The invention is directed to a method for growing plants. In addition, the invention is directed to assimilation lighting systems comprising solid-state light (abbreviated as SSL) sources to be used in such methods.

Traditional assimilation lighting systems are based on sodium or metal halide lamps to illuminate plants. In the recent period, such assimilation lighting systems have been replaced by solid-state light sources (e.g. LED-based), primarily to save the amount of energy required to provide the plants with sufficient light.

Besides the energy saving aspect, solid-state lighting system also enable optimization of the spectral distribution of the light provided to the plants.

Examples of such lighting systems are described in US 2010/0020536 and WO 2009/022016, wherein lighting systems comprising a blue light source and a red light source. Typically, a blue light source provides light of a wavelength in the range of 375 to 500 nm, while the red light sources provides light of a wavelength in the range of 600 to 750 nm. This spectral range is beneficial for plant growth as red light is very efficient for photosynthesis while blue light stimulates for instance stomata opening and induces chloroplast development. In a process called non- cyclic photophosphorylation two photosystems (PSII and PSI) are working together to transport an electron towards creating NADPH, each requiring a number of photons of 680nm (PSII) or 700nm (PSI). Furthermore PSI and PSII contain several pigments that use the mechanism of stokes shift to catch and transform a higher energy photon to a photon that the respective photosystem can use to transport the electron. These complexes of pigments are called light harvesting complexes.

Since PSI and PSII function in series, it is efficient to illuminate both systems in a balanced ratio. To provide an appropriate balanced irradiation of PSI and PSII, it has been suggested to complement the assimilation lighting with far- red light having a peak wavelength of 730 nm. Based on the Emerson-effect, it is believed that far-red light only stimulates PSI (PSII being insensitive to such light). The present inventor realized that a drawback of using such far-red light however, is that plants can poorly, if at all, absorb this light. Since the far-red light source is generally replacing another light source (e.g. a red light source), it may even be less efficient to complement the assimilation lighting with the far-red light.

In addition, far-red light having a peak wavelength of 730 nm has also been used in the art to influence the phytochrome balance in the plants.

Phytochrome is a photoreceptor present in most plants, which is controlling many aspects of the plant morphology such as (but not limited to) stem elongation, detection of day/night cycle and flowering. Phytocrome comes in an active (Pfr) and an inactive (Pr) form, and activation happens by light. The inactive form of phytochrome is sensitive to a broad range of wavelengths of light, but Pr has an absorption peak at 660 nm. Once activated, the active (Pfr) form will revert back to the inactive form after a period of darkness, but can also be inactivated by far-red light with an absorption peak at 730nm. The ratio between active and inactive phytochrome is important for plant development. A measure for the ratio between active and inactive phytochrome is called the Phytochrome Photostationary State (PSS). The light absorption of Pr and Pfr are different for each wavelength (see J.C. Sager et al. Transactions of the ASAE, 31 (1988) 1882-1889). In the art, far-red light having a peak wavelength of 730 nm is used because Pfr has its highest absorption peak at that wavelength, while at the same time Pr has almost no absorption at that wavelength, so the PSS value is very sensitive to changes in the amount of 730 nm light. For example, cucumbers can be grown with an LED spectrum of 5% blue, 80% red and an additional 15% of far-red (730 nm). The 15% far-red is used to create a more desirable PSS value, and thus a more beneficial plant development. However, a drawback of this approach is that the far-red replaces the red and blue LEDs such that eventually less light will be available to the plant for photosynthesis.

It is an object of the present invention to provide an improved assimilation lighting system. In particular, it is an object of the present invention to provide a system that can provide a balanced illumination of PSI and PSII and/or a desired PSS value, while maintaining efficient lighting by limiting or preventing illumination with light of wavelength outside the absorbance of the plant.

Accordingly, the present invention is directed to an assimilation lighting system for growing plants, said lighting system comprising at least a deep-red solid-state light source adapted for emitting light having a peak wavelength in the range of 680 to 710 mn.

