharge drain water.
Text: Tijs Kierkels
Images: Wilma Slegers and Wageningen University & Research
Plants, insects, fungi and people perceive light colour and intensity via different organs and pigments.
The human eye is particularly sensitive to green light, while plants have various pigments that absorb light and control different processes. Insects are sensitive to light in a different way again. The advent of LED technology, with a wide range of light colours to choose from, opens up new opportunities for use in greenhouse horticulture.
But which combination of light colours is needed for optimum plant growth and development, and what effect does adding LED to the sunlight and high-pressure sodium spectrum have? Does using LED lighting on its own produce other reactions in the crop? And what does this mean in terms of plant cultivation cells in urban farming? Does growing plants using only LED lighting enable you to produce vegetables and flowers without using gas (i.e. fully electric)? These are just some of the questions that arise when considering the ways in which LED lighting could be used. The Denkkader Licht (Thinking about Light) project looks at these opportunities and uses.
Intensive lighting in winter is common in Phalaenopsis growing. A long-running Dutch research project is seeking to answer the question of whether less lighting could be used at that time of year. The trials in the first three years revealed that turning the lights up or down could cut electricity bills by as much as 30% without loss of quality or production. The results from the fourth trial year, with practical trials at Ter Laak Orchids, show that the limit has been reached in terms of quality – at least in the top segment.
The trials run in 2014-2016 by the specialist Dutch research companies Plant Lighting and Plant Dynamics, with support from growers, delivered some surprising insights. One of these was that timing is more important than the light sum in the vegetative and generative phases. They also discovered that a long day of 16 hours produces more CO2 uptake than a day of 11½ hours. Dimming the lights at the beginning and end of the lighting period seems to be possible without loss of production or quality. That generates electricity savings of more than 30%.
Dimming in cooling phase
Based on these results, two follow-on studies were run in the winter of 2016/2017. The first took place in climate chambers equipped with daylight simulators and SON-T lights from Plant Lighting in Bunnik, the Netherlands. In these chambers, the dimming treatment, which can cut electricity usage by up to 30%, was also applied in the cooling phase for the first time. In another room, the plants were lit in line with the biorhythm, starting at 05:00 instead of 01:00. This can save as much as 43% in electricity because it makes better use of free daylight.
Researcher Sander Hogewoning explains: “The CO2 uptake was the same in all three treatments. Yet something caught our eye: CO2 uptake ended relatively late: it didn’t stop until 2½ hours after the lights went on. So the plant rhythm is not always the same, and we have no idea why that is. This indicates that it is important to keep on taking measurements with sensors, otherwise you are taking a risk. We also noticed that the plants were a week behind in the treatments with dimmed light. That can be explained by the fact that the plant temperature was 0.5°C lower on average because the SON-T lamps were used less. In practice, you would compensate for that by turning up the heat. Although the percentage of double spiked plants was just as high, the percentage of branched ones was lower. With these treatments, you’re reaching the limits in terms of quality.”
The limits were explored and found in the second trial as well. This study took place in the Ter Laak Orchids trial greenhouses in Wateringen in the west of the Netherlands, in both the vegetative and cooling stages. The generative stage took place in the production greenhouse. The researchers and growers chose four varieties for the trials: Sacramento, Donau, Jewel and Las Palmas. Martin van Dijk of Ter Laak: “We grow more than 100 varieties here, so we wanted to know what effect a different lighting regime would have on different varieties. The quality and the number of double spikes must remain the same. That’s essential for us.”
In the one 80 m² trial greenhouse, the plants were lit for the usual 16 hours. The plants in the other trial greenhouse were also lit for 16 hours but with the dimming treatment used in the previous trial. The only difference was that the start was delayed until 03:00 in order to make better use of the daylight. To check the quality, the root weight and above-ground weight were measured three times. At the end of the generative stage, the researchers counted the number of double spikes and the number of flowers.
The same or marginally lower
The results? The quality of the plants was the same or marginally lower. With the dimming treatment, the roots in three of the four varieties were lighter than in the control treatment, although the weight of the leaves and flowers was comparable. The number of flower buds was also the same.
