i
ABSTRACT
Use of chemical growth regulators is the main tool used for making dwarf and compact plant in pot plant industry, although there are ever increasing concerns about the negative consequences of such chemicals to the environment. Therefore, the aim of this study was to investigate the light quality as an alternative tool to reduce stem elongation in pot plants.
Similarly, the effectiveness of end-of-the-day red (EOD-R) was also evaluated in terms of reducing plant height. Three experiments, i.e. long day, short day and growth chamber were conducted in controlled growing conditions in SKP (Senter for Klimaregulert Planteforskning), Norwegian University of Life Science (UMB). Two cultivars i.e. ‘Advent Red’ and ‘Christmas Eve’ were taken as plant materials. The three different light sources used in the greenhouse experiments were high pressure sodium (HPS), light emitting diodes (LED) and mixture of high pressure sodium light and light emitting diodes on equal proportion. The LED light consisted of 80% red light and 20% blue light. In the EOD-R treatment, plants were exposed to low intensity red light (5 µ mol/m2/s) for 30 minutes at the end of the day.
Similarly, in growth chamber experiment, only high pressure sodium light and light emitting diodes were used to compare the effects on plant height. The cultivar ‘Christmas Eve’ was found less sensitive to the light qualities than the cultivar ‘Advent Red’. Plants in LD experiment did not produce visible cyathia in three light qualities. Plant height in LED was significantly shorter than HPS light both in growth chamber and greenhouse. Similarly, EOD- R was effective to reduce the stem length in HPS, which reduced around 13% plant height than the plants without EOD-R. The results suggest that LED can be used to shorten the plant height in cultivar ‘Advent Red’ and EOD-R in HPS can be equally effective as drop treatment in terms of plant height reduction.
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ACKNOWLEDGEMENT
First of all, I wish to express my sincere gratitude to my first supervisor Prof. Dr. Hans Ragnar Gislerød, for his valuable guidance, scientific advices and orientation during the entire research period. His encouragement is duly appreciated. I am grateful and deeply indebted to my co-supervisor associate Prof. Sissel Torre for her generosity in giving valuable comments and suggestions throughout the research, which made the accomplishment of this work possible.
Special thanks to Ida Kristin Hagen and Dag Wenner for their assistance in technical needs. I would like to thank all of my colleagues and NEPSA family who made my stay in Norway an enjoyable experience with homely environment.
Finally, I would like to express my heartiest gratitude for the moral support and encouragement that I received from my parents. My heartfelt thanks go to my wife and son for their love and understanding, which made life more comfortable and enjoyable.
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T ABLE OF CONTENTS
ABSTRACT ... i
ACKNOWLEDGEMENT ... ii
ABBREVIATION ... v
1. INTRODUCTION ... 1
2. LITERATURE REVIEW ... 2
2.1. Cultural practices in poinsettia production ... 2
2.1.1. Effect of DIF on plant height ... 2
2.1.2. Effect of drop on plant height ... 4
2.2. Effects of light quality on stem elongation ... 5
2.2.1. Use of light emitting diodes (LED) ... 5
2.2.2. Use of light filtering plastic films ... 7
2.2.3. Use of ultraviolet -B radiation (UV-B) ... 10
2.2.4. Use of end-of-the-day red light (EOD-R) ... 11
3. MATERIALS AND METHODS ... 12
3.1. Plant culture and lighting system ... 12
3.2. Data collection ... 15
3.3. Data analyses ... 16
4. RESUTLS ... 17
4.1. Effect of light quality on stem elongation ... 17
4.2. Effect of EOD-R on stem length ... 19
4.3. Effect of light quality on number of internodes and flowering time ... 21
4.4. Effect on number of leaves ... 24
4.5. Effect on leaf area ... 24
4.6. Effect on petiole length ... 25
4.7. Effect on dry weight ... 25
5. DISCUSSION ... 26
5.1. LD experiment in greenhouse ... 26
5.2. SD experiments in greenhouse ... 26
5.3. Effect of light quality on number of internodes, flowering time, leaf area and dry weight ... 28
5.4. Growth chamber experiment ... 30
iv
6. CONCLUSIONS ... 31 7. REFERENCES ... 32
v
ABBREVIATION
% Percent
°C Degree Celsius
DIF Difference between day and night temperature
DT Day temperature
EOD End-of-the-day
EOD-FR End-of-the-day far red light EOD-R End-of-the-day red light
FR Far red
GAs Gibberellins
HPS High pressure sodium
LD Long day
LED Light emitting diodes
NT Night temperature
PE Polyethylene
PGR Plant growth regulator PPF Photosynthetic photon flux PPM Parts per million
R Red
SD Short day
UV Ultraviolet rays
1
1. INTRODUCTION
Strong, dwarf and compactness are one of the most important criteria in pot plant production.
Chemical plant growth regulators are widely used for proper control of stem elongation and plant height in many ornamentals (Moe et al. 2002). However increasing concern over the impacts of these compounds on human health and natural environment is apparent (Lund et al.
2007), Thus, controlling plant height without using plant growth retardants is getting attention of many physiologists. Out of several techniques, light quality manipulation is one of the most common practices in greenhouse production. Light manipulation can be done either by using specific light sources (Appelgren 1991; Bula et al. 1991) or by screening of daylight by selective plastic films (Cerny et al. 2003; Clifford et al. 2004) or by manipulating the red (R)/far-red (FR) ratio (Maliakal et al. 1999).
Major radiation sources in plant-growing facilities are fluorescent, metal halide, high-pressure sodium, and incandescent lamps which have various limitations and are not an optimum radiation source for plants (Bula et al. 1991). These sources contain unnecessary wavelengths that are of low quality for promoting growth (Kim et al. 2004). To identify the effect of light quality is always difficult since these sources of light content broad spectrum of lights.
Therefore light emitting diodes (LED) which have pronounced peak and narrow bandwidth wavelength, combined to their operational advantages such as small size, low mass, long functional life and high electrical energy conversion efficiency, these are a potential irradiation source for intensive plant culture systems (Ménard et al. 2005). Therefore, researches on light quality by using LED are very important. LED light can substitute of plant growth regulators (PGR) for control of plant height.