The present inventor realized that it may not be required to provide light that can only be (in part) absorbed by PSI (and not by PSII), but that it is sufficient to provide light that results in a higher absorption yield of PSI compared to the yield of PSII (herein also referred to a PSII/PSI yield ratio). It was realized that as such, the PSII/PSI yield ratio can be influenced while maintaining an overall good absorbance of the light. Accordingly, it was advantageously found that in accordance with the present invention, the deep-red light results in a higher yield of PSI than of PSII and that as such, the illumination of PSI and PSII can be balanced, without sacrificing or spilling emitted light that can not be absorbed by the plant, resulting in an overall improved efficiency (see also Laisk et al.,

Biochimica et Biophysica Acta 1837 (2014) 315-325). The deep-red light is thus believed to balance the desired ratio of PSII/PSI yield ratio and a good absorbance of the light by the plant. Accordingly, in a preferred embodiment, said deep-red solid-state light source is adapted for emitting light having a peak wavelength in the range of 680 to 695 nm, preferably in the range of 682 to 692 nm such as about 690 or 685 nm. At a peak wavelength of 685 nm, the PSII/PSI yield ratio is about 0.65.

Moreover, the present inventor found that the deep-red solid-state light source in accordance with the present invention can influence the PSS value while remaining in the spectral range that plants can use for photosynthesis, i.e. the photosynthetically active radiation (PAR) spectrum. For example, according to the present invention it is possible to influence the PSS value with the deep-red solid- state light source because the Pr absorption is less at this wavelength than at a lower wavelength (e.g. at a wavelength in the range of 640 to 675 nm), while the Pfr absorption is higher at a wavelength in the range of 680 to 695 nm than in the range of 640 to 675 nm. Favorably, 680 to 695 nm is still well within the PAR spectrum.

Accordingly, an aspect of the present invention is the use of the assimilation lighting system for providing a balanced illumination of PSI and PSII and/or for influencing the phytochrome photostationary state (PSS) value of one or more plants. The light emitted by solid-state light sources is generally not of a single wavelength but it generally has a small distribution or emission spectrum of wavelengths. This may due to a mix of different sources that each have slight physical and chemical variations in the composition and or by interactions of the emitted light. The solid-state light sources as used in the present invention, each have their own spectrum and hence each have their own peak wavelength.

However, parts of the spectra may overlap. The peak wavelength of a solid-state light source according to the present invention is the wavelength where the spectrum reaches its highest intensity.

The deep-red solid-state light (SSL) source preferably has a relatively narrow emission spectrum to efficiently utilize its energy input. Accordingly, the deep-red SSL source preferably has a spectrum of which 85%, preferably 90%, of the emitted photons have a wavelength within a range between -30 and +30 nm of the peak wavelength. In addition, the deep-red solid-state light (SSL) source preferably has a spectrum wherein at least 70% of the emitted photons has a wavelength below 700 nm.

The deep-red solid-state light source (herein also referred to as the deep-red light) is typically complemented in the assimilation lighting system by other light sources. These other light sources can be traditional sodium or metal hydride lamps, but this is not preferred. Preferably, the system only comprises solid-state light sources as the light source or sources. Typical solid-state light sources include semiconductor light-emitting diodes (LEDs), organic light-emitting diodes (OLED), perovskite light-emitting diodes (PeLEDs), polymer light-emitting diodes (PLED) or a combination thereof. These types of light sources can also be used for the deep-red light source of the present system. Preferably, LEDs are used for the deep-red light source, more preferably for all light sources.

For an optimal use of the deep-red light it is preferably mixed in a particular ratio with the other light sources in the system. In a preferred embodiment, which gives good results in terms of plant growing yield, the deep-red solid-state light source is adapted to emit light of a relative intensity of up to 50%, preferably in the range of 15 to 30%, more preferably in the range of 20 to 25% such as about 22%, based on the total amount of light emitted by the lighting system. The complemental solid-state light sources can comprise solid-state so- called“white-light” sources, blue-light sources, red-light sources or a combination thereof. Examples of solid-state“white-light” sources include single light-emitting diodes (LEDs) light sources with one or more phosphor materials that partially or fully convert the LED emission (phosphor converted LEDs). Preferably, the solid- state white-light source is adapted to emit light in the range of 375 to 680 nm, as this matches the absorption spectrum of plants. In a further preferred embodiment, the solid-state white-light source has a peak wavelength in the range of 375 to 500 nm and having a peak wavelength in the range of 550 to 650 nm.