Another indicator of quality is the percentage of multiple spikes. With the dimming treatment, only Jewel showed significantly lower results by the end of the generative stage. The percentage was slightly lower in the other two varieties but not to a statistically significant extent. The differences are not massive, but they do show that the limits of dimming were reached in this trial as well.
Hogewoning: “My conclusion after these trials is that dimming saves a lot of electricity and produces the same or slightly lower quality. We advise growers in the top segment not to push the boundaries when looking for savings but to stop a little way from the limit. However, the quality differences are small. Growers with fewer lamps will find that switching the lights on later saves them money. By making better use of free daylight, they will reach their light sum more easily at the time of day that is most important for the plants. And finally, bear in mind that any differences in quality are very much magnified because we are simulating winter for 30 weeks of cultivation. In reality it’s not always December.”
With the benefit of hindsight, the growers involved are making various choices depending on the capacity of lamps, but also depending on whether they have a CHP plant or have to buy in electricity, which is expensive. At Ter Laak Orchids they are biding their time. Van Dijk: “With the results of the study and the experience we gained last year, we are waiting to see what the results of next year’s sensor study will be. This winter we plan to turn the lights up and down incrementally, but starting at 01:00 as normal.”
Honselersdijk-based Levoplant has been switching the lights on later since last year. Cultivation manager Erwin van Vliet, a long-standing member of the supervisory committee: “We used to start at 01:00 and we would stop suddenly at 16:00, sometimes even earlier. Now we start at 04:00 in October and finish at 19:00. In November and December we start at 03:00 in order to achieve our light sum. We also turn the lights up and dim them incrementally. That works very well for us because it makes the climate in the greenhouse more uniform. An additional advantage last year was that we had less of an issue with premature spiking. The quality is every bit as good as before.”
Are these insights resulting in energy savings? Van Vliet: “We are not saving as much as in the study. We are lighting for longer, although we are saving energy by dimming. Before the study, the trend was heading towards 100% lighting for 16 hours. But we now know that really isn’t necessary. So we need to continue to develop our knowledge – by working together.”
The Pot Orchid Growers Cooperative this year invested in the development of robust, affordable sensors to provide ongoing information on the plants’ light usage. Van Dijk and Ter Laak are certainly convinced of the benefits. “We are installing a wireless network in the new Daylight Greenhouse we are building to allow for the use of wireless sensors in the future.”
The fourth year of the research into lighting Phalaenopsis in winter has confirmed the previous years’ results. The orchid needs a long day, but that can be achieved by gradually turning up and dimming the lights in the vegetative, cooling and generative stages. Switching on the lights later saves more electricity. The quality of the plants in the dimming treatments is the same or marginally lower. The limits of the savings thus seem to have been reached.
Text: Karin van Hoogstraten. Images: LD Photography.
Light plays an important role in demanding crop science research applications. A lot of research requires accurate replication of real-time outdoor light conditions to achieve different goals.
But it is impossible to replicate those conditions to the extent to which data collected indoors would be relevant. LightDNA was created to solve this problem. The system consists of Valoya’s latest technology: the 8-Channel Light, a high-power LED fixture with eight channels of light, an internet connected microcomputer and both local and cloud-based software for processing the light data.
Optimised LED configuration
The configuration of LEDs is optimised to meet outdoor light conditions with 90% or higher accuracy (380-780 nm range).
Other advantages are: light-weight, compact design suitable for various applications; strong light intensity up to 2000 µmol/m2/s with uniform light output; low heat emission with active cooling system and energy saving.
Stand number: 12.424
With the second crop in the Winterlight greenhouse at the Energy Innovation and Demo Centre (IDC) in Bleiswijk (NL) coming to an end, it’s time to draw some initial conclusions. The predominant feature of the greenhouse is its extremely high light transmittance.
Growers don’t only stand to gain from this in the winter but in the dark autumn months too: the 10%-plus light gain the designers were aiming for has turned out to be a reality. This not only means that all the partners involved in the project did a fantastic job, but also that the models used in the design process, such as RAYPRO, have proved their worth.