Poinsettia is the main pot plant crop for the Christmas market in Norway (Moe et al. 1992b) with a yearly sale of 5.2 million poinsettia plants (Vik et al. 2001). The main aim of this study was to investigate the effect of light quality and to evaluate the effect of red light in the end of the dark period (EOD-R) on stem elongation of poinsettia.
2
2. LITERATURE REVIEW
2.1. Cultural practices in poinsettia production
Poinsettia is a popular ornamental plant known as a symbol of the Christmas season, mostly grown in greenhouses. From several studies it has been shown that the growth and flowering of poinsettia is dependent on day length and photoperiod. Poinsettia is a short-day plant (SDP), which naturally flowers in response to long nights (Wang et al. 2003) and critical day length to initiate flowers is 12.5 hours (Kristoffersen 1969). Transfers of poinsettia from long day (LD) to short day (SD) induce the metabolic changes and a transition from a vegetative to generative occurs (Kristoffersen 1994). Mother stocks of poinsettia are mainly grown in long day condition, since a higher number of cuttings can be taken form a vegetatively grown plants.
In poinsettia, stem length is of special interest since excessive stem elongation reduces the sale value of the crops and most of the cultivars need application of chemical growth regulators (Kristoffersen 1994). Cycocel is one of the most common plant growth regulators used for height control in poinsettia (Moe 1994). Due to negative consequences of chemical growth regulators to the environments and human beings, alternative methods of height control have been investigated. Among them, day-night temperature difference (DIF) and temperature drop (a 2 hours temperature decrease in the beginning or the end of the night) have been found to be of practical interests.
2.1.1. Effect of DIF on plant height
Temperature has significant effects on plant growth and development. The difference between day temperature and night temperature influences internode length and plant height in many plant species (Moe & Heins 1990). Negative DIF is a condition when there is high night temperature and low day temperature where reverse is called positive DIF. There are several studies that show the effect of positive and negative DIF on internodes length. The effects of DIF on stem elongation are a result of increased cellular elongation rather than division (Myster & Moe 1995). According to Moe & Heins (1990), plants grown in positive DIF are taller than the plants grown in negative DIF. In an experiment on Dendranthema grandiflora
3
to compare the effect of negative DIF and zero DIF on plant height and internode length, it was found that the plant height and internode length were significantly shorter at negative DIF (Table 1) (Jacobsen & Amsen 1992). Similarly, a interaction between light quality and DIF was observed in long day plant (Moe & Heins 1990). Campanula responded to negative DIF only when there was high R/FR ratio but with same treatment with low R/FR ratio, the negative DIF was not effective.
Table 1. Plant height and internode length of Dendranthema grandiflora ‘Surf’ and ‘Saphire’.
Similar effect of negative DIF was also observed by (Patil et al. 2001). A change from positive DIF (DT/NT -21/13 °C) to negative DIF (DT/NT - 13/21 °C) reduced shoot length in Pisum sativum, Fuchsia hybrida, Petunia hybrida ‘Ultra Blue’ and Petunia hybrida. ‘Silvana Purpur’ by 36%, 30%, 28% and 15% respectively. The results obtained by Moe et al. (2002) are consistent with earlier reports on the effects of changing day and night temperature. It was found that a lower DT than NT reduced plant height significantly in Cucumis sativus and Fuchsia x hybrida cv. ‘Beacon’. Similarly, Kubota et al. (2000) observed promotive action of positive DIF on stem elongation in petunia whereas negative DIF inhibited it.
However some plant species reacted in an opposite way to DIF. Kalanchoe showed a strong elongation of the flower stems to the temperature DIF (-6C) (Cuijpers & Vogelezang 1992).
The physiological basis for the effects of DIF on stem elongation is believed to involve gibberellins (GAs). There are several reports that indicate low endogenous GA content in negative DIF treated plants (Tangerås 1979) and GA applications overcome inhibition of stem elongation in negative DIF grown plants of Cumpunulu (Moe 1990).
4
Although DIF is a successful method for height control in the winter months with their cool climate (Moe et al. 2002), its application is sometimes difficult especially during year with high day temperature due to high solar irradiation (Patil et al. 2001). Moreover, in some studies it has been seen that the DIF alone was not sufficient to get acceptable plant height and still needed plant growth regulators (Cuijpers & Vogelezang 1992). Moreover, in poinsettia, negative DIF has a strong retarding effect on stem elongation, but negative effects on the keeping quality have been found. Negative DIF during the growth period increased the abscission of cyathia and more bract necrosis than zero or positive DIF in ‘Lilo’ cultivar (Moe et al. 1992a)
2.1.2. Effect of drop on plant height
Stem length is also sensitive to short period of temperature drop (2-4 hours) and the effect is greatest in the last part of the night or the first period of the day (Moe et al. 1992b; Myster &
Moe 1995). There are several studies that show the positive effect of temperature drop to reduce internodes length. Uber & Hendriks (1992) found a significant effect of temperature drop in stem elongation of Euphorbia pulcherrima. It was observed that a moderate drop from 24 to 16°C reduced stem length by 5-25 %, while a strong drop from 24 to 8 C resulted strong reductions in stem length. Even, a 2 hours drop from 24 to 8°C reduced stem length by about 50%. In another study by Moe et al. (1992b), it was found that a 2-hour temperature drop during the last 2 hours of the night or the first 2 hours of the day reduced the total plant height in poinsettia. But a temperature drop in the middle of the night had small effect on the plant height. The results obtained by Grindal & Moe (1994) in begonia agreed with previous results in poinsettia where a temperature-drop in the morning reduced the plant height and a 2 h temperature increase in the morning increased the plant height.
Effects of temperature drop was evaluated in other crops such as cucumber and tomato (Moe et al. 1992c). It was found that with a temperature drop of 10 or 9°C for 2 hours resulted 24%
and 38% reduction in plant height in cucumber and tomato respectively. Similarly, in an another similar kind of experiment, by Grimstad (1993) found 24% and 36% reduction in plant height in cucumber and tomato with 10 and 9°C temperature drop respectively.
However there are some plant species such as pot herbs (Melissa officinalis and Ocimum basilicum) and some bedding plants, they were not much affected by a 2-hour drop in the morning (Moe et al. 1992c).
5
Cuijpers & Vogelezang (1992) compared the effects of DIF and drop and found that the chrysanthemum and poinsettia reacted differently. In chrysanthemum, DIF (-5°C) was more effective in reducing plant height than temperature drop but there was only small difference between DIF and drop in terms of reducing plant height in poinsettia. Cuijpers & Vogelezang (1992) concluded that the potential of DIF and drop as an alternative to growth regulators is dependent on crops.
