Phytochromes are a class of plant photoreceptors that modulate development throughout the life cycle of a plant. They interconvert between two major forms upon photon absorption: the inactive form (P_r), which is most sensitive to red photons, and the active form (P_fr), which is most sensitive to far-red photons [1]. Although P_r and P_fr are named for the region that they are most sensitive to, both forms absorb across the entire biologically active range of radiation (300 to 800 nm). Historically, a metric called phytochrome photoequilibrium (PPE) has been used to predict phytochrome-mediated developmental responses [2]. PPE is an estimate of the fraction of active P_fr to the total phytochrome pool, and it is calculated from the spectral photon distribution (SPD) of the incident light and photoconversion cross-sections (which predict the likelihood of photon absorbance and subsequent phytochrome conversion) for P_r and P_fr at each wavelength [2–8]. Photoconversion cross-sections are closely related to absorption spectra and, when multiplied by the photon intensity at specific wavelengths, they provide an estimate of the rates of conversion between the two forms of phytochrome [3, 4]. Several studies have separately derived the photochemical parameters necessary to calculate these photoconversion cross-sections. Fig 1A shows four sets of P_r cross-section values (from 650 to 800 nm) that are derived from 1) Seyfried and Schäfer [5], 2) Kelly and Lagarias [6], 3) Lagarias et al. [7] and 4) Sager et al. [8]. Shinomura et al. [9] used a spectrograph (a device that uses prisms to provide narrow bandwidths of radiation) to determine the action spectrum of seed germination in Arabidopsis thaliana and found that it closely matched the absorbance spectrum of P_r (Fig 1A). Phytochrome absorbance spectra above 800 nm have not been rigorously determined, but Schäfer et al. [10] predicted the photoconversion cross-sections out to 1100 nm using action spectra responses for both the inhibition of mesocotyl elongation and the promotion of coleptile elongation. These data showed a sustained decrease in the relative photoconversion cross-section out to 1100 nm. This indicates that, although the ability of photons to activate P_r into P_fr decreases rapidly above 700 nm (and even 800 nm), responses ought to still occur beyond 700 nm with high enough photon intensities.

Photoconversions for the conversion of P_fr back to P_r are also available from many of these studies [5–8] (Fig 1B). These show a higher sensitivity than P_r cross-sections between 700 to 800 nm, but similar to P_r, rigorously determined P_fr photoconversion cross-sections are not available for wavelengths above 800 nm.

A common NIR LED for night vision has a peak at about 850 with a full width at half maximum (FWHM) of about 35 nm and outputs photons down to about 700 nm (S1 Fig). This LED is used in security cameras for night vision in greenhouses and controlled environment agriculture. Because of their regular use in controlled plant growth environments, especially during the dark periods, it is valuable to investigate the potential role they may play in altering plant growth and development. The photons from this LED may affect plant growth and development either by 1) the activation of P_r to P_fr (Fig 1A), or 2) the inactivation of phytochrome from P_fr to P_r (Fig 1B). The activation of phytochromes (i.e. convert P_r to P_fr) can be assessed by the inhibition of floral initiation in short-day plants (SDP), while the inactivation of phytochrome (i.e. convert P_fr to P_r) can be assessed with stem elongation.

SDP undergo floral initiation when the period of un-interrupted darkness is longer than a critical length [11]. The application of photons for 4 hours or less during the dark period, called a night-break or night-interruption, is a common practice to delay or inhibit flowering of SDP in ornamental crop production [12, 13].

Similarly, low levels of constant light throughout the dark period, here called nighttime photons, can also disrupt flowering in SDP. P_fr plays a vital role in this process [14]; however, the mechanisms governing the response are only partially understood. It involves complex interactions between phytochromes (phyA, phyB and phyC in the SDP rice [15]) and 1) the circadian oscillator [16], 2) transcriptional regulation [17], and 3) possibly post-transcription stabilization [18].

Early studies that contributed to the discovery of phytochrome investigated the action spectrum of floral inhibition by night-break lighting in SDP. Using a spectrograph, these studies found strong inhibitory responses to red photons (600 to 700 nm), and minimal responses to the yet unnamed far-red photons, especially beyond 720 nm, although they still observed some inhibition at 770 nm; no response was observed at 840 nm [19]. Following the landmark flip-flop seed germination study with red and far-red photons [20], studies found that far-red could reverse red night-break inhibition of flowering in SDP [21, 22]. Although flowering was able to be re-induced by a far-red pulse after a red pulse, it differed from the germination response in that each additional cycle had a reduced response. After four cycles of alternating between red and far-red pulses, flowering was almost entirely inhibited [22]. Follow-up studies determined that high doses (dependent on intensity and duration) of far-red (with or without prior night-break with red) inhibited flowering of SDPs compared to a control without night-break lighting [22–25].

