Edited by: Carl-Otto Ottosen, Aarhus University, Denmark
Reviewed by: Erik Harry Murchie, University of Nottingham, United Kingdom; Shardendu Kumar Singh, AeroFarms, United States
*Correspondence: Shuyang Zhen,
This article was submitted to Crop and Product Physiology, a section of the journal Frontiers in Plant Science
†Present address: Shuyang Zhen, Department of Horticultural Sciences, Texas A&M University, College Station, TX, United States
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Far-red photons regulate shade avoidance responses and can have powerful effects on plant morphology and radiation capture. Recent studies have shown that far-red photons (700 to 750 nm) efficiently drive photosynthesis when added to traditionally defined photosynthetic photons (400–700 nm). But the long-term effects of far-red photons on canopy quantum yield have not yet been determined. We grew lettuce in a four-chamber, steady-state canopy gas-exchange system to separately quantify canopy photon capture, quantum yield for CO2 fixation, and carbon use efficiency. These measurements facilitate a mechanistic understanding of the effect of far-red photons on the components of plant growth. Day-time photosynthesis and night-time respiration of lettuce canopies were continuously monitored from seedling to harvest in five replicate studies. Plants were grown under a background of either red/blue or white light, each background with or without 15% (50 μmol m−2 s−1) of far-red photons substituting for photons between 400 and 700 nm. All four treatments contained 31.5% blue photons, and an equal total photon flux from 400 to 750 nm of 350 μmol m−2 s−1. Both treatments with far-red photons had higher canopy photon capture, increased daily carbon gain (net photosynthesis minus respiration at night), and 29 to 31% more biomass than control treatments. Canopy quantum yield was similar among treatments (0.057 ± 0.002 mol of CO2 fixed in gross photosynthesis per mole of absorbed photons integrated over 400 to 750 nm). Carbon use efficiency (daily carbon gain/gross photosynthesis) was also similar for mature plants (0.61 ± 0.02). Photosynthesis increased linearly with increasing photon capture and had a common slope among all four treatments, which demonstrates that the faster growth with far-red photon substitution was caused by enhanced photon capture through increased leaf expansion. The equivalent canopy quantum yield among treatments indicates that the absorbed far-red photons were equally efficient for photosynthesis when acting synergistically with the 400–700 nm photons.
Plants capture light as fuel for photosynthesis and perceive fluctuations in their radiation environments as signals that trigger changes in plant shape, biochemical composition, developmental stages and resource allocation (
Additionally, reduced branching and tillering, lower leaf to stem dry mass ratio, smaller proportion of biomass allocation to the roots, hyponastic (upward bending) leaves with reduced chlorophyll (chl) content, and earlier flowering are among the most frequently observed responses to low R:FR ratios in shade-avoiding species adapted to open habitats (
Unlike the shade-avoiding species, some plants tolerate shade without showing a strong, or any phytochrome-mediated stem extension growth. Instead, those shade-tolerant species maximize radiation capture through leaf expansion, which is accompanied with thinner leaves and increased fractional biomass allocation to leaves (
Because crop productivity exhibits a strong linear correlation with radiation intercepted by canopies (
One important difference between simulated shade and natural vegetation shade is that natural shade significantly reduces total photon flux (
While the photomorphogenic effects of FR photons are largely well characterized, far-red photons (
Following canopy photon capture and photosynthetic efficiency, the third determinant of daily carbon gain and productivity is the conversion efficiency of carbon fixed in gross photosynthesis into biomass,
Our objective was to quantify the effects of far-red substitution for 400–700 nm photons on radiation capture, canopy quantum yield, carbon use efficiency, and biomass allocation of a model crop lettuce. This quantitative approach can provide a mechanistic understanding of the effect of far-red photons on plant growth. Continuous measurement of canopy quantum yield for CO2 fixation can provide additional evidence for changing the definition of photosynthetic active radiation to include far-red photons (700–750 nm).