Today, temperature drop is common in practices but the effect can be very weak in certain years when the outdoor temperature is too high. Also, to drop the temperature in the greenhouse in the morning may not be the right strategy when it comes to energy saving in greenhouses.
2.2. Effects of light quality on stem elongation
Plants have several light receptors involved in controlling plant morphogenesis. Blue light absorbing cryptochromes and phototropins and red and far-red light absorbing phytochromes are the most important light receptors. Phytochrome consists of two inter-convertible forms:
the inactive Pr form that absorbs R light and the active Pfr form that absorbs FR light (Cerny et al. 2004). Absorption of red light changes the red-absorbing form to the far-red absorbing form; conversely, absorption of far-red light changes the far-red absorbing form to the red absorbing form (Mathews 2005).
2.2.1. Use of light emitting diodes (LED)
Light is not only the primary energy source for plants but also as a signal for their morphogenesis (Ubukawa et al. 2004). The most effective components of the spectrum of light are red (R), far-red (FR), and blue (Tsegay et al. 2005). The effect of light quality on stem elongation has been documented in many studies in different plant species. Shimizu et al. (2006) reported that the blue light inhibited shoot elongation in Chrysanthemum. They observed that the internode extension of chrysanthemum increased by an average of 1.6 mm·d-1 when plants were exposed to a night interruption delivered by fluorescent lamps, while the plants grown with night interruption delivered by the blue LED increased by an average of 1.0 mm.d-1. In another study, Kim et al. (2004) observed the greatest stem length
6
of chrysanthemum plants under red LED and far-red LED when the plants were grown under six different light qualities: fluorescent, blue LED, red LED, red and blue LED, red and far- red LED, and blue and far-red LED.
Effect of light quality on plant morphology has been documented in several other crops. In in- vitro culture of Azorina vidalii (Wats.) Feer, the low R/FR ratio (0.6) enhanced plant length than high R/FR ratio (1.1) and control (9.7) (Fig. 1a) (da Silva & Debergh 1997). The R/FR ratio had also significant influence in leaf area. The highest leaf area observed with low R/FR (Fig. 1b)
(a) (b)
Figure 1. Plant height (a) and leaf area (b) in different light treatments (da Silva & Debergh 1997)
Similarly, it was found that the blue light deficient environment promoted stem extension(by 10% to 100%) in five long day plant species (Runkle & Heins 2001). In cucumber and tomato, it was found that the blue light enhancement by blue LED lamp treatments reduced internode length on both plant species (Ménard et al. 2005). In another study, it was found that the exposure to light from red significantly increased the stem elongation while blue light strongly inhibited the elongation in Pelargonium (Appelgren 1991). The contrasting result of
7
blue light was found in eggplant (Hirai et al. 2005) and petunia (Fukuda et al. 2002) where they observed a promotive action of blue light on stem elongation.
In a study, Hoenecke et al. (1992) investigated the effect of ‘Blue’ photon levels for lettuce seedlings that were grown under red LED. The elongation of the hypocotyl was significantly affected by the incident blue-photon flux level. As seedlings were exposed to increasing flux of blue photons from 0 to 60 µmol/m/2/s, hypocotyl elongation decreased from 30 to 2 mm.
(Fig. 2).
Figure 2. Relationship between Lettuce seedling hypocotyl length and flux of blue photons at two photosynthetic photon flux levels provided by red LED and blue fluorescent lamps (Hoenecke et al. 1992).
2.2.2. Use of light filtering plastic films
Several studies (Li et al. 2000; Lykas et al. 2005; Wilson & Rajapakse 2001) have been done to investigate the effect of light filtering plastic film on plant morphogenesis. Clifford et al.
(2004) reported in poinsettia that plant height was approximate up to 20 % shorter than control plants when plants were grown under the photo-selective film that reduces the transmission of far red light. It was found that in gardenia (Gardenia jasminoides), the height of potted plants grown in short tunnels, under the light filtering plastic film (Red/FR = 3.1 and Blue/FR = 28.9), was significantly lower (59%) compared to plants grown under the common
8
plastic film (R/FR = 1.3 and B/FR = 0.8) 75 days after transplanting (Fig. 3) (Lykas et al.
2005).
Figure 3. Mean height of the plants grown in short tunnels covered with light filtering plastic film (♦), and common plastic film (□) (Lykas et al. 2005).
In another experiment of plant response to photo-selective plastic films with three concentrations of a far-red (FR) light absorbing dye in chrysanthemum and bell pepper, it was found that the plant height progressively decreased as the dye concentration increased (Li et al. 2000). It was indicated that the photo-selective film with a R:FR ratio of 2.2 caused about 20% height reduction in chrysanthemum and 30% height reduction in bell pepper after 4 weeks of treatment. And in both chrysanthemum and bell pepper, plants grown in CuSO4
chamber had the shortest plant height (Fig. 4).The strongest effect of CuSO4 on plant height was due to higher interception of FR wavelengths of sunlight.
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Figure 4. Weekly height increase of chrysanthemum and bell pepper seedlings grown in different photoselective film chambers. BCE -1 is the control film. YCE-1 #80, #75, and #65 are photoselective films with the FR absorbing dye at 0.08, 0.13 and 0.22 g m-2, respectively.
CuSO4 is the chamber covered with panels filled with 4% CuSO4.5H2O liquid. The average PPF inside chambers was 620±120 µ mol/m2/s. (Li et al. 2000).
Another similar kind of experiment investigated the plant response to a photo-selective plastic film with a red (R) or far-red (FR) absorbing property in perennial salvias (Wilson &
Rajapakse 2001). Films were designated AFR (FR light-absorbing film), AR (R light- absorbing film) and control (clear plastic film). It was found that solar light transmitted through the AFR film reduced plant height by 17-36%, depending on the species. Similarly, in petunia, reduction in plant height observed in FR-filtering films (Cerny-Koenig et al. 2005;
10
Cerny et al. 2003; Fletcher et al. 2005). The R-rich treatment inhibited elongation of the main stem in petunia as compared to the control and the FR-rich treatments (Kubota et al. 2000).
