Li 190

A dwarf tomato cultivar, ‘Micro-Tom’ (Solanum lycopersicum L.), was used for the experiment. Tomato seeds were sown in a urethane sponge (M-urethane, M Hydroponics Laboratory Co. Inc., Aichi, Japan) and maintained at 25 °C for 3 days in the dark. Seeds germinated on the 3rd day, and then seedlings were cultivated under white LED lamps (LDL40S-N19/21, Panasonic Corporation, Osaka, Japan) in a room with a controlled environment at Matsudo campus, Chiba University. The following conditions were maintained—the PPFD was set to 200 μmol m⁻² s⁻¹ at the canopy level using a quantum sensor (LI-190, LI-COR Inc., Lincoln, NE, USA) and air temperature of 25/20 °C (light/dark period) with 70% relative humidity, 1000 μmol mol⁻¹ CO₂ concentration, and 16 h/8 h (light/dark) photoperiod were maintained. All plants were cultivated using 1/2 OAT house A nutrient (OAT Agrio Co., Ltd., Tokyo, Japan) from 7 days after germination. pH and electrical conductivity of the nutrition solution were set at 6.3 and 1.3 dS m⁻¹, respectively. The nutrition solution was renewed every 4 days. Seedlings were grown under these conditions until the third true leaf (from the bottom) fully expanded, which occurred 24 days after sowing, after which the seedlings were transplanted into the treatment area.

Net ecosystem CO₂ exchange was observed from 2015 to 2020 by an open-path eddy covariance measurement system installed above an alpine meadow at 2 m. The sensor was broken at the beginning of 2019, so there was a long data gap in 2019 and the data in 2019 were discarded. The open-path eddy covariance system has a three-dimensional sonic anemometer (CSAT3; Campbell Scientific Inc. (CSI), Logan, USA) and an open-path CO₂/H₂O infrared gas analyzer (LI-7500A; Li-COR Inc, Lincoln, NE, USA). Flux data are logged with a data logger at 10 Hz (CR5000, Campbell Scientific, UT, USA). HMP45C temperature probe (Vaisala, Finland) was used to measure air temperature. Soil volumetric water content (VWC) at a depth of 5 and 10 cm was measured using a CS655 probe (CSI, Logan, USA). Precipitation was measured by a tipping bucket rain gauge (TE525, CSI, Logan, USA). Photosynthetic photon flux density (PPFD) was measured using a photosynthetic active radiation sensor (LI190, LI-Cor, USA). This eddy covariance tower is one of the ChinaFlux (China Flux Observation and Research Network) and FLUXNET long-term observation site.

The test species Lemna minor L. (strain ID: 5544, Rutgers Duckweed Stock Cooperative) was obtained from Ghent University, Belgium. After disinfection by 0.5% NaOCl (v/v), the L. minor cultures were maintained in Swedish Standard (SIS) medium (Moody and Miller 2005 ) in a growth chamber at 24 °C under continuous white light from fluorescent lamps (L 36W/77-G13, Centra Osram, Berlin, German) with a PAR of 80 ± 5 µmol m⁻² s⁻¹ following the OECD TG221 guideline (OECD 2006 ), with stock thalli sub-cultured twice a week. The combinations of UVB, UVA and PAR were carefully chosen to assess the effects of different UVB irradiances under artificial conditions with relatively low photosynthetic photon flux density rather than mimicking growth conditions in nature. The irradiance was measured with a LI-COR quantum sensor Model LI-190 (Lincoln, NE, USA) connected to a LI-COR LI-250 photometer unit.

Environmental data were recorded every minute from three sensor stations distributed in the Miniplot facility. Data were uploaded to an SQL database. The used sensors were LI-190 (LI-COR Inc., Nebraska USA) for photosynthetic photon flux density (PPFD) and HMP110 (Vaisala, Helsinki, Finland) for air temperature and humidity. Environmental data were linked to LIFT measurements taken in the same minute.

Leaf Area Index (LAI) was estimated from hemispherical photographs taken with a Panasonic Lumix DMC-GH1 camera (Panasonic Corporation, Kadoma, Japan) and Sigma fisheye 4.5 mm and 1:2.8 aperture lens (Sigma Corporation, Rokonkoma, NY, USA). Hemisfer software (Schleppi et al., 2007 (link)) was used to calculate LAI values from the images and pixels were divided into sky and canopy using threshold values that were checked manually.
Canopy light interception (Δ light) was estimated using an Li-190 (LICOR, Nebraska, USA) quantum sensor connected to a Li-250A light meter. First we estimated PAR under the canopy around the sampling point (PAR_UC). PAR_UC was measured in a cross sampling scheme centered on the tree used for leaf traits and optical measurements, with 5 repeat measurements in direction North to South and 5 repeat measurements in the direction East to West. Then we measured (5 repeats of) PAR in an open and non-obstructed place, such as an access road, in similar sky conditions and as close to the original sampling point as possible (PAR_OPEN). We calculated Δ light using averaged repeat measurements as:

Daily canopy light interception fraction and season-long interception efficiency, 𝜀_i, were determined as the fraction of available PAR that was absorbed (APAR) by the canopy. APAR was calculated as
where I_o was incident PAR measured above the canopy with an upright quantum sensor, I_t was transmitted PAR measured at soil level using a line sensor, and I_r was reflected PAR measured with an inverted quantum sensor above the canopy. All data were collected using line (model SQ-311) and quantum (model SQ-110) sensors (Apogee Instruments, Logan, UT, USA) that had been calibrated with a high precision quantum sensor (LI-190, LI-COR, Lincoln, NE, USA) at the beginning of the season. All data were logged every 10 s using a datalogger (model CR3000 in 2012 and model CR10X in 2013, Campbell Scientific, Logan, UT, USA). Measurements began on DOY 180 in 2012 and DOY 189 in 2013 and corresponded to the V5 developmental stage. The energy conversion efficiency (𝜀_c) was determined as the slope of accumulated aboveground biomass energy regressed on accumulated APAR from early vegetative stages to peak biomass energy. 𝜀_p was determined as the ratio of seed energy: total aboveground plant energy at harvest maturity. Yield and seed mass were determined after harvesting and threshing seeds from pods of four complete rows per plot in each experiment.