Lettuce (
Sixteen uniform seedlings were moved into a steady-state gas exchange system with four acrylic chambers (100 L/chamber with four plants per chamber; 36 cm × 47 cm × 59 cm; w × l × h), similar to the multi-chamber gas exchange system described by
Spectral distributions of four light treatments composed of red/blue (RB; peaks at 443 and 663 nm), white (peak at 450 nm with secondary peak at 567 nm), and far-red (FR; peak at 730 nm) photons. The numbers following each type of light (
Effect of Spectral Quality on Growth, Morphology, and Chlorophyll Content of Lettuce.
Spectra and photon flux density | ||||
---|---|---|---|---|
RB 350 | RB 300 + FR 50 | White 350 | White 300 + FR 50 | |
(PPE) | (0.87) | (0.82) | (0.84) | (0.70) |
Shoot fresh mass (g) | 130b | 179a | 129b | 167a |
Shoot dry mass (g) | 8.6b | 11.3a | 8.5b | 11.2a |
Shoot dry/fresh mass (%) | 6.7a | 6.4a | 6.6a | 6.8a |
Root dry mass (g) | 1.61a | 1.85a | 1.61a | 1.95a |
% root | 16.1a | 14.7a | 16.6a | 15.3a |
Leaf area (cm2) | 3091b | 4423a | 3111b | 4228a |
Leaf mass per area (g m−2) | 28.2a | 25.5c | 27.5ab | 26.5bc |
Chl content (μmol m−2) | 272a | 218b | 267a | 213b |
Leaf photon absorption | 0.927 | 0.798 | 0.838 | 0.686 |
PPE, Phytochrome photoequilibrium; RB, red and blue; FR, far-red. The number following each type of light (e.g. RB 350) indicates photon flux density in μmol m−2 s−1. Biomass and leaf area were expressed on a per chamber basis (i.e. the sum of four plants). Data represent means from five replicate studies with different letters indicate significant differences between means (P ≤ 0.05; n = 5). Leaf photon absorption of incident photons under each spectral treatment (mol absorbed mol−1incident) was integrated from 400 to 750 nm.
Chamber walls were lined with highly reflective Mylar to eliminate side lighting and increase light uniformity. The reflective walls also help to simulate light environment of a canopy because they reflect FR photons similar to neighboring plants. Two of the treatments had FR photons (700–750 nm) substituting for ~15% of the conventionally defined white or RB photosynthetic photons. All four treatments contained 31.5% blue photons and equal total photon flux of 350 μmol m−2 s−1 from 400 to 750 nm. Photoperiod was 14/10 h light/dark. Daily light integral was 17.64 mol m−2 d−1.
Plants were grown under elevated CO2, with the CO2 concentration [CO2] in the pre-chamber air streams enriched to 800 μmol mol−1. CO2 enrichment is a common practice in controlled environment crop production. Controlling CO2 to a constant elevated level also reduces noises in the gas exchange measurements caused by un-steady [CO2] in the ambient air in urban areas. Air flow rate through the chambers was gradually increased from 11 to 37 mmol s−1 (15 to 50 standard L min−1) from seedling to mature plant stage. Air inside the chambers was mixed with fans (793 L min−1; Model A36-B10A-15T2-000, Globe Motors). The [CO2] inside the chambers ranged from 740 (with mature plants) to 800 μmol mol−1 (seedlings) during the light periods and from 800 (seedlings) to 820 μmol mol−1 (mature plants) during the dark respiration periods. Flow rates through the chambers were adjusted depending the size and photosynthetic rate of canopies to maintain the [CO2] inside each chamber within 5 μmol mol−1 of each other. Chamber air temperature was controlled with resistance heaters and maintained constant at 25 ± 0.1°C day/night. Plants were watered daily with the hydroponic nutrient solution described above (
The pre- and post-chamber air streams were sampled every second using two infra-red gas analyzers (reference and differential IRGA; LI-6252; LI-COR, Lincoln, NE). Each chamber was sampled for 30 s, and then the tubing that connected the chambers to the differential IRGA was purged for 40 s before the next chamber was sampled. Canopy gas exchange rate [net photosynthetic rate during the light period (Pnet, light) and dark respiration rate (Rdark; negative values) in μmolCO2 m−2ground area s−1] was calculated from the mass flow rate and delta [CO2] between the pre- and post-chamber air streams and recorded using a datalogger (CR1000; Campbell Scientific, Logan, UT, USA) throughtout the entire course of the study.