Oyaert et al. (1999) showed that the inhibition of stem elongation in chrysanthemum increased with increasing pigment concentration under the blue polyethylene (PE) films, where maximum of 22% growth reduction was observed as compared to the control.
Another experiment of modifying daylight by screening light through light quality selective plastic was done by Moe et al. (2002). The film type YXE-10 which created a high red (R)/far-red (FR) ratio (1.6) resulted in 45-50% shorter stem length of cucumber compared to a control film (R/FR ratio = 1.1).
2.2.3. Use of ultraviolet -B radiation (UV-B)
The UV light is now emerging as a new tool to greenhouse horticulture, especially for bringing change in plant morphology. Zuk-Golaszewska et al. (2003) investigated the effect of UV-B on Avena fatua and Setaria viridis. It was found that the mean plant height of Avena fatua was not affected by different levels of UV-B radiation while the plants of Setaria viridis were found much more susceptible to the UV-B radiation, resulted in shorter plants at the levels of 8 and 12 kJ m-2 d-1 UV-B radiations than 0 and 4 kJ m-2 d-1. Kumari et al. (2009) observed that the plant height significantly decreased with increasing UV-B radiation in Acorus calamus, and resulted compact plants. It was found that plant height decreased by 15.6% (ambient UV-B + 1.8 kJ m-2 d-1) and 20.4% (ambient UV-B + 3.6 kJ m-2 d-1) than control (only ambient UV-B). Zhao et al. (2003) reported that the reduction of plant height in UV-B rich environment was due to shortening of internodes.
In G. hirsutum under high level of UV-B exposure (16 kJ m-2 d-1), Kakani et al. (2003) reported a highly significant UV-B induced reduction in plant height. Plants exposed to high level of UV-B were 47% shorter than control plants (no UV-B radiation). Similarly, plants exposed to ambient UV-B radiation (8 kJ m-2 d-1) were shorter than control plants but the difference was not significant.
11 2.2.4. Use of end-of-the-day red light (EOD-R)
Use of EOD-R is another tool to bring changes in plant morphology, specially making short plant in pot plant industry. End of the day exposure of the red light influences plant morphology by altering the amount of Pfr present at the beginning of the dark period. The amount of Pfr at the beginning of the dark period is considered to have critical role in regulating plant morphology such as plant height (Rajapakse et al. 1993).
Several studies found that the use of EOD-R reduced stem elongation. Exposure to EOD-R light reduced the plant height by 11% compared to non EOD-R treated plant in chrysanthemum (Rajapakse et al. 1993). Similarly, final leaf dry weight also significantly reduced in EOD-R treatment. In cucumber, EOD-R treatment resulted plants with shorter stem length, hypocotyl and internodes than EOD-FR treatment (Xiong et al. 2002). Besides, there was a significant interaction between EOD and DIF treatment on stem elongation, since the effect of negative DIF relative to positive DIF was more prominent under EOD-R compared to EOD-FR.
According to Hisamatsu et al. (2008), gibberellins is believed to play the role in changing plant height in EOD-FR. They noticed in chrysanthemum that EOD-FR enhanced shoot elongation which was mediated by increase responsiveness to GA.
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3. MATERIALS AND METHODS
3.1. Plant culture and lighting system
The experiments were conducted in SKP (Senter for Klimaregulert Planteforskning) Norwegian University of Life Science (UMB). Shoot cuttings of poinsettia were transplanted in the pot containing standard peat mix (Floralux, Nittedal) at two true leaves stage on 12 August 2009. Two cultivars of poinsettia i.e ‘Advent Red’ and ‘Christmas Eve’ were selected for the research. Pinching was done to facilitate the side shoots on 03 September 2009 and three side shoots were allowed to grow.
Experimental set-up
Three different experiments were conducted: two in the greenhouse and one in a closed growth chamber without any influence of natural light. The experimental set up of the greenhouse experiments is shown in table 2. The ventilation in greenhouse opened at 23°C.
Table 2. The experimental set up used in the greenhouse experiments.
Light sources Intensity (µ mol/m2/s)
R/FR ratio RH (%) Temperature (°C)
CO2
(ppm)
Mix light 100 13.4* 70 21 800
LED 100 - 70 21 800
HPS 100 4.3 70 21 800
* Average from three measurements
Experiment 1. Long day (LD) experiment in greenhouse:
The plants were grown with 20 hr lighting provided by three different light qualities. The first treatment had mixture of 50% light emitting diodes (LED) and 50% high pressure sodium (HPS) light and is called mix light treatment. Second treatment and third treatment consisted of pure LED and pure HPS light respectively. The LED used in this experiment was a Red/Blue LED which was the mixture of red LED and blue LED of 80% and 20%
respectively, using LED system (VA- 24150T, Sola Co. Grimstad). The spectral distribution of the Red/Blue LED and HPS are shown in Figure 5a and 5b. The instrument LI-COR
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radiation sensors (LI-250 light meter) was used to measure the light intensity while R/FR ratio was measure by Skye 660/730 sensor. Leaf temperature was measure by Thermocouple thermometer (HD 9016) by taking the value from around 1 cm above the leaf surface and found that leaf temperature was higher by 1.5°C than the air temperature in HPS treatment than other two light treatments.
14 (a)
(b)
(c)
Figure 5. Spectral distribution of Red/Blue LED (80% red and 20% blue light) (a) high pressure sodium light (b) and red LED (c).
0,00E+00 8,00E-02 1,60E-01 2,40E-01 3,20E-01
300 400 500 600 700 800
W/0,5nm
nm
Red/Blue LED
0,00E+00 8,00E-02 1,60E-01 2,40E-01 3,20E-01
300 400 500 600 700 800
W/0,5nm
nm
HPS
0,00E+00 8,00E-02 1,60E-01 2,40E-01 3,20E-01
300 400 500 600 700 800
W/0,5nm
nm
Red LED string
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Experiment 2. Short day (SD) experiment in greenhouse:
In this experiment the plants were covered by black poly-ethylene plastic to give a light duration of 10 hours. There were two SD experiments; SD without EOD-R and SD with EOD-R. The three treatments were arranged in a same way as in LD experiment i.e. a mixture of LED and HPS, pure LED and pure HPS. In SD with EOD-R experiment, plants were exposed to an end of the day (EOD) red (R) light. Plants were covered by black poly-ethylene plastic after 10 hours lighting and then exposed to low intensity red light (5 µ mol/m2/s) for 30 minutes at the end of the day. The red LED string (Philips LED power drivers, item 9137006208) was used to supply red light. The spectral distribution of the red LED string is shown in Figure 5c. The total amount of light level was maintained by 100 µ mol/m2/s.
