End-of-day far-red light quality and dose requirements for tomato rootstock hypocotyl elongation

Plant morphology control is a critical technique in commercial greenhouse transplant production. Light treatment at the end of the day affects a phytochrome- regulated response affecting plant height among other characteristics and has been studied by biologists for many years. Recognizing the need to produce long hypocotyls in vegetable grafting, effects of end-of-day far-red (EOD-FR) light on tomato rootstock hypocotyl elongation were investigated. Two commercial rootstock cultivars, Aloha (Solanum lycopersicum) and Maxifort (S. lycopersicum · S. habrochaites), were used for the experiments examining responses to EOD-FR light quality (red to far-red ratio (R/ FR)) and EOD-FR light dose in a greenhouse environment. In the EOD-FR light quality experiment using 'Aloha' seedlings, incandescent light (R/FR = 0.47) induced significant hypocotyl elongation (20%) compared with the untreated control. Incandescent light with a spectral cut filter (reducing R/FR to 0.05) induced a greater hypocotyl elongation (44%) than unfiltered light, confirming the importance of use of light with a lower R/FR (or purer FR) light source in EOD-FR treatments. In the experiment on EOD-FR light dose-response, hypocotyl elongation of both 'Aloha' and 'Maxifort' was increased by increasing FR intensity as well as FR treatment duration at a lower dose range. The dose saturation curve of hypocotyl length was well described using a Michaelis-Menten-type model with FR dose (in mmolm -2 d -1 ) as an independent variable. The model-based estimation of 90% saturating FR light dose for 'Aloha' and 'Maxifort' was 5 to 14 mmolm -2 d -1 and 8 to 15 mmolm -2 d -1 , respectively, although practical near saturation dose seems to be 2 to 4 mmolm -2 d -1 for both cultivars. None of the EOD-FR treatments affected plant dry weight, stem diameter, or plastochron index. Hence, elongation was achieved without compromising growth and development. EOD-FR was shown to be an effective non-chemical means allowing transplant propagation industry to produce long hypocotyls for grafting use. Phytochromes are photoreceptor proteins making possible the perception of the exter- nal 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 re- sponses 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 elonga- tion (Blom et al., 1995; Decoteau et al., 1988; Kasperbauer and Peaslee, 1973). Tomato seed- lings with end-of-day (EOD) FR light treat- ment 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 trans- duction pathway in Arabidopsis thaliana ,c u- cumber (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 phyto- chrome genes and intermediate molecules such as gibberellic acid (Moe et al., 2003; Naganati et al., 1991; Olsen and Junttila, 2002).