After briefly introducing the original LASS system, this report describes some modifications which were made of the LASS system. Finally, we describe procedure of a Pn-quantification method developed for successive evaluation of Pn for 30 PCW-LED lights with different relative SPDs.
Original LASS system
The original LASS system  comprises a light source unit, an LED temperature control system, and a spectral irradiance distribution (SID) control system. The light source unit consists of an LED module and a hollow conical reflection condenser. The LED module is equipped with 625 monochromatic LEDs with 32 peak wavelengths (385–910 nm). The condenser is used to mix lights from different LEDs and to gather them to the light outlet. The LED temperature control system is assembled to stabilize the LED module temperature. The SID control system can produce light with an SID (combined SID) that accurately approximates a given SID (target SID) within a 380–940 nm wavelength. Because an SID can convert to an SPD at the same wavelength range, as described in a review of the literature , the SID control system is therefore an SPD control system. This function can be achieved because it can control SIDs on the light outlet by adjusting the voltages applied to each type of monochromatic LED. When a desirable SID is obtained, users can save the voltage set (voltages of the 32 types of LED) in the SID control system. The LASS system can then readily re-irradiate a light with any combined SID by loading its voltage set and applying the voltages to the LEDs. Furthermore, the SID control system can automatically supply more than dozens of produced lights successively. Moreover, the irradiation period of each light can be set at intervals of more than 2 s.
Modified LASS system
Although the original LASS system can produce lights with different SPDs, accurately approximating those of the corresponding selected PCW-LED lights (a typical one is presented in Additional file 4, Figure S2A), the extent of the differences between some combined SPDs and the corresponding target SPDs in several wavelength ranges, such as 440–460 nm, 510–550 nm, and 580–620 nm (a typical one is presented in Additional file 4, Figure S2B), was not negligible. These differences might engender biases between the Pn measured under the produced lights and that measured under the corresponding PCW-LED lights. Another point to be improved was the low maximum-PPFDs of the produced lights with the original LASS system for PCW-LEDs, which were approximately 150 µmol m− 2 s− 1. Considering that the PPFD used in a PFAL can reach 300 µmol m− 2 s− 1 , the modified LASS system is expected to be capable of producing PCW-LED lights at a PPFD of over 300 µmol m− 2 s− 1.
Therefore, we made the following modifications to the original LASS system to meet our requirements. First, we selected monochromatic LEDs of three types and PCW-LEDs of three types (Fig. 7; Additional file 5, Table S3) to be installed on the modified LED module. Their peak wavelengths differed from those of the original LED module. The three PCW-LEDs to be installed were selected because their relative SPDs included some wavelength ranges that we can find only rarely in commercial monochromatic LEDs. In addition, their high photon fluxes per LED contribute to increased maximum PPFDs of the produced lights.
Then, we replaced monochromatic LEDs of eight types with selected LEDs of eight types with similar peak wavelengths to the original LED module; the eight types of LEDs have higher photon fluxes per LED. After those modifications, the modified LED module consists of monochromatic LEDs of 27 types (426–826 nm peak wavelengths) and PCW-LEDs of three types. Their relative SPDs, model codes, and peak wavelengths are shown in Fig. 7 and Additional file 5, Table S3.
Lastly, we coated the inner surface of the hollow conical reflection condenser with a water-based high-reflectance barium sulfate coating (Avian-B500; Avian Technologies LLC, New London, NH, USA) to increase the distribution uniformity of the combined PFDs at the light outlet.
The RMSEs [mol m− 2 s− 1 nm− 1] of the combined SPDs and the target SPDs were used to evaluate the extent of their mutual difference. For lower RMSE, better combined SPD is obtained.
Plant materials and growth conditions
Cos lettuce (cv. Cos Lettuce; Takii Seed Co. Ltd., Kyoto, Japan), red-leaf lettuce (cv. Mother-red; Takii Seed Co. Ltd.), and green-leaf lettuce (cv. Mother-green; Takii Seed Co. Ltd.) plants were subjected to Pn measurements at 20 and 25 DAS. They were cultivated in a growth chamber at 25 ± 1 °C (mean ± SE) under a 16-h/8-h light/dark period cycle. A PCW-LED array (HMW120DC6 (1N-40Y); Kyoritsu Densho Co., Ltd., Osaka, Japan), shown as PCW-LED light 16 in Fig. 2, was applied to provide a PPFD of 150 ± 10 µmol m− 2 s− 1 on the cultivation surface. The CO2 concentration in the growth chamber was 1000 ± 50 µmol mol− 1 controlled by an infrared CO2 controller (ZFP9; Fuji Electric Co., Ltd., Tokyo, Japan). Lettuce plants were cultivated in a nutrient solution (half-strength Otsuka-A nutrient solution; OAT Agrio Co. Ltd., Tokyo, Japan) with an electrical conductivity of 150 ± 0.1 mS m− 1.