Experiment 3. Growth chamber experiment:
The growth chamber experiment was designed to investigate the effect of pure HPS and LED on plant height. The cultivar ‘Christmas Eve’ was used for this experiment. Plants were transplanted in small plastic pots with peat mix on 12 August 2009 and then transferred to the growth chamber after 4 weeks. Temperature of 21°C, CO2 of 800 ppm and light level of 100 µ mol/m2/s was maintained in both treatments. Like greenhouse experiment, leaf temperature of 1.5°C was found higher in HPS treatment. Plants were grown in short day condition (10 hours lighting). In this experiments also, pinching was done and three side shoots were allowed to grow.
3.2. Data collection
In experiment 1 and 2, the total length of three side shoots was measured every second week.
At the end of the experiment, total number of internodes in main shoot, total number of leaves in main and side shoots, total leaf area, petiole length plant diameter, fresh weight and dry weight of the plant were recorded. The plant diameter was measured by making two perpendicular lines on the top of the plants. Similarly, for petiole length, three fully grown leaves at the middle of the main shoots were chosen. Leaf area was measured by LI-COR 3100 leaf area meter (LI-COR Inc; Lincoln, NE, USA). The final harvest of the plant was
16
taken when visible cyathia appeared. To determine dry weight, plants were oven-dried at 70°C until they reached constant mass
In experiment 3 the side shoots length during the growing period and plant fresh and dry weight were measured at the end of the experimental period.
3.3. Data analyses
Data were analyzed by analysis of variance using Minitab statistical software version 15 and differences among treatment means were tested by general linear model at P = 0.05.
4. RESUTLS
4.1. Effect of light quality on
Experiment 1. Long day experiment Plant morphology was affected by
interaction between cultivars and light sources was found response in the two cultivars.
In cultivar ‘Advent Red’, plant height was found the end of the experiment (Fig. 6
and mix light treatment (Fig. 7
treatments with mix light, HPS light and under LED was shortest (Fig.
Figure 6. Poinsettia (Cultivar- condition.
17 n stem elongation
Long day experiment:
Plant morphology was affected by the different light qualities under LD and
interaction between cultivars and light sources was found (P = 0.005) indicating a different
lant height was found shortest in the LED treatment
(Fig. 6). The LED treatment was followed by the treatments HPS 7a). There was a significant difference in final
, HPS light and LED light (P = 0.001) and the plants developed 7b).
-‘Advent Red’) grown with HPS, LED and mix light
under LD and a significant indicating a different
LED treatment from start to was followed by the treatments HPS final plant height in the and the plants developed
grown with HPS, LED and mix light in long day
(a)
Figure 7. Increase in shoot length over time with different light qualities length after 10 weeks (b) in cultivar
value of four observations, n = 4.
same letters are not significantly difference at P = 0.05.
In cultivar ‘Christmas Eve’, it was found that the plant height in the treatment shortest in all measurements (Fig
mix light treatment (Fig. 8b) (P = 0.03
(a)
Figure 8. Increase in shoot length over time with different light qualities length after 10 weeks (b) in cultivar
are the standard error of the means.
= 0.05.
18
(b)
n shoot length over time with different light qualities length after 10 weeks (b) in cultivar ‘Advent Red’ in long day condition.
value of four observations, n = 4. Error bars are the standard error of the means.
same letters are not significantly difference at P = 0.05.
, it was found that the plant height in the treatment (Fig. 8a) and was significantly different with (P = 0.03).
(b) Increase in shoot length over time with different light qualities length after 10 weeks (b) in cultivar ‘Christmas Eve’ in long day condition.
are the standard error of the means. Bars with same letters are not significantly difference at P (b)
(a), and total shoot . Data are the mean Error bars are the standard error of the means. Bars with
, it was found that the plant height in the treatment of LED was ) and was significantly different with HPS but not with
(b)
Increase in shoot length over time with different light qualities (a), and total shoot n long day condition. n = 4. Error bars Bars with same letters are not significantly difference at P
Experiment 2. Short day experiment 4.2. Effect of EOD-R on stem length
In cultivar ‘Advent Red’, a significant effect of the main light source and EOD on stem elongation, but there was not significant interaction between EOD factors (Fig. 9a). Stem length in plants without
the plant in with EOD-R treatments.
But, cultivar ‘Christmas Eve’
in stem length was found with and without EOD
light source was found. Similarly, there was no interaction between EOD light environments (Fig. 9b).
(a)
Figure 9. Effect of EOD-R on stem elongation in cultivar Advent Red (a) and Christmas Eve (b).
While comparing two cultivars Eve’ was shorter than the cultivar
without EOD-R conditions. Interaction between cultivar and light sources was found in SD experiment without EOD-R (P = 0.004)
EOD-R light (P = 0.3).
19
experiment with and without EOD-R:
R on stem length
a significant effect of the main light source and EOD on stem elongation, but there was not significant interaction between EOD
). Stem length in plants without EOD-R was found significantly R treatments.
was found less sensitive to EOD-R and no significant difference with and without EOD-R. However, a significant
arly, there was no interaction between EOD-R and with diff
(b)
R on stem elongation in cultivar Advent Red (a) and Christmas Eve
While comparing two cultivars in SD experiments, it was found that the cultivar was shorter than the cultivar ‘Advent Red’ in three light qualities
Interaction between cultivar and light sources was found in SD P = 0.004) while there was no interaction in SD experime
a significant effect of the main light source and EOD-R was found on stem elongation, but there was not significant interaction between EOD-R and main light R was found significantly higher than
R and no significant difference significant effect of the main R and with different
R on stem elongation in cultivar Advent Red (a) and Christmas Eve
iments, it was found that the cultivar ‘Christmas in three light qualities in both with and Interaction between cultivar and light sources was found in SD while there was no interaction in SD experiment with
Experiment 3. Growth chamber
In growth chamber, only the cultivar ‘Christmas Eve’
HPS and LED on stem elongation.
poinsettia plants (Fig. 10). It was found that plants with height in all measurements (Fig. 11
light (P = 0.001) (Fig. 11b).