On the crop side, the two high-power crops we grew also yielded good results, despite the thrips problems we had in the first crop. With a few growing weeks to go, the tally is currently 268 cucumbers with an average fruit weight of 407 grams, bringing the total yield to more than 109 kg/m2. We are pleased with the outcome on the energy front, too. In this greenhouse, which is single glazed and has two high-transparency screens and a dehumidifier with heat recovery, we used less than 20 m3/m2 gas between the end of December and mid-November. But this did mean that we had to buy in around 13 kg of CO2.
The Greenhouse Horticulture and Bulbs, Trees and Fruit business units are launching an Evergreen project to carry out research into bulb production using fewer crop protection products.
Plant growth promoting rhizobacteria can facilitate plant growth directly by supplying the plant with nutrients (nitrogen, phosphate and minerals) or indirectly by inhibiting the growth of plant pathogens by producing antibiotics and siderophores. These bacteria can offer a green alternative to chemicals provided they are able to colonize the rhizosphere of the bulbs in a field situation.
In this project we are investigating the effect of two strains of rhizobacteria on growth stimulation and on disease suppression of a fungal pathogen (Fusarium) that is common in tulips and daffodils. We are also investigating whether rhizobacteria colonisation can be encouraged by adding extra substrate (whey) to the soil.
We launched a container trial in November 2017. For the experiment, naturally contaminated soil was collected from a grower. We will be repeating the trial in a field situation next year to test whether colonisation of these bacteria in the rhizosphere is successful and replicable.
We know much more about photosynthesis than about the flowering of the plant. This sometimes leads to surprises, especially with new crops. The grower has to take into account the juvenile phase, effect of temperature, light, size of the plant, day length, and the interaction between hormones, sugars and other compounds in the plant.
Before a plant can flower it first has to become an adult. Many plants have a juvenile phase. Even under optimal conditions they are unable to flower during this stage.
This is logical because a plant flowers in order to reproduce. Therefore the flowers must be of sufficient quality to actually achieve this. They have to be developed to the extent that they can be pollinated, for example by insects. And after pollination all kinds of processes need to start for the fertilisation and development of seeds and fruits. All of this costs a lot of energy. So from the plant’s point of view it’s considered ‘wise’ to postpone these processes until sufficient assimilates are available in the plant.
From juvenile to adult phase
The duration of the juvenile period varies enormously, from a few days to a few decades in trees. Of course, for the grower this can be very unprofitable if you have to wait a very long time for it to become productive. That’s why it’s good that a cutting or graft taken from a plant that is already in the adult phase also remains an adult.
The switch from juvenile to adult phase happens quite abruptly. The moment at which this occurs can depend on the size of the plant, age, number of leaves and growth factors.
From a wide range of research it’s clear that a hormonal factor plays a part in the transition from vegetative to generative. Suddenly the apical bud changes in shape as a forerunner to flowering. For a long time researchers looked for a hormone that stimulated flowering. The unknown flowering hormone was even given a name, namely florigen. But, it is now clear that florigen does not exist.
Although gibberellins play a role in many plants – this group of hormones was for a long time the leading candidate for the role of florigen – the situation is still ambiguous. In some plants gibberellins actually slow down flowering. Bearing this in mind, it’s also remarkable that growth inhibitors, such as daminozide, that slow the activity of gibberellin, do prevent the long and thin development of flowering plants, but not the flowering itself.
Another hormone group, the cytokinins, plays an important role in the induction of flowering. But again no general rules apply.
It seems that an interaction between hormones, such as gibberellins, cytokinins and ethylene, as well as sugars and other substances, such as polyamines, causes the induction of flowering. It’s different for every crop. The limited knowledge about the mechanism of flowering makes it difficult to effectively influence flowering. This is especially the case for new ornamental crops. Usually, the practical research focuses on achieving the most appropriate cultivation measures, without knowing exactly what happens inside the plant.
Leaves under first truss
Fortunately, a lot of research has already been done on the major horticultural crops. One of the many crops examined is tomato. A grower would like the plant to start producing quickly, and in terms of the tomato this means: the number of leaves under the first truss has to be limited.