To eliminate the mole fraction dilution of CO2 analysis by water vapor, the air streams sampled by the IRGAs were first passed through nafion dryers (Perma Pure, Lakewood, NJ, USA) and then columns of magnesium perchlorate to completely remove water vapor.
To determine the photosynthetic value of FR photons, far-red LEDs in the two FR substitution treatments were turned off for 30 to 40 min during the days prior to harvest.
Pnet, light and Rdark was averaged over the 14 h of light period and 10 h of dark period respectively, on a daily basis. The averaged values (in units of molCO2 m−2ground area h−1) were used in the subsequent calculations of gas exchange parameters (Eqs 1–3).
Canopy gross photosynthesis (Pgross; g d−1) was calculated as:
Where |Rdark| was the absolute value of dark respiration. 14 was the light period in hours, and 0.17 was the chamber ground area in m2. 30 represents grams dry mass per mole of CO2 assimilated, assuming a carbon content of 0.4 g g−1 in plant tissues. This calculation of Pgross also assumes that respiration rate was similar in the light and dark. There is ongoing research and discussion on whether respiration rate in the light is similar, higher, or lower than respiration in the dark (
Daily carbon gain (DCG; a measure of canopy growth rate in g d−1) was calculated as:
Where 14 was the light period in hours, and 10 was the dark period in hours.
Carbon use efficiency (CUE; the ratio of daily net carbon gain to the total amount of carbon fixed in gross photosynthesis) was calculated from Eqs (1) and (2) as:
Plants were taken out of the gas exchange chambers for 2 to 3 min daily for top down photos of the canopies, taken with a digital camera placed 130 cm above the plant base. Images were analyzed using a Python program, available as open source at
Plants were destructively harvested 17 to 20 days after transferred into the gas exchange chambers, when the largest canopies were near canopy closure. Shoot fresh weight and total leaf number of all four plants in each spectral treatment were recorded. Leaf chlorophyll content (μmol m−2) was measured on five representative leaves per plant and averaged over four plants per treatment (MC-100 chlorophyll meter; Apogee Instruments, Logan, UT). Total leaf area per chamber was measured using a leaf area meter (LI-3000; LI-COR, Lincoln, NE, USA). Shoots and roots were oven-dried to a constant mass (at least 48 h at 80°C). Root percent mass was calculated as root dry mass/(total shoot and root dry mass). Leaf mass per area (g m−2), a measure of leaf thickness, was calculated as leaf dry mass/total leaf area.
Leaf light absorptance was determined on the day of harvest using a spectroradiometer (PS300; Apogee Instruments) similar to the method of
Canopy photon capture (mole of photons absorbed per mole of incident photons) was estimated by multiplying the fraction of canopy ground cover by the leaf photon absorption under each spectral treatment. This gave a good estimate in small canopies with only one layer of leaves. However, the canopy photon capture would be under-estimated in larger canopies that had over-lapping layers of leaves. To account for this, the number of leaf layers in each canopy at harvest was calculated as total leaf area at harvest (m2) divided by the projected canopy ground cover (m2). The projected canopy ground cover (or projected canopy leaf area) was determined from top down photos (
Using lettuce grown under spectral treatment RB 300 + FR 50 as an example, a single layer of leaves absorbed 0.798 moles of photons (integrated over 700 to 750 nm) per mole of incident photons (
Canopy quantum yield for CO2 fixation (moles of CO2 fixed in gross photosynthesis per mole of photons absorbed from 400 to 750 nm) was calculated as:
Note that Pgross here is expressed in units of moles of CO2 fixed per mole of incident photons for the simplicity of the equation. To convert Pgross from g d−1 in Eq. (1) to molCO2 mol−1incident photon, the Pgross values in g d−1 need to be first divided by 30 g mol−1 (grams of dry mass per mole of carbon assimilated), then divided by 0.17 m2 (chamber ground area), and then divided by17.64 mol incident photons m−2 d−1 (total daily photon flux density at canopy level).