Figure 10. Poinsettia grown in growth chamber with LEDs and HPS light.
(a)
Figure 11. Increase in shoot length over time with different light qualities length after 10 weeks (b) in growth chamber.
means. Bars with same letters are not significantly difference at P = 0.05 20
Growth chamber experiment:
th chamber, only the cultivar ‘Christmas Eve’ was used to investigate the effect of on stem elongation. The clear effect of HPS and LED
It was found that plants with LED light were measured surements (Fig. 11a) and were significantly lower than the plants with
Poinsettia grown in growth chamber with LEDs and HPS light.
(b)
Increase in shoot length over time with different light qualities
length after 10 weeks (b) in growth chamber. N = 4. Error bars are the standard error of the . Bars with same letters are not significantly difference at P = 0.05.
was used to investigate the effect of LED was observed in were measured smaller than the plants with HPS
Increase in shoot length over time with different light qualities (a) and total shoot ars are the standard error of the
21
4.3. Effect of light quality on number of internodes and flowering time
In cultivar ‘Advent Red’, there was no significant difference in number of internodes between mix light, LED and HPS treatments (Table3) in the LD experiment. But in SD without EOD- R experiment, the numbers of internodes in HPS treatment were significantly lower than the two other treatments. There was a significant difference in number of internodes between the treatments in SD with EOD-R experiment, where mix light treatment had maximum number of internodes which is followed by LED and HPS.
Similarly in cultivar ‘Christmas Eve’, it was found that the treatment HPS light had significantly higher number of internodes than other two treatments while an opposite result was found in SD and EOD-R experiments (Table 4). In these experiments, HPS light treatment had significantly lower number of internodes than other two treatments and there was no difference between these two treatments.
In cultivar Advent Red, the first visible cyathia appeared in HPS light treatment, which was before 7 days than in LED and mix light treatments under SD with EOD-R and SD without EOD-R.
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Table 3. Comparison of three treatments in number of internodes, total number of leaves, leaf area, petiole length and dry weight under LD, SD and EOD-R conditions in cultivar ‘Advent Red’. N = 4. Values in parentheses are standard deviations. The mean values within one column followed by the same letter are not significantly different at p = 0.05.
No. of internodes
Treatments LD SD SD+EOD-R
Mix light 19.41 (1.00) A 18.41 (1.17) A 17.75 (0.87) A LED 17.41 (0.83) A 18.00 (0.72) A 16.08 (0.84) B HPS 18.34 (1.66) A 14.50 (0.34) B 14.34 (0.60) C
No. of leaves
Treatments LD SD SD+EOD-R
Mix light 50.75 (0.50) A 49.75 (0.95) A 46.75 (3.59) A LED 47.50 (1.00) B 44.50 (3.87) B 46.50 (3.41) A HPS 53.00 (1.82) A 48.00 (1.41) AB 48.00 (1.41) A
Leaf area
Treatments LD SD SD+EOD-R
Mix light 4288.75 (69.83) A 4234.00 (145.41) A 3825.25 (442.60) A LED 3855.00 (140.14) B 3504.75 (470.63) B 3695.00 (508.70) A HPS 4630.00 (269.20) A 3961.50 (198.29) AB 3974.00 (140.00) A
Petiole length
Treatments LD SD SD+EOD-R
Mix light 8.82 (0.12) A 7.47 (0.52) B 6.32 (0.98) A LED 8.65 (0.31) A 7.85 (0.64) B 7.10 (0.50) A HPS 9.05 (1.08) A 9.32 (0.50) A 7.42 (0.26) A
Dry weight
Treatments LD SD SD+EOD-R
Mix light 50.00 (0.48) A 48.22 (2.28) A 42.32 (5.46) A LED 34.75 (3.44) B 35.70 (3.44) B 32.62 (0.72) B HPS 48.52 (5.09) A 43.90 (0.88) A 39.52 (2.34) A
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Table 4. Comparison of three treatments in number of internodes, total number of leaves, leaf area, petiole length and dry weight under LD, SD and EOD-R conditions in cultivar
‘Christmas Eve’. n = 4. Values in parentheses are standard deviations. The mean values within one column followed by the same letter are not significantly different at p = 0.05.
No. of internodes
Treatments LD SD SD+EOD-R
Mix light 18.91 (1.03) B 20.34 (0.27) A 20.67 (1.27) A LED 18.83 (0.43) B 20.00 (0.68) A 19.67 (0.38) A
HPS 20.84 (0.41) A 13.91 (1.58) B 16.34 (1.58) B
No. of leaves
Treatments LD SD SD+EOD-R
Mix light 58.25 (2.98) A 50.50 (3.10) A 49.50 (1.73) A LED 52.00 (2.45) B 49.50 (1.29) A 48.50 (1.29) A HPS 60.50 (2.88) A 47.50 (2.88) A 48.25 (4.03) A
Leaf area
Treatments LD SD SD+EOD-R
Mix light 4187.00 (343.10) A 3312.50 (374.10) A 3172.75 (188.70) A LED 3481.50 (263.50) B 3222.75 (19.40) A 3144.00 (122.00) A HPS 4520.00 (245.80) A 3042.50 (304.8) A 3097.00 (476.40) A
Petiole length
Treatments LD SD SD+EOD-R
Mix light 4.70 (0.45) A 5.27 (0.34) A 5.65 (0.44) A LED 4.32 (0.47) A 4.92 (0.41) A 4.62 (0.39) B HPS 4.90 (0.20) A 5.75 (1.32) A 5.65 (0.44) A
Dry weight
Treatments LD SD SD+EOD-R
Mix light 53.17 (4.22) B 41.7 0 (1.81) A 38.25 (2.90) A LED 45.32 (3.40) C 32.80 (2.28) B 30.65 (1.23) B HPS 62.50 (0.29) A 40.57 (2.00) A 42.20 (1.54) A
24 4.4. Effect on number of leaves
Leaf number in cultivar ‘Advent Red’ was found significantly lowered in LED treatment than mix light and HPS light treatments in LD experiment (Table 3). And no significant difference in leaf number between mix light and HPS was found. Similarly, in SD experiment, number of leaves in LED treatment was significantly lower than mix light treatment but there was no difference in number of leaves between mix light and HPS, and LED and HPS treatment.