In Figure 1, the emission spectrum of a particular embodiment of the invention is compared to a conventional white-light and red-light system, the latter having a peak wavelength of about 660 nm. By substituting the red-light source in the system with a deep-red light source having a peak wavelength of about 685 nm, an emission spectrum is obtained that is optimized to (more) selectively activate the PSI.

Alternatively, or in addition to the white-light source, the deep-red light may be complemented by blue-light sources, red-light sources or a combination thereof. Accordingly, the assimilation system may further comprise

a) a blue solid-state light source adapted for emitting light having a peak wavelength in the range of 375 to 500 nm; and/or

b) a red solid-state light source adapted for emitting light having a peak wavelength in the range of 600 to 680 nm.

In a preferred embodiment, said blue solid-state light source is adapted for emitting light having a peak wavelength in the range of 425 to 475 nm, preferably in the range of 440 to 460 nm such as about 450 nm; and/or said red solid-state light source is adapted for emitting light having a peak wavelength in the range of 640 to 675 nm, preferably in the range of 650 to 670 nm such as about 660 nm.

In Figure 2, the emission spectrum of a particular embodiment of the invention is compared to a conventional blue-light and red-light system. By complementing the system with a deep-red light source having a peak wavelength of about 685 nm, an emission spectrum is obtained that is optimized to (more) selectively activate the PSI thereby obtaining a better PSII/PSI balance. Notably, all emitted light has a wavelength of less than 710 nm, therewith minimizing or preventing spilling of emitted light.

The correct emission spectrum of the light sources can be set using conventional techniques, e.g. by influencing the desired band gap of solid-state light sources. For instance, blue-light LEDs can be based on GaN while the red- light and deep-red light LEDs can be based on AlGalnP.

For the embodiment wherein the lighting system comprises the blue- light, red-light and deep-red light sources, it was found that particular good results are obtained when said blue light solid-state source is adapted to emit light of a relative intensity up to 25%, said red light solid-state source is adapted to emit light of a relative intensity in the range in the range of 45 to 97%, and said deep- red solid-state light is adapted to emit light of a relative intensity of up to 50%.

In yet a further preferred embodiment, the intensity ratio of the light sources based on the total amount of light emitted by the system may be as follows: a) said blue light solid-state source is adapted to emit light of a relative intensity in the range of 3 to 15% such as about 10%;

b) said red light solid-state source is adapted to emit light of a relative intensity in the range of 55 to 97% such as about 65%; and/or

c) said deep-red solid-state light is adapted to emit light of a relative intensity in the range of 5 to 30%, preferably in the range of 20 to 25% such as about 22%, based on the total amount of light emitted by the system.

The intensity ratio of the various light sources can be set in various ways. In case the system comprises separate units comprising one or more of said various light sources, switching on or off one or more of these units can change the overall emission spectrum. In a particular embodiment of the invention however, the assimilation lighting system comprises a light unit comprising all of said light sources. This is preferred for each of installation and control of the system.

Moreover, it may allow for an even distribution of the emitted spectrum over the plants. The even, homogeneous emission be achieved in an embodiment wherein each of said three light sources comprises a plurality of light sources, which light sources are essentially evenly distributed over a mount of the unit onto which the sources are mounted. A homogeneous distribution is particularly preferred for illumination of the plants at close distance. At a longer distance, for instance in a greenhouse wherein the plants may be illuminated by a grid of light units, a homogeneous distribution of the light sources in each individual unit may be of less importance since each light unit is relatively small an may be considered as a point source.

Unless indicated otherwise, light intensities (including relative intensities of the various light sources) are herein expressed or based on photon flux. The intensity of the light sources can thus be expressed in mmol photons per second ( mmol/s). By setting the amount of individual light sources in a particular unit, the intensity ratio of the light sources can be set as well as the light intensity of the unit can be influenced. Typically, a light intensity per unit in the range of 1500 to 3000 mmol/s such as about 2000 mmol/s is preferred when units are mounted on the roof structure of a greenhouse. A lower intensity would require more units per surface area. For applications such as vertical farming, an light intensity of 100-200 mmol/s is preferred because the close proximity to the plants. It may be appreciated that for the application of the light units in greenhouses, the actual light intensity applied is generally lower. For instance, for growing certain lettuce species, maximum 75 mmol/s/m ² is typically applied such that one light unit per 27 m ² suffices. Other plant species however, e.g. cannabis plants may require much higher light intensities such as about 600-700 mmol/s/m ² . The light unit of the present invention can provide sufficient light to illuminate all types of plants, even in the full absence of sunlight.

