Abstract
Light-emitting diode (LED) lamps signify one of the most important advances in artificial lighting for horticulture over the last few decades. The objective of this study was to compare the cultivation of four horticultural plants using a conventional white LED tube (T0) light against one with a good spectral fit to the maximum photosynthetic response (T1) at two intensities. The experiment was carried out with two types of young lettuce, tomato, and bell pepper plants. In a controlled environment chamber, six and four lamps per square meter were used to achieve high (H) and low (L) intensity, respectively. We measured the lighting parameters illuminance (lux) and photosynthetic photon flux (PPF) intensity (µmol·m−2·s−1). The dry and fresh weight, leaf area (LA), and specific index were measured to gauge plant growth. The photosynthetic activity and energy efficiency (EE) were recorded for each species over 60 days of cultivation. The results clearly demonstrate that, compared with conventional LED lamps, the specific horticultural LED lamps with an improved light spectrum increased the EE of the evaluated vegetables by 26%. At both the studied light intensities, plant growth was clearly more closely linked to the spectral fit of the light to the maximum photosynthetic response recorded by McCree (1972) than to PPF or illuminance (lux). We therefore suggest that a specific, detailed spectral distribution study be conducted to predict the effect of the specific quantity and quality of light used in this study on a single parameter of plant growth.
Fluorescent lamps are widely used in horticulture, particularly in in vitro culture. However, LED lighting systems have several beneficial properties, including their spectral composition, durability, long operating lifetime, wavelength specificity, relatively cool emitting surface, and high EE. Therefore, LED lighting is both energy efficient and beneficial to plants (Li et al., 2013; Massa et al., 2008; Ouzounis et al., 2015; Urrestarazu, 2013). Morrow (2008) suggests that solid-state lighting LED is one of the leading advancements in horticultural lighting in the last decade.
Light intensity is one of the most important environmental factors for plant growth (Naoya et al., 2008). In contrast with high-intensity light, low-intensity light is frequently reported as a factor for photoinhibition (Long et al., 1994). In addition, the effect of low light levels on plant growth and photomorphogenesis is well known; for example, they can lead to increased specific leaf area (SLA) and plant height (Fan et al., 2013; Steinger et al., 2003).
In PPF, blue and red light have ≈90% absorption, and green light has 70 to 80% absorption (Terashima et al., 2009). The proportions of both red and blue light PPF have been studied by several authors (Li et al., 2013). Johkan et al. (2012) suggest that high-intensity green LED light promotes lettuce growth, and in particular, short-wavelength green light (510 nm) is available for active plant growth.
On the other hand, the use of illuminance, a measure of radiation expressed as the PPF intensity from 380 to 780 nm [the photosynthetic radiation spectrum (PAR)], is often used in horticulture for determining the sufficiency of light intensity for proper plant cultivation. Multiple studies have been conducted with blue and red light (e.g., Fan et al., 2013), with different combinations of them (e.g., Li et al., 2013) and with the addition of a very specific section of the green spectrum (Johkan et al., 2012). Little information on continuous-spectrum LED lamps fit to a theoretical model of the maximum photosynthetic response has been recorded since McCree’s (1972) experiments on cultivated plants.
The objective of this study was to evaluate the agronomy and EE of LED lamps manufactured for horticultural use with a continuous spectrum fit to the maximum photosynthetic response vs. conventional white lighting at two different light intensities.
Materials and Methods
Plant material and growth conditions.
The experiment was carried out at the University of Almeria (Spain) in an 1800 mm wide × 800 mm deep × 2200 mm high growth chamber. The light racks were spaced 50 cm apart, and the plants were exposed to a 16/8 h photoperiod (day/night) at a temperature of 28 to 18 °C (day/night) and 85% to 80% relative humidity. Lettuce [cv. Astorga (Lettuce 1) and cv. Cervantes (Lettuce 2)], tomato (cv. Simona), and pepper (cv. Dulce italiano) plants with six true leaves were transplanted to a 0.750-mL pot containing coir substrate. The physical characteristics of the coir substrate have been previously described by Morales and Urrestarazu (2014). The fertigation was replenished if 10% of the readily available water was consumed (Rodríguez et al., 2014; Urrestarazu, 2004). The nutrient solution used was similar to that recommended by Sonneveld and Straver (1994).
Five plants per species and five cultivars per replicate were considered after 35 d of treatment. The total LA was determined using the AM350 portable LA meter (ADC BioScientific Ltd., Hertfordshire, United Kingdom). The plants were separated into leaves, stems, and roots, immediately weighed to determine their fresh weight, dried at 75 °C for 48 h, and reweighed.
Net photosynthesis rate and EE.
The net photosynthesis (Pn) rate was measured using the LCi Portable Photosynthesis System (ADC BioScientific Ltd., Hertfordshire, United Kingdom). The objective was to monitor the environment and measure the plants’ photosynthetic activity. The LCi system can measure within the intervals of 0 to 2000 ppm (CO2) and 0 to 75 mbar (H2O) with a precision of ±2%. The Pn experiment was repeated 12 and 33 d posttreatment. Five measurements per treatment, species, and cultivar were conducted on completely expanded mid-leaf blades.