Numbers of leaves were not significantly difference between the three light treatments in SD with EOD-R condition.
Leaf number was found significantly lower number in LED treatment than mix light and HPS treatments under LD condition in cultivar ‘Christmas Eve’ (Table 4). While in SD without EOD-R and SD with EOD-R experiments, there was no difference in leaf number between the treatments.
4.5. Effect on leaf area
Leaf area was also found to be significantly lowered in LED treatment than mix light and HPS treatments but there was no difference in leaf area between the treatments mix light and HPS in cultivar ‘Advent Red’ under LD condition (Table 3). Under SD without EOD-R condition, again the LED treatment had significantly lowered leaf area than mix light treatment but no difference between LED and HPS in term of leaf area. Similarly, there was no difference between LED and HPS. Moreover, the plants developed under the three light treatments are not significantly different in leaf area under SD with EOD-R condition.
In cultivar ‘Christmas Eve’, leaf area was found significantly lower in LED treatment than mix light and HPS treatments and these two treatments were not significantly different each other in terms of leaf area under LD (Table 4). But under SD without EOD-R and SD with EOD-R conditions, treatments were not significantly different each other in leaf area.
25 4.6. Effect on petiole length
There was no difference in petiole length between the three light treatments in cultivar
‘Advent Red’ in LD and SD with EOD-R conditions (Table 3). But in SD without EOD-R experiment, petiole length in HPS light treatment was significantly longer that mix light and LED treatments.
In cultivar ‘Christmas Eve’, there was no significant difference in petiole length among the treatments under LD and SD without EOD-R experiments (Table 4). But in SD with EOD-R experiment, only the treatment LED had significantly shorter petiole than mix light and HPS treatments.
4.7. Effect on dry weight
Dry weight in cultivar ‘Advent Red’ was significantly lowered in LED treatment than mix light and HPS treatments under LD, SD without EOD-R and SD with EOD-R conditions (Table 3).
In cultivar ‘Christmas Eve’, all treatments are significantly different in terms of dry weight in LD. HPS treatment had maximum dry weight followed by mix light and LED (Table 4). But in SD without EOD-R and SD with EOD-R experiments, the LED treatment had significantly lower dry weight than other two light treatments.
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5. DISCUSSION
5.1. LD experiment in greenhouse
The plants the LD experiment did not produce visible cyathia under any of the light sources.
Poinsettia is a short-day plant (SDP), which naturally flowers in response to long nights (Wang et al. 2003) and critical day length to initiate flowers is 12.5 hours (Kristoffersen 1969). Kristoffersen (1969) also observed no bracts in cultivar ‘Vikings’ at 18 and 21°C with 13.5 hours day length. However, the morphology and general growth was very much affected by the light source. Plants in LED treatment under LD were stunted in growth, leaves were distorted with chlorotic lesions and the plants appeared extremely rough and misshapen.
Similar negative symptoms were not observed with LED under SD conditions. The probable reasons for less growth under LD with LED compared to SD can be due to differences in RH and night temperatures between SD and LD since the plants in LD experiments were not covered by plastic. There also might be nutrient deficiency in LD plants, since it probably is a difference in RH and transpiration between plants from SD and LD. However, it can also be that the plants are getting stressed by long duration of LED exposure (LD) and accumulate high amounts of starch in the leaves. The starch content was not measured in this experiment but high accumulation of starch under high levels of blue light has earlier been described (Heo et al. 2010). Similar kind of injury appeared in the leaves of tomatoes under continuous lighting (Demers et al. 1998; Globig et al. 1997). However, the plants in HPS light under LD condition were significantly bigger in all aspects i.e. height, internodes numbers, leaf area, leaf size (Table 4&5); therefore HPS light under long day condition is optimum for raising mother stock. More numbers of cuttings can be taken from vegetatively grown plants under HPS.
5.2. SD experiments in greenhouse
The effect of LED, mix light and HPS in stem reduction is observed as high, medium and low respectively in both SD experiments; without EOD-R and with EOD-R in both cultivars (Fig.
9). One of the factors behind this response is due to the FR radiation, which is presented in a lower proportion or almost none in LED, medium in mix light and higher in HPS treatment. It is well known that the FR radiation induced stem elongation in many plant species and
27
ponsettia is clearly responsive to R/Fr ratio (Mata & Botto 2009). Another factor of interest is the low levels of blue light in HPS lamps. Blue light, known to be detected by both cryptochrome and phyochrome, has been shown to suppress elongation growth in Arabidopsis (Zhao et al. 2007). It is not possible to know if the plants under LED are shorter due to more blue light or because of the high R/Fr ratio but it can also be a combination of both factors.
The effect of light quality on stem elongation of cultivar ‘Advent Red’ was clearly seen in SD experiments; both without EOD-R and with EOD-R (Fig. 9a), where the plants with LED light was found significantly shorter. The LED consisted 80% red light and 20% blue light.
There are several reports which showed similar kind of results in different plant species. Kim et al. (2004) investigated different kinds of LED and found that the LED with 50% blue and 50% red light had shorter stem length than blue LED, red LED and fluorescent lamp with 16 hours day light. However, the effect of red LED on stem elongation probably dependent on species and cultivars. Appelgren (1991) observed that the red light promoted stem elongation in Pelargonium. The results obtained by Hahn et al.(2000) in Rehmannia glutinosa were in accordance with the findings in Pelargonium, where the shoots of the plantlets under red LED were twice as long as for plantlets growing under blue LED, mix LED (half blue half red) and fluorescent light without ventilation condition. Contrary to these results, Heo et al. (2002) observed that the stem length in marigold was highest in monochromatic blue light, being three times higher than fluorescent light. Poinsettia is highly sensitive to the R: FR ratio (Clifford et al. 2004) therefore any change in this ratio will change in plant height (Mata &
Botto 2009).
While comparing the effect of EOD-R on stem elongation, it has been clearly seen in cultivar
‘Advent Red’ (Fig. 9) that plants with EOD-R had shorter stem length than the plants without EOD-R. Our results are consistent with earlier reports on use of EOD-R in petunia (Ilias &
Rajapakse 2005), where plants were exposed to red light for 15 minutes at the end of the photoperiod and found that EOD-R reduced the stem elongation. The results obtained by Rajapakse et al. (1993) in chrysanthemum were also in agreement with our results, where EOD-R light reduced height by 11%. The probable reason behind the reducing effect on stem elongation by EOD-R might be due to the conversion of Pr form to more active Pfr form. The amount of Pfr at the beginning of the dark period is thought to play a critical role in regulating plant morphology such as plant height (Rajapakse et al. 1993).