In a preferred embodiment, the intensity ratio of the various light sources can be controlled by a controller that is adapted to individually control the light output of each light source. In this particular embodiment, the system thus further comprises a controller suitable for independently controlling the light output of the deep-red light source and the other light sources, for instance the white-light, blue-light and/or red-light sources if present. The controller can function by selectively switching off or on a number of individual light sources in the unit (if the unit comprises a plurality of light sources). Alternatively, or additionally, the controller can be adapted to independently dim the light output using dimming method known in the art.

The system in accordance with the present invention preferably further comprises a means for cooling the light sources. As is described in WO 2009/022016 (which is incorporated herein in its entirety), solid-state light sources function particularly well upon cooling (in contrast to sodium or metal hydride lamps). In a particularly preferred embodiment, the means for cooling comprise a passage such as a tube adapted for a cooling fluid flowing through the tube. The fluid may be air or another type of gas, but is preferably a liquid such as water. In addition to the passage, the means for cooling may also comprise other means such as one or more heat sinks. In a particularly preferred embodiment, the unit comprising all light sources as described heroin-above, comprises a mount comprising an aluminum heat sink that is in heat connection with a passage adapted for a cooling fluid flow.

In another aspect, the present invention is directed to an enclosure for growing plants comprising the assimilation lighting system according to any of the previous claims. The enclosure may comprise a greenhouse further comprising one or more windows for admitting sunlight in the greenhouse to complement the lighting system of the invention. Sunlight also has inter alia a wavelength in the range of 680 to 720 nm and if the intensity of the sunlight is sufficient (e.g. during daytime in summer), it may not be required to illuminate the plants in the greenhouse at full power, if at all, with the deep-red light source. In such case, illumination of plants by the assimilation lighting system can accordingly be reduced. At other moments, in in the absence of sufficient sunlight, for instance in during twilight, dusk, night, dawn or daytime in winter, said illumination of plants is carried out in part or fully by the assimilation lighting system.

In another embodiment, the enclosure comprises an indoor farming facility and/or vertical farming facility. Such facilities differ from greenhouse in that, although they may comprise one or more windows as well, these windows are typically not specifically meant to provide sunlight to the plant as is the case for the greenhouse. If any windows are present at all, the windows do not provide sufficient light for plants to be farmed in the facility. For this reason, assimilation lighting is required at all time, independent of the amount of sunlight outside the facility.

Yet another aspect of the invention is directed to a method for growing plants comprising illuminating of plants by the assimilation lighting system. In a particular embodiment, the plants are illuminated by the system in the absence of sufficient sunlight, for instance in during twilight, dusk, night, dawn or daytime in winter due to the plant being grown in the indoor farming facility and/or vertical farming facility as described above.

The method may comprise reducing the amount of illumination by the system (e.g. by dimming or switching off the light sources) in case there is sufficient sunlight present (for instance during daytime in summer).

In Figures 3 and 4, particular embodiments of a light unit (1) according to the present is illustrated. The light unit (1) is provided with a mount (2) onto which a plurality of light sources can be mounted. The light unit (1) in Figure 4 comprises such a plurality of light sources (5).

Figure 5 depicts a plurality of light sources (5) that can be mounted.

The light unit (1) as depicted in Figures 3 and 4 further comprises a tube (3) adapted for a cooling fluid flowing through the tube and a power connector (4) to connect the unit to the electric power grid.

The plurality of light sources can be distributed over the mount a for instance illustrated in Figures 7 and 9. In Figure 7, of a total of 528 light sources, 248 light sources are adapted to emit light of 660 nm, 64 light sources are adapted to emit light of 685 nm and 24 light sources are adapted to emit light of 450 nm. This distribution results in an emission spectrum as shown in Figure 14.

For comparison, in Figure 6, of a total of 528 light sources, 408 light sources are adapted to emit light of 660 nm, 96 light sources are adapted to emit light of 730 nm and 24 light sources are adapted to emit light of 450 nm. This distribution results in an emission spectrum as shown in Figure 13.