Treatment, LED devices, and light intensities.
Two PPF intensities, low (L) and high (H), were tested using two types of LED lamps. The L and H intensities consisted of four and six lamps per square meter, respectively. The resulting light data are shown in Table 1. Three replicate measurements were taken of the spectral scan LED light 20 cm below the panel where the plants were grown. The HD 2302.0 LightMeter (Delta OHM®, Veneto, Italy) was used to measure the quantitative light. The LP 471 PAR and LP 471 PHOT quantum radiometric probes were used to measure the photon flux intensity density (µmol·m−2·s−1) and illuminance (lux), respectively.
Light data with two light-emitting diode (LED) lamps and light intensities [4 (L) and 6 (H), unit m−2] and electric energy expenditure (kWh·m−2).
Two types of 18 W LED lamp tubes were used. L18 T8 Roblan® (Toledo, Spain) was used as a control (T0), and L18 EU AP67 Valoya® (Helsinki, Finland), an agronomic LED tube (T1) made for horticultural growth, was evaluated as the test group. The light spectra of T0 and T1 are shown in Fig. 1A and B and were recorded with the UPRtek MK350S LED Meter (Miaoli County, Taiwan).
(A) Spectral photon flux distributions for 380 to 780 nm lighting treatment for the white light-emitting diode (LED) lamps used as a control and (B) the LEDs specifically designed for agricultural use. (C) Absorbance of different pigments recorded by Nelson and Cox (2013). (D) The relative photosynthetic response recorded by McCree (1972). (E) Relative spectral sensibility measured with the LP 471 PHOT by Delta Ohm®. (F) Relative spectral sensibility measured with the LP 471 PAR by Delta Ohm®.
Citation: HortScience 51, 3; 10.21273/HORTSCI.51.3.268
(A) Spectral photon flux distributions for 380 to 780 nm lighting treatment for the white light-emitting diode (LED) lamps used as a control and (B) the LEDs specifically designed for agricultural use. (C) Absorbance of different pigments recorded by Nelson and Cox (2013). (D) The relative photosynthetic response recorded by McCree (1972). (E) Relative spectral sensibility measured with the LP 471 PHOT by Delta Ohm®. (F) Relative spectral sensibility measured with the LP 471 PAR by Delta Ohm®.
Citation: HortScience 51, 3; 10.21273/HORTSCI.51.3.268
(A) Spectral photon flux distributions for 380 to 780 nm lighting treatment for the white light-emitting diode (LED) lamps used as a control and (B) the LEDs specifically designed for agricultural use. (C) Absorbance of different pigments recorded by Nelson and Cox (2013). (D) The relative photosynthetic response recorded by McCree (1972). (E) Relative spectral sensibility measured with the LP 471 PHOT by Delta Ohm®. (F) Relative spectral sensibility measured with the LP 471 PAR by Delta Ohm®.
Citation: HortScience 51, 3; 10.21273/HORTSCI.51.3.268
Statistical analysis and LED devices with different light intensities.
A randomized complete block design was implemented. For each treatment, species, and cultivar, four blocks of five plants per block were constructed. The mean data were analyzed at different levels of significance. The data were subjected to analysis of variance, and their means were compared by a Tukey’s test using Statgraphics Centurion® 16.1.15 (Warrenton, VA) and Microsoft Office 2010.
Results and Discussion
Effect of light quality on light parameters.
Given identical energy consumption and an equivalent number of LED lamps per area, there was a substantial variation in both illuminance (lux) and PPF (µmol·m−2·s−1) (Table 1). Compared with the T0 plants, the T1 plants exhibited a significant 37% and 39% decline in illuminance and PPF (PAR), respectively.
Figure 1 shows the spectral photon flux distributions for the 380 to 780 nm lighting treatments. The T1 lamps exhibited a closer fit to the photosynthetic efficiency curve. We obtained MSE values of 1, 0.47, and 0.64 for McCree (1972), T0, and T1, respectively.
Effect of light quality on the parameter of plant growth.
In all cases, with the exception of the roots of the lettuce 1 and 2 cultivars, significantly or highly significantly increased vegetative growth was found with increased light intensity (Table 2).
Effect of the light intensity and spectral composition on growth parameters of four vegetables.
The spectrum of the LED lamps caused an important and significant effect on the majority of the growth parameters of the four species: the light treatment with a closer spectral fit to the highest photosynthetic response (T1) was more conducive to growth. Only in the roots of the plants, there was no clear and significant effect in this regard. Although the illuminance and PPF were noticeably higher in T0 than T1, the plant growth parameter values of T0 were not higher than T1 in any of the four species. Lettuce 1 had the highest significant mean increase in total biomass (fresh and dry weight, greater than 100%), and the average increase in dry biomass was 57%.
