Efficacy of End-of-Day Far-Red Light In Controlling Tomato Rootstock Height

Pre-plant soil fumigants are used widely to combat soil-borne pathogens in open-field vegetable production. Recently, vegetable grafting was introduced to North America as a potential alternative with the benefit of increasing yields. Grafted tomato seedlings, however, are small in size and the position of graft union is sometimes too close to the ground when transplanted, and may expose the vulnerable scion to pathogens. Preparing rootstock seedlings with a longer hypocotyl length will allow higher positions of the graft union. End-of-day far-red (EOD-FR) is known to induce hypocotyl elongation. To apply this knowledge in rootstock preparation in greenhouse environments, we need to know the plant requirement of the FR light quality (i.e., Red to Far-Red ratio or R:FR) and the minimum FR dosage for maximum response to induce hypocotyl elongation. In the first experiment, ‘Aloha’ and ‘Maxifort’ tomato rootstock seedlings were subject to EOD-FR light treatment at 2.5 μmol m^-2 s^-1 FR photon flux for 12 minutes for fourteen days, using two FR light sources (unfiltered incandescent light with 0.5 R:FR and filtered incandescent light with 0.05 R:FR). EOD-FR treatment with unfiltered incandescent light was unable to induce hypocotyl elongation compared to the untreated control for both cultivars. However, filtered incandescent light significantly increased the hypocotyl length by 24% for ‘Maxifort’ and 47% for ‘Aloha’ compared to the respective untreated control seedlings. In the second experiment, ‘Aloha’ and “Maxifort’ rootstock seedlings were subject to EOD-FR treatments for two different durations (6 or 12 minutes) at two FR photon fluxes (1.3 or 2.5 μmol m^-2 s^-1) at the same R:FR (0.05). The four treatments generated a range of FR dosages between 0 (darkness control) and 1.7 mmol m^-2. Hypocotyl lengths of the both cultivars increased in a near-linear fashion with increasing FR dosage. This confirmed that both EOD-FR treatment duration and magnitude of FR photon flux are factors determining the degree of EOD-FR response in tomato rootstock seedlings. Having a low R:FR in EOD-FR treatment was critical in inducing hypocotyl elongation in tomato rootstock seedlings, suggesting a possible application of FR light emitting diodes (LED) as effective EOD-FR light source. Hypocotyl length also increased with FR dosage in the EOD-FR treatment. The minimum dosage for maximum response, however, has yet to be found at a dosage range greater than 1.7 mmol m^-2.