Table 1 summarizes the effect of far-red night-break lighting compared to controls without night breaks reported in studies spanning 63 years. The older studies report stage of floral development as an index, and the newer studies typically report time to flowering. Additionally, the older studies typically used a spectrograph with filters while the newer studies apply far-red photons with LEDs that have a peak at about 730 nm. It should be noted that this LED outputs some photons below 700 nm, while the NIR LED does not output photons below 700 nm. Some studies show a delay in flowering (see [26]), indicating that photons above 700 nm are able to activate phytochrome into P_fr and inhibit flowering. By contrast, some studies under similar conditions show no significant response (see [27]). These contradictions may be due to differences in the duration of the dark period, intensity of the far-red, duration of the night-break and sensitivity of the species (Table 1). Floral initiation is a complex molecular process, and different species/cultivars will have different thresholds for a photo-molecular process to occur. Therefore, it is important to choose species known to be sensitive to night-break/nighttime photons when investigating the ability of photons from NIR LEDs to activate phytochromes and inhibit flowering. Vince-Prue [11] listed soybean (Glycine max) and Cannabis sativa as among the most photosensitive species to nighttime photons.

Species	Reported FR intensity	FR source	Night break length	Photoperiod conditions (day/night)	Effect on flowering development	Citation	Comment
Xanthium pensylvanicum Wallr.	unclear	filtered sunlight with output near 735 nm	12 min	12h/12h	Reduced stage of flowering from 6 to 4 (scale from 0 to 7) - 33% reduction	Downs [22]	Provided after R (about 50 μmol m-2 s-1)
Chrysanthemum morifolium cv. Indianapolis Yellow		Filtered incandescent			Reduced stage of flowering from 4.1 to 0.4 (scale from 0 to 10) - 90% reduction
Chrysanthemum morifolium cv. Shasta	unclear		81 min	9h/15h	Reduced stage of flowering from 3.7 to 0 (scale from 0 to 10)—inhibited	Cathey and Borthwick [23]
Chrysanthemum morifolium cv. Honey Sweet					Reduced stage of flowering from 3.7 to 0 (scale from 0 to 10)—inhibited
Chenopodium rubrum	14 μmol m-2 s-1	Spectrograph centered at 730 nm (about 720 to 740 nm)	16 min	8h/16h	Reduced stage of flowering from 9 to 6.7 (scale from 0 to 9) - 26% reduction	Kasperbauer et al. [24]
	about 50 μmol m-2 s-1	Filtered incandescent quantified from 710 to 800 nm		12h/12h	Reduced stage of flowering from 6.7 to 4.9 (scale from 0 to 7) - 27% reduction
Xanthium pensylvanicum Wallr.			1 h	8h/16h	Reduced stage of flowering from 6.8 to 3.1 (scale from 0 to7) - 55% reduction	Mancinelli and Downs [25]
				4h/20h	Reduced stage of flowering from 6.7 to 0.9 (scale from 0 to 7) - 87% reduction
Oryza sativa (rice)	18000 μmol m-2	acrylic filtered fluorescent. Shortest wavelength ≈ 710 nm, peak ≈ 765 nm	"flash"	10h/14h	no effect	Ishikawa et al. [27]
Tagetes erecta (African Marigold) cv. America Antigua Yellow	1.3–1.6 μmol m-2 s-1	LED peak at about 730 nm, quantified from 700 to 800 nm	4 h	9h/15h	9 day delay	Craig and Runkle [12]	only significant in one of two replicate studies
Chrysanthemum morifolium Ramat. cv. Reagan	62.5 μmol m-2 s-1	LED peak at about 740 nm, quantified from 300 to 900 nm	4 h	12h/12h	1.7 day delay	Higuchi et al. [28]
Chrysanthemum ×morifolium cv. Adiva Purple	1.3–1.6 μmol m-2 s-1	LED peak at about 735 nm, quantified from 700 to 800 nm	4 h	9h/15h	no effect	Craig and Runkle [13]	Data for Dahlia should be interpreted with caution because there was incomplete flowering in SD and FR NB treatments
Dahlia hortensis cv. Carolina Burgundy					11 day delay
Dahlia hortensis cv. Figaro Mix					8 day delay
Tagetes erecta (African Marigold) cv. America Antigua Yellow					10 day delay
Chrysanthemum seticuspe	20 μmol m-2 s-1	LED peak at about 740 nm	10 min	8h/16h	no effect	Higuchi et al. [29]	Supplementary data
Chrysanthemum morifolium Ramat. cv. Iwa no hakusen	6.6 μmol m-2 s-1	LED peak at 728 nm, quantified from 400 to 800 nm	6 h	12h/12h	Reduced stage of flowering from 0.86 to 0.27 (scale from 0 to 1) - 96% reduction	Liao et al. [30]	R NB similar to SD
Chrysanthemum morifolium Ramat. cv. Jimba					no effect
Dendranthema grandiforum cv. Gaya Yellow	10 μmol m-2 s-1	LED peak at 730 nm	4 h	10h/14h	no effect	Park and Jeong [26]

Photons from NIR LEDs could also potentially affect plant growth and development by inactivating P_fr back into P_r. Far-red photons are often reported to increase stem elongation [31], a process that is modulated through the inactivation of phytochrome [32].

We investigate the ability of photons with wavelengths greater than 700 nm from NIR LEDs applied over a 24 h photoperiod to 1) delay flowering in two sensitive short-day species, and 2) elongate stems. We found that at high enough doses, photons from NIR LEDs can affect both of these plant responses, indicating a role of long wavelength photons in modulating plant growth and development.