In Figure 9, of a total of 40 light sources, 26 light sources are adapted to emit light of 660 nm, 10 light sources are adapted to emit light of 685 nm and 4 light sources are adapted to emit light of 450 nm. This distribution results in an emission spectrum as shown in Figure 10.

The light units (1) can be placed in a greenhouse as illustrated in Figures 11 and 12 (A and B). A plurality of light units (1), connected by cooling tubes (6) between the light units (1) are position over the crops (7). The light units are placed inside the greenhouse, below a transparent panel that is suitable to allow sunlight to enter the greenhouse.

As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and/or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and/or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features.

For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

The invention can be illustrated by the following non-limiting examples.

Example 1

In an experimental setup, a comparative, first batch of plants basilicum plants (Ocimum basilicum) were illuminated by a conventional lighting system (as illustrated in Figure 7 (comparative)) comprising a blue-light source (450 nm) of 10% photon flux and a red-light source (660nm) of 90% photon flux, based on the total photon flux of the light. The conventional lighting system emits light with a spectrum as shown in Figure 8.

Another, second batch of plants were illuminated by a lighting system according to the invention (as illustrated in Figure 9) comprising a blue-light source (450 nm) of 10% photon flux and a red-light source (660nm) of 67.5% photon flux, and a deep-red light source (685 nm) of 22.5% photon flux, based on the total photon flux of the light. The lighting system according to this experiment emits light with a spectrum as shown in Figure 10.

Both batches of plants were otherwise treated equally.

After 2 weeks, the second batch of plants has shown a significantly higher growth progress.

Comparative example 1

An assimilation light with blue, red and far-red lights as illustrated in Figure 6 has been designed to have approximately 4.5% blue light, and a PSS value of 0.74 could result in the following fixture:

Blue (400-500 nm): 91 mmol/s, (4.5%)

Mid (500-600 nm): 3 mmol/s, Red (600-700 nm): 1594 mmol/s

Far-red (700-800 nm): 314 mmol/s (15.7%) PSS Value: 0.741

PAR photon flux: 1688 mmol/s

Total photon flux: 2002 mmol/s

Power consumption: 666W

The corresponding spectrum is illustrated in Figure 13.

Example 2

As in comparative example 1, an assimilation light with blue and red is designed to have a PSS value of 0.74. However, as illustrated in Figure 7, the assimilation light further is provided with deep-red lights having a wavelength of about 690 nm resulting in the following fixture:

Blue (400-500 nm): 92 mmol/s, (4.4%) Mid (500-600 nm): 3 mmol/s,

Red (600-700 nm): 1776 mmol/s

Far-red (700-800 nm): 217 mmol/s (10.4%) PSS Value: 0.741

PAR photon flux: 1871 mmol/s

Total photon flux: 2088 mmol/s

Power consumption: 668W

The corresponding spectrum is illustrated in Figure 14.

This light of this example has the same amount of blue, and the same PSS value as the light of Comparative Example 1, but almost 11% more PAR light. As such, the Emerson effect in this fixture will furthermore boost the

photosynthesis. The increase in PAR light is partly due to the higher efficiency of the deep-red light, but also partly due to less light outside PAR (far-red) needed to achieve the desired PSS value. Example 3

In an experimental setup, young tomato plants of the variations Merlice and Brioso were grown in a complete dark and conditioned growing chamber either one of the four different light spectra shown in Table 1. The light intensity was set at 100 mmol m ² s ^-1 by controlling the height of the lamp above the plants.

Table 1

The plants were illuminated for 18 hours with at a d/n temperature of 21 °C, at a relative humidity (RH) of 70% and a CO ₂ of 600 ppm.

During the experiment, the photosynthesis of the plants was determined using a Licor LI-6800. A spectral measurement was carried out using a Jeti Specbos 1211UV. Per treatment, 6 leaves per cultivar were measured. In addition, at the end of the experiment, after 6 weeks, the plant length, the number of leaves, the leave surface area, and the fresh/dry weights of the leaves and stems were determined. The results are shown in Figures 15-18.

In the light recipes where far-red (FR) was added, the PAR light level was kept the same as in the other recipes, this means that the power consumption of the HB/ER25/FR and HB/FR lamps was approximately 17% higher than the other recipes. The results show that the tomato plants grow more efficiently using deep-red light (corrected for the energy usage). In addition, the plant grown without the FR light in the spectrum remain advantageously more compact and sturdier.