Phytochromes are photoreceptor proteins making possible the perception of the external light environment by plants. The light reception region within phytochrome is the chromophore that has two interconvertible isoforms with different peak light absorption spectra in the red [wavelengths of 600 to 700 nm (R)] and far-red [700 to 800 nm (FR)] regions (Hendricks et al., 1962; Siegelman et al., 1966). Irradiation with R or FR light changes the phytochrome isoform ratio and alters biochemical and physiological responses such as germination (Mancinelli et al., 1966), stem elongation, and pigment synthesis (Hendricks and Borthwick, 1967).
Red or FR light at the end of the day (photoperiod) is known to affect stem elongation (Blom et al., 1995; Decoteau et al., 1988; Kasperbauer and Peaslee, 1973). Tomato seedlings with end-of-day (EOD) FR light treatment had a greater height and leaf length than those with EOD-R light treatment (Decoteau and Friend, 1991). EOD-FR lighting was also used in fundamental plant biology research, investigating the phytochrome signal transduction pathway in Arabidopsis thaliana, cucumber (Cucumis sativus), and other species such as aspen (Populus tremula ×tremloides) in which stem elongation under EOD-FR was used as a positive test in identifying phytochrome genes and intermediate molecules such as gibberellic acid (Moe et al., 2003; Naganati et al., 1991; Olsen and Junttila, 2002).
Our study focuses on application of EOD-FR lighting in controlling morphology of tomato rootstock seedlings. Vegetable grafting has been widely used worldwide to obtain resistance to soilborne pathogens and pests (Kubota et al., 2008). Methyl bromide fumigation has been limited by the Montreal Protocol. As a result, alternatives such as grafting became increasingly viable. Producing seedlings with long hypocotyls is desired in vegetable grafting for the reasons described subsequently. The position of the graft union must be high enough to prevent the vulnerable scion from coming into direct contact and exposure with the soil. However, the tomato graft union is often below the cotyledons of rootstock seedlings, especially when rootstock axillary shoot growth needs to be avoided. Longer hypocotyl lengths of rootstock would both allow easier grafting and reduce the risk of scion exposure. Also, the authors are aware that commercial propagators often encounter inconsistencies in seedling morphology, possibly as a result of changes in light quality during twilight hours or light quality altered by the artificial lighting used in or near the propagation facilities. Hence, it may also be possible to use EOD light treatments as a means of increasing plant hypocotyl lengths as well as the consistency of seedling heights.
In this study, EOD-FR treatment was examined as a non-chemical means to extend hypocotyl length of tomato rootstock seedlings. Although supplemental lighting has limited use in commercial vegetable seedling production, EOD lighting may be more economically feasible than supplemental photosynthetic lighting as a result of its lower intensity requirements and shorter application duration. In the present study, we examined two critical questions that need to be answered for applications of EOD-FR treatment in vegetable seedling production: 1) light quality (R/FR ratio) and 2) minimum dose (intensity × duration) requirements to assure the maximum response of rootstock elongation.
Incandescent lamps emit light rich in FR (at ≈0.4 to 0.5 R/FR ratio) and are often used for enriching FR light in plant growth chambers. They are also widely used for night break treatment (duration in hours) for modulating photoperiodic responses of floricultural species. In phytochrome studies, when incandescent lamps were used as the light source, they were always combined with a spectral cut filter to eliminate non-FR radiations. Lund et al. (2007) examined effects of R/FR ratios (0.4 and 0.7) in the EOD-FR treatment on stem elongation of chrysanthemum plants, showing a greater degree of stem elongation at a R/FR ratio of 0.4 than 0.7. In our experiment, we compared the effects of EOD-FR lighting with a 0.05 to 0.47 R/FR ratio achieved by use of incandescent lamps and spectral filters.
In EOD experiments, FR or R light was provided at relatively low intensity for a short duration. For example, EOD-FR light intensity used in previously reported experiments varied from 0.8 μmol·m−2·s−1 (for Cucumis sativus; Moe et al., 2003) to 222 μmol·m−2·s−1 (for Vigna sinensis; Martinez-Garcia et al., 2000) for a lighting duration ranging from 4 min (for Pharbitis nil; Fredericq, 1964) to 30 min (for Cucumis sativus; Moe et al., 2003). Because phytochrome, as a photoreceptor, responds to the intensity (photon flux) as well as the spectral quality of light, understanding phytochrome kinetics (or dose–response) would help in selecting the necessary light intensity, quality, and treatment duration. Such understanding is especially important in horticultural applications where growers may consider use of light sources with optimized spectra such as light-emitting diodes (LEDs). However, only a limited number of plant species, including Avena sativa, Helianthus annuus, and Vigna radiate (Gorton and Briggs, 1982), Cucumis sativus (Gaba and Black, 1985), and Zea mays (Gorton and Briggs, 1980), were well studied for kinetic response to EOD-FR light treatments. As far as we know, there is no report on the level of saturation dose of EOD-FR light on tomato or its wild relatives. Therefore, we conducted an EOD-FR light dose experiment to find out the relationship between hypocotyl length and EOD-FR dose (1 to 8 mmol·m−2·d−1) using FR light of a 0.05 R/FR ratio. Dose was varied using both intensity and duration to find out whether the response was intensity- or duration-dependent.
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