Data were analyzed using regression (linear and sigmoid) in Sigmaplot (Systat Software, San Jose, CA) and ANOVA in Statistical Analysis Systems (SAS Institute, Cary, NC, USA). Mean separation was performed using Fisher’s protected least significant difference (LSD,
Lettuce grown under the two control treatments without far-red (white 350 and RB 350) had nearly identical dry mass and leaf area at harvest. In contrast, both treatments with 15% FR substitution, either with a white or RB background light, had 29 to 31% increase in total biomass (shoot and root) and a 36–43% increase in leaf area (
Canopy Pnet, light and Rdark under all four spectral treatments increased exponentially from seedling to young plant stage over the first two weeks, followed by slower increases during the third week as plants quickly approached canopy closure (
Canopy net photosynthetic rate [Pnet; net gas exchange rates in light (positive) and dark (negative) expressed as μmolCO2 m−2ground area s−1] of lettuce under red/blue (RB) and white, with or without far-red substitution. The numbers following each type of light (
Daily carbon gain (DCG, a measure of canopy growth rate) of lettuce under four spectral treatments from seedling to mature plants. RB, red and blue; FR, far-red. The numbers following each type of light (
A decrease in Pnet, light was detected within 5 min (note that each chamber was sampled every 4 min and 40 s) when the far-red LEDs in the two FR substitution treatments were turned off (
Carbon use efficiency (CUE) was lower for seedlings and gradually increased as plants matured (
Carbon use efficiency (CUE = daily carbon gain ÷ gross photosynthesis) of lettuce grown under red/blue (RB) and white light, with or without far-red (FR) substitution. The numbers following each type of light (
Fractional ground cover and canopy photon capture increased over time in a similar manner as gas exchange rates and daily carbon gain (
Fraction of ground cover and canopy photon capture of lettuce under four spectral treatments. Fraction of ground cover was obtained from green pixel analysis of top down photos of the canopies. Canopy photon capture (moles of photon absorbed from 400 to 750 nm per mole of incident photons) was estimated from ground cover, leaf photon absorption of the incident spectra and leaf area index. RB, red and blue; FR, far-red. The numbers following each type of light (
The higher fractional ground cover translated into higher canopy photon capture in the FR substitution treatments, although to a smaller extent (
Leaf photon absorptance of lettuce grown under four spectral treatments. RB, red and blue; FR, far-red. The numbers following each type of light (
Canopy quantum yield (moles of CO2 fixed in gross photosynthesis per mole of absorbed photons integrated over 400 to 750 nm) was similar among all four light treatments, with an average of 0.057 ± 0.002 (SD) (
Canopy quantum yield (moles of carbon fixed in gross photosynthesis per mole of photons absorbed from 400 to 750 nm) of lettuce grown under red/blue (RB) and white light, with or without far-red (FR) substitution. The numbers following each type of light (
Canopy Pgross of all four treatments increased linearly with increasing photon capture and had a common slope; daily carbon gain of all four treatments also showed a common linear correlation with canopy photon capture (
Canopy gross photosynthesis (Pgross) and daily carbon gain (DCG) as a function of canopy photon capture of lettuce. Canopy Pgross (and DCG) of all four treatments increased linearly with increasing photon capture and had a common slope. RB, red and blue; FR, far-red. The numbers following each type of light (
Beneficial leaf expansion elicited by far-red has been increasingly explored as means to improve production efficiency of high-value leafy green vegetables and ornamentals in greenhouses and indoor vertical farms (
Far-red supplementation comes with an energy cost and it remains unclear whether it is cost effective to add far-red photons or to increase the traditionally defined photosynthetic photons (400 to 700 nm) by the same amount. Far-red photons contain lower energy and thus can be generated with higher efficacy (moles of photons per unit of electric energy) than red, blue, and green photons using current LED technology (
The increased leaf expansion in the far-red substitution treatments was presumably caused by a decrease in phytochrome photoequilibria (PPE) compared to the control treatments (white or red/blue light) (
Phytochrome photoequilibria cannot be directly determined in green leaves due to the masking effect of chlorophylls (which also fluoresce) (
The increased yield in response to FR supplementation is often attributed to increased photon capture (
Leaf thickness and chlorophyll content affect leaf optical properties (
In addition, the far-red photons were less efficiently absorbed by the leaves, which reduced canopy photon capture (
All gas exchange systems measure the carbon assimilation rate in moles of carbon (CO2) fixed per unit time. To convert Pgross and DCG from moles of carbon to grams of biomass, we assumed that the canopy carbon content of lettuce was 0.4 g carbon g−1dry mass. Carbon fraction in plants has often been assumed to be 43 to 45% (
Growing conditions also affect carbon fraction.