28
If we look on individual light factors, from figure 9a, it is clear that the effect of EOD-R on stem elongation is more pronounced in HPS light treatment but not in mix light treatment in cultivar Advent Red. Moreover, the effect of EOD-R on stem elongation was higher in HPS light than LED. It gives very useful information and open ups that possibilities of using EOD- R with HPS light to reduce stem length in cultivar ‘Advent Red’. If we compare the effect of EOD-R with earlier experiments done with drop treatment to reduce stem length in poinsettia, EOD-R was found equally effective as drop treatment, although drop is commonly used by commercial growers in greenhouse production. Moe, R et al. (1992b) observed that the total plant height was reduced by 5 cm in cultivar ‘Starlight’ and 3.5-4 cm in cultivar ‘Lillo’ by drop treatment. The reduction in total plant height by 4.8 cm by EOD-R was found in our HPS light treatment (Figure 9). Thus, there is a possibility to substitute drop treatment by EOD-R which offers efficient energy use in greenhouse by closing ventilation. Similarly, drop treatments need huge amount energy to lower the temperature and to raise the temperature after the treatment.
No interaction between EOD-R and different light quality was observed. This result indicates that the effect of EOD-R on stem elongation is independent on the light quality of the supplemental light sources. However, in a study conducted by Xiong et al. (2002) , the interaction between DIF and EOD was found while plants were growing in positive and negative DIF with combination of EOD-R and EOD-FR. The effect of EOD-FR was not investigated in out studied but in several studies it showed the stimulating effect for stem elongation (Ilias & Rajapakse 2005; Xiong et al. 2002).
5.3. Effect of light quality on number of internodes, flowering time, leaf area and dry weight
No significant difference was found in numbers of internodes in cultivar ‘Advent Red’ under LD condition. There are several other reports that show similar effects of light quality on number of internodes. Kim et al. (2004) found no effect of light quality on number of nodes in chrysanthemum. And he further concluded that the stem elongation was due to internodes elongation rather than number of internodes. In another experiment to investigate the effect of light quality on the morphogenesis of in vitro cultures of Azorina vidalii (Wats.) Feer by da Silva & Debergh (1997), they suggested that light spectra affect the internodes length but not the number of internodes. The result obtained by Appelgren (1991) was consistent with
29
previous reports where she observed no difference in number of nodes with different light qualities. However in our experiment, numbers of internodes were found to be significantly different in SD without EOD-R and SD with EOD-R treatments in cultivar ‘Advent Red’ and
‘Christmas Eve’. The numbers of internodes in HPS light were significantly lower than in mix light and LED treatment. Surprisingly, lower numbers of internodes in HPS light treatment gave higher total plant height which showed that the higher plant height in HPS was due to internodes elongation rather than higher number of internodes.
Cyathia appeared first in HPS light than LED and mix light treatments in cultivar Advent Red. This might be due to higher leaf temperature in HPS light treatment than LED and mix light treatments. HPS light contained lower proportion of blue light. Our results are in agreement with results obtained by de Graaf-van der Zande and Blacquiere (1991) where they observed that earlier flowering in the plants those treated with low levels of blue light.
Similarly, Mata and Botto (2009) observed delay in flowering time in plants grown under a high R/FR ratio, which was in accordance with our results.
Leaf area was minimum in LED treatment in all three experiments but only significantly lower with other two light treatments in LD experiment (Table 3 &4) in both cultivars. But an opposite result was observed in chrysanthemum that the leaf area was significantly higher in LED with mixture of 50% red and 50% blue light than pure blue and red LED (Kim et al., 2004). Similarly, dry weight was found to be significantly lowered in LED treatments in three experiments in both cultivars. The lower dry weight in LED experiment might be due to smaller plant height and fewer numbers of leaves.
There are not many studies that compare the effect of HPS light with LED. But there are several studies which compared the effect of fluorescence light with LED light (Hahn et al.
2000; Heo et al. 2002; Kim et al. 2004). Similarly, in our research we used LED, which was the mixture of 80% red and 20% blue, while in most of the researches, monochromatic red or blue light was used. Kim et al. (2004) recorded significantly higher dry weight in LED with mixture of 50% red LED and 50% blue LED than blue and red LED. Similarly, bulblets of Lilium, grown in mixture of red and blue LED light (1:1 photon flux density) had higher dry weight than those grown in red LED, blue LED and fluorescent light (Mei-Lan Liana et al.
2002). Contrary to the results reported by Kim et al. (2004) and Mei-Lan Liana et al (2002),
30
Heo et al. (2002) found that the dry weight was found significantly higher in monochromatic red light and was followed by fluorescence light in marigold.
5.4. Growth chamber experiment
There was a clear effect of LED light on stem elongation and the plants were much more compact compared to HPS (Fig. 11). The experiment conducted in growth chamber avoided the possibility of influence of outside FR radiation. Therefore, the most important reason for short plants in LED light treatment in growth chamber was due to lower proportion of FR radiation than in HPS. One of the reasons for higher stem length with high FR light is due to the reduction in ethylene levels (Kurepin et al. 2007b). Similarly, in another experiment, with a low R/FR ratio significantly increased the growth of Helianthus annuus and gave significantly higher amount of endogenous IAA, GAs and cytokinins (Kurepin et al. 2007a).
Therefore higher shoot length with FR radiation is associated with low ethylene production and higher IAA, GAs and cytokinins. Another reason might be due to lower proportion of blue light in HPS treatment.
31
6. CONCLUSIONS
From the results of the experiments, the following conclusions could be drawn:
• HPS light under LD is suitable to grow mother stocks. LED in LD condition should be avoided since it damaged the plants.
• The LED makes poinsettia shorter than HPS light.
• Cultivar ‘Christmas Eve’ is less sensitive than cultivar ‘Advent Red’ to light quality and EOD-R.
• The EOD-R on stem reduction is more effective in HPS light, which offer around 13%
plant height reduction. Therefore, it opens the possibility to substitute drop treatment in greenhouse commercial production.
32
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