In theory, a certain amount of assimilates must first be present in the tomato plant before it can start to flower. Indeed, research shows that any procedure taken to increase the amount of assimilates speeds up flowering. More light means fewer leaves under the first truss. A higher temperature at a low light intensity also leads to more leaves under the truss because the plant consumes more energy at a higher temperature.
As well as having a minimum quantity of assimilates, distribution is also important. At a lower temperature the top of the plant – the apex – has an advantage as it competes with the leaves.
This knowledge is difficult to translate into other crops. In fact the influence on flowering should be examined separately for each crop.
Short day = long night
A special phenomenon is the sensitivity of a flower to day length. In this respect, the origin of the plant makes a big difference. At the equator the length of day and night are the same and tropical plants are not day length sensitive. Plants from higher altitudes, that flower in the spring or even in the autumn, do tend to be sensitive to day length.
Sensitivity to day length exists as a result of natural selection. Therefore it’s also possible to remove this sensitivity by selection. By consistently selecting and further propagating the most insensitive plants it’s possible to solve this inconvenience. This doesn’t work sufficiently well with all crops, so we do encounter short day plants such as poinsettia, chrysanthemum and kalanchoe and long day plants such as gypsophilia, trachelium and carnation.
The naming is actually wrong. A short day plant is actually a long night plant because it’s all about the length of the dark period. And if this is broken – even for just a very short period – the whole effect of the dark period is lost.
Length of dark period
The plant registers the length of the dark period in its leaf but flowering takes place elsewhere. Therefore there has to be some communication between the leaf and the point where flowering occurs. This is carried out by a hormone that is produced in the leaf and then travels to the point of flowering.
How does the plant measure the length of the dark period? Previously researchers thought that the pigment phytochrome slowly broke down during the night into another form and that this was a signal to the plant to start flowering. But it’s more complicated than that. There is an interaction between the endogenous rhythms in the plant (‘the biological clock’). As a result the same length of darkness can sometimes produce different effects, whereby temperature can also play a role.
Some short day plants need just one long night. One of the most well known short day plants in horticulture is the chrysanthemum and it actually needs several weeks of long nights. If a grower stops the dark period prematurely, abnormalities occur. Just after a few short days the growth point becomes generative and stops producing leaves. Yet a grower has to continue with the long night regime for several weeks. It’s likely that multiple genes are involved in the flowering of chrysanthemum and it’s not simply a transition from vegetative to generative based on one gene that can be turned ‘on’ or ‘off’.
Once the plant has switched from vegetative to generative and then flower buds have actually formed, many things can still go wrong. The buds can dry out or fall off and the flower may not open properly. This is mostly a question of how well the flower bud and the flower have been supplied with water, minerals and assimilates.
The flower has to compete with other parts of the plant and sometimes loses the fight. Optimal climate conditions, providing enough light and water, reducing the competition with the young leaves (by picking leaves) are all ways to ensure that flowering is successfully achieved.
A plant can only flower when it is mature. In horticulture, we bypass the juvenile phase by using cuttings and grafting. The transition from vegetative to generative appears to be controlled by a hormones. Flowering is the result of an interaction between several substances, for example, gibberellins. We still know too little about flowering which is sometimes difficult when working with new crops. A lot of research has been carried out on the major horticultural crops such as tomato and chrysanthemum. The latter is the best-known short day plant, although we should call it a long night plant.
Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Images: Theo Blom (University of Guelph, Ontario, Canada).
The Precision Horticulture Programme was launched this year. It is a form of horticulture in which plants receive precisely the treatment they need.
In traditional horticulture, what happens is usually determined on the greenhouse scale, whereas in Precision Horticulture the need is determined per square metre or even per plant. This enables the grower to generate maximum yields in terms of quality and quantity, predictability and sustainability with minimal losses at the local level with minimal inputs (fertilisers, crop protection products, energy and plant material).
Precision Horticulture uses advanced technology such as location recognition, sensors to observe crop characteristics and applications to perform precision crop and dosing operations.
Artificial intelligence is used to establish relationships between observations and desired action. The programme was launched under the Horticulture and Breeding sector and consists of a collection of projects for developing the necessary technology and applications.