End-of-day Far-red Lighting with a Low Daily Light Integral Increases Stem Length But Does Not Promote Early Leaf Expansion for Petunia ×hybrida Seedlings

• Published in: HortScience Vol. 58; no. 9; pp. 1010 - 1017
• Main Authors: Percival, Anthony C., Craver, Joshua K.
• Format: Journal Article
• Language: English
• Published: American Society for Horticultural Science (ASHS) 01.09.2023
• Subjects: controlled environment agriculture; light-emitting diode; plug; shade avoidance syndrome
• ISSN: 0018-5345; 2327-9834
• DOI: 10.21273/HORTSCI17132-23

Greenhouse production of high-quality young annual bedding plants (plugs) at northern latitudes often requires supplemental lighting to compensate for a low natural daily light integral (DLI), but radiation interception by plugs is limited by a low leaf area index. Some species show an increase in leaf area in response to growth under a low ratio of red to far-red radiation (R:FR), and an early increase in leaf area may allow for more effective radiation capture by seedlings and a reduction in wasted radiation. Thus, the objective of this study was to examine the effects of end-of-day far-red (EOD-FR) radiation treatments varying in intensity, R:FR (600–700 nm/700–780 nm), and duration on early leaf expansion and plug quality for petunia ( Petunia × hybrida ) ‘Wave Purple’ and ‘Dreams Midnight’. Seedlings were grown in 128-cell trays in a common greenhouse environment under a simulated winter DLI (∼5.3 mol·m −2 ·s −1 ) and received one of four EOD-FR treatments, control conditions (no EOD-FR or supplemental lighting), or supplemental lighting (target photosynthetic photon flux density of 70 μmol·m −2 ·s −1 ). The EOD-FR treatments were provided for 3 weeks on cotyledon emergence and included the following: 10 μmol·m −2 ·s −1 of far-red radiation for 30 minutes with a R:FR of ∼0.8 (EOD FL ), 10 or 20 μmol·m −2 ·s −1 of far-red radiation for 30 minutes with a R:FR of ∼0.15 (EOD 10:30 and EOD 20:30 , respectively), or 20 μmol·m −2 ·s −1 of far-red radiation for 240 minutes with a R:FR of ∼0.15 (EOD 20:240 ). Destructive data were collected 14 and 21 days after cotyledon emergence. Seedlings that received EOD-FR treatments did not show any increase in leaf area compared with control or supplemental lighting treatments. Stem length generally increased under EOD-FR treatments compared with supplemental lighting and control treatments; greater elongation was observed when the R:FR decreased from 0.8 to 0.15, and when treatment duration increased from 30 minutes to 240 minutes. However, at a R:FR of 0.15 and a treatment duration of 30 minutes, an increase in far-red radiation intensity from 10 to 20 μmol·m −2 ·s −1 did not promote further stem elongation resulting in similar stem lengths for both cultivars under EOD 10:30 and EOD 20:30 . Results of this study indicate that under low DLIs, EOD-FR radiation applied in the first 3 weeks of seedling production does not promote early leaf area expansion, and generally decreases seedling quality for petunia. As responses to far-red radiation may vary based on study taxa, incident radiation, and DLI, future research examining EOD-FR–induced morphological changes is warranted.