The correlation between vegetative growth and illuminance and PAR (Table 1) was unclear, which corroborates the findings of Barnes et al. (1993), who reported on two necessary measures for PAR: 1) PPF, which assigns values to all photons from 400 to 700 nm, and 2) yield photon flux, which weights photons in the approximate range of 360 to 760 nm according to the photosynthetic response of the plant.
The average photosynthetic responses of crop plants reported by McCree in 1972 (including tomato and lettuce) are shown in Fig. 1D. The closer spectral fit of T1, particularly between 580 and 710 nm (Fig. 1B), might explain its improved results over T0, i.e., a significant reduction in PPF after 580 nm. However, the MSE values of 47% and 65%, associated with the T0 and T1 spectra with the best fit to the maximum photosynthetic response, more closely fit the higher growth of T1 than T0. In lettuce, even small spectral changes (from 500 to 530 nm) in LED lamps vary the behavior of vegetative growth in comparison with white fluorescent lamps (Johkan et al., 2012).
With the exception of pepper in T1, the LA significantly or highly significantly increased when the light intensity was increased (Table 3). However, while the significant mean increase was 36% for T0, it was 113% for T1. The greatest increase in LA due to increased light intensity occurred during treatment T1 with tomato seedlings (170%). With the exception of tomato grown at low light intensity, the effect of the spectrum of the LED lamps in T1 on LA was a mean 57% and 63% increase for low and high light intensity, respectively.
Effect of the light intensity and spectral composition on leaf area growth parameters of four vegetables.
The studies of Johkan et al. (2012) found no major differences in the LA of young tomato plants using various spectra when they used a PPF of 100 µmol·m−2·s−1, whereas we recorded a considerable spectral effect in T1 vs. T0 (with the exception of tomato in treatment L). This may be due to the closer spectral fit between 580 and 710 nm, or the closer overall fit of any of the spectra to the maximum photosynthetic response. It is noteworthy to mention that Li et al. (2013) obtained improved results using a combination of red and blue LEDs over white fluorescent lamps.
With the exception of the tomato cultivars of T1, a clear and significant decrease in the SLA was associated with an increase in light intensity. This SLA decrease was also reported to be in the range of 50 to 550 µmol·m−2·s−1 in young tomato plants by Fan et al. (2013), who used blue and red LEDs; they also suggested that SLA always decreased, which may have reduced the absorption of light energy. In the present study, the effect of the spectrum on SLA did not show a clear trend: it increased SLA in lettuce 2 and pepper, decreased SLA in tomato at low light intensity, and had no effect on lettuce 1 and tomato SLA under high lighting conditions.
Effect of light quality on Pn and EE.
All the Pn measurements increased significantly in T1 with increasing light intensity; however, this effect was less clear for white LED lamps (T0) (Table 4). In the T0 lettuce 1 and pepper plants, no differences were found in Pn compared with lettuce 2. Similarly, at the highest light intensity, all the Pn values increased with the treatments in T1 compared with T0, with the exception of lettuce 2. In contrast, at the lowest light intensity, only the young tomato plants were affected by the T1 treatments. Fan et al. (2013) also reported a Pn increase in the range of 50 to 150 µmol·m−2·s−1. Again, the improvements associated with the spectra in T1 vs. T0 might be justified by the distribution of the T1 spectra, which leads to a greater photosynthetic response.
Effect of the light intensity and spectral composition on photosynthesis (Pn) and energy efficiency (EE) of four vegetables.
With the exception of T0 and lettuce 1, increasing the light intensity with LED lamps always significantly or highly significantly increased the EE. Although it varied proportionally for each species, the mean significant EE of the four horticultural species for T1 was 4 g higher dry matter per kilowatt of energy consumed than T0, which represents a 26% increase in significant average efficiency.
The significant average increases in EE of the four species under increased light intensity were 67% and 76% for T0 and T1, respectively, whereas within the L and H intensities, the comparable EE increases were 28% and 44%, respectively. The improvements of LED over conventional spectra are likely due to the closer fit of their PAR spectrum to the spectrum of the maximum photosynthetic response recorded by McCree (1972).
Our results are in agreement with those of Fan et al. (2013), who found that EE increased by 50% from 50 to 150 µmol·m−2·s−1 but decreased above 300 µmol·m−2·s−1.
Conclusions
The results clearly demonstrate that, compared with other conventional LED lamps, the LED lamps specifically designed for horticultural use were more beneficial to the evaluated vegetables and had 26% higher EE.
Plant growth was more closely linked to the maximum photosynthetic response recorded by McCree than to PPF intensity (µmol·m−2·s−1) or illuminance (lux). Therefore, a specific detailed spectral distribution is necessary to predict the effect on a single parameter of plant growth. At the same time, within a specified quantity and quality of light, significant species- and cultivar-specific plant growth is observed.
When the light intensity was increased in T1 from 50 to 100 µmol·m−2·s−1, this minimally doubled the LA compared with T0, a very significant increase found in all the horticultural species tested.
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