Measurements of canopy gas exchange rate coupled with canopy photon absorption enable the calculation of canopy quantum yield, a measure of photosynthetic efficiency expressed as moles of CO2 fixed in gross photosynthesis per mole of photons absorbed. We calculated quantum yield using wavelengths from 400 to 750 nm. To our knowledge, this is the first report of the long-term effect of far-red photons on canopy quantum yield. The lack of difference in canopy quantum yield among the four spectral treatments indicates that the absorbed far-red photons were equally efficient as red/blue or white photons (
On an incident photon flux basis,
Respiration is often assumed to be a fixed fraction of gross photosynthesis in modeling maximum productivity per unit input of solar energy (
However, reports have conflicted on whether CUE stays constant among species, environmental conditions, and developmental stages (
In this study, CUE of young/newly matured lettuce plants (~16–22 days after emergence) was similar under all four treatments, with an average of 0.61 ± 0.02. This is consistent with the values reported for lettuce of similar age by
Overall, we found that neither canopy quantum yield nor carbon use efficiency were affected when substituting far-red for traditionally defined photosynthetic photons. This indicates that the absorbed far-red photons (700–750 nm) were equally efficient for photosynthesis when acting synergistically with 400–700 nm photons. Crop yield increased with far-red substitution due to increased leaf expansion and canopy radiation capture mediated by phytochromes during long-term plant cultivation. These data, coupled with previous studies, provide compelling evidence that the current definition of photosynthetically active radiation should be extended to include photons from 700 to 750 nm.
All datasets presented in this study are included in the article/
SZ and BB designed the experiment. SZ performed the experiment, analyzed data, and wrote the first draft. SZ and BB discussed the data and revised the manuscript.
This work was supported by the Utah Agricultural Experiment Station, Utah State University; by the NASA-CUBES project award number NNX17AJ31G; and by the USDA-NIFA-SCRI award number 2018-51181-28365 (LAMP Project). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Aeronautics and Space Administration (NASA) or the USDA.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We thank Jakob Johnson for helping with the image analysis of canopy ground cover and Alec Hay for technical support.
The Supplementary Material for this article can be found online at:
Canopy net photosynthetic rate (Pnet; μmolCO2 m−2ground area s−1) of lettuce under red/blue (RB) and white, with or without far-red substitution. The numbers following each type of light (
Relationships between estimated phytochrome photoequilibria (PPE) and leaf area of lettuce grown with or without far-red photons under two background light spectra—white and red/blue (RB). The decrease in the estimated PPE under each background light was due to a 15% far-red substitution. All four spectral treatments had equal total photon flux of 350 μmol m−2 s−1 (400 to 750 nm) and 31.5% blue photons.
CUE, carbon use efficiency; DCG, daily carbon gain; LAI, leaf area index, LED, light emitting diode; Canopy Pgross and Pnet, canopy gross and net photosynthetic rate; PAR, photosynthetically active radiation from 400 to 700 nm; PPFD, photosynthetic photon flux density integrated over 400 to 700 nm; PFD, photon flux density; Rdark, dark respiration.