Procedure of the rapid and mostly automated Pn-quantification method
Step 1: Producing PCW-LED lights with the modified LASS system
The general procedure to produce light with the modified LASS system (Fig. 8) is the following: 1) preparing the target-SID data, which include spectral irradiance at every 1 nm between 400 and 800 nm; 2) determining the voltage sets to gain the desired SID based on the target-SID data, and saving it to the LASS system.
The target-SID data of 30 types of PCW-LEDs (Fig. 2; Additional file 6, Table S4) were converted from their SPDs, as calculated from the relative SIDs of the selected PCW-LEDs at a PPFD of 150 µmol m− 2 s− 1. The relative SIDs are usually shown as figures in their specification sheets. A free web-based program  was used to obtain relative SIDs at every 1 nm between 400 and 800 nm, from the figures shown in specification sheets.
The target-SID data were then imported to the SID control system to determine the appropriate voltage set to obtain the desired SIDs, and therefore the desired SPDs. The SPDs were measured using a spectroradiometer (MS720; Eko Instruments Co. Ltd., Tokyo, Japan) at the same surface of a leaf enclosed in the leaf chamber of the LI-6400 (Fig. 1). It is noteworthy that, in the Pn measurements, a steel plate was used as a connector to place the leaf chamber rigidly under the light outlet of the modified LASS system, just as it was placed in light production (Fig. 1). The steel plate was installed on the top of the leaf chamber and trepanned a hole, of which the shape and size were approximately the same as those of the light outlet.
Step 2: Measurement of Pn under the produced lights
Before Pn measurement started, the irradiation order of the 30 produced lights was determined randomly (Fig. 9). The irradiation period of each produced light was set as 0.5 h. During Pn measurement, the LASS system automatically supplied the produced light 16 (Fig. 2) for 1 h. Then the system supplied all 30 produced lights successively to an identical leaf according to the set order and period. Each Pn measurement was completed in 16 h. An example of Pn measurement is presented in Fig. 10.
A plant was used for Pn measurement approximately six hours after the beginning of the light period at 20 or 25 DAS. The timing was set based on our preliminary experiment results, which showed that the Pn measured under one produced light kept increasing for approximately six hours after the beginning of the light period. An LI-6400 portable photosynthesis system (Li-Cor Inc. Lincoln, NE, USA) was used for Pn measurements. In the leaf chamber of the LI-6400 during the Pn measurements, the leaf temperature (mean ± SE), CO2 concentration, relative humidity, and airflow rate were, respectively, 25.0 ± 0.1 °C, 1000 ± 1 µmol mol− 1, and 65 ± 8% and 500.3 ± 0.2 µmol s− 1. During Pn measurement, the LI-6400 was set to record a dataset including Pn, CO2 concentration, etc., automatically every 3 s and to match its two CO2 and H2O analyzers for reference and sample lines, respectively, after every ten datasets were recorded. On average, the LI-6400 recorded ten datasets (30 s) and matched its analyzers (approximately 30 s) in approximately one minute. Therefore, approximately 300 datasets were compiled for each produced light. The last 100 datasets were used to calculate the mean Pn of that produced light. In all, five replications for each produced light with each lettuce cultivar were conducted at 20 and 25 DAS.
Significant differences in mean Pn among the 30 produced lights were tested using a t-test with Holm-Bonferroni correction for multiple comparisons, with significance inferred for p < 0.05, using software (R 3.6.1, R Core Team, 2019). Correlation analyses between the mean Pn, which was the mean value of the Pn under the produced lights at 20 DAS and that at 25 DAS, and blue-light (400–500 nm) photon flux densities (PFDs), green-light PFDs (501–600 nm), red-light PFDs (601–700 nm), and far-red-light PFDs (701–800 nm) of the 30 PCW-LEDs were tested using Pearson correlation analysis, with significance inferred for p < 0.05, using the same software.