Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO2 Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR

Abstract

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.

Keywords: canopy photosynthesis; carbon use efficiency; far-red photons; phytochrome equilibrium; quantum yield; radiation capture.

Figure 1

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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹. All four treatments had equal total photon flux of 350 μmol m⁻² s⁻¹ (400 to 750 nm) and 31.5% blue photons.

Figure 2

Canopy net photosynthetic rate [P_net; net gas exchange rates in light (positive) and dark (negative) expressed as μmol_CO2 m⁻²_{ground area} s⁻¹] of lettuce under red/blue (RB) and white, with or without far-red substitution. The numbers following each type of light (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹. Plants were seeded in a greenhouse and moved into a multi-chamber gas exchange system on day zero, four days after seedling emergence. Downward pointing arrows indicate when far-red LEDs were turned off. This representative dataset of P_net was used to calculate daily carbon gain shown in Rep 1 of Figure 3 carbon use efficiency in Figure 4, and canopy quantum yield in Figure 7.

Figure 3

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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹. Plants were moved into the gas exchange system from a greenhouse on day zero (four days after emergence in Rep 1 and two days after emergence in Rep 2).

Figure 4

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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹. Dashed lines indicate a typical CUE value of 0.6 that has been reported for mature plants.

Figure 5

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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹.

Figure 6

Leaf photon absorptance of lettuce grown under four spectral treatments. RB, red and blue; FR, far-red. The numbers following each type of light (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹.

Figure 7

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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹.

Figure 8

Canopy gross photosynthesis (P_gross) and daily carbon gain (DCG) as a function of canopy photon capture of lettuce. Canopy P_gross (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 (e.g. RB 350) indicate photon flux density in μmol m⁻² s⁻¹. Circles represent data from Rep 1, and triangles represent data from Rep 2 (see DCG in Figure 3).