Effects of end-of-day far-red light and relative humidity on flowering and stem elongation of petunia (Petunia x hybrida) seedlings

Far-red light at the end of the day (EOD-FR) can promote the flowering of long-day plants. However, it can cause excessive stem elongation that reduces the quality of transplants. Therefore, flowering and stem elongation must be balanced when EOD-FR is used in bedding-plant production. Here, we examined the interactive role of relative humidity (RH) to suppress excessive elongation caused by EOD-FR and the combined effects of EOD-FR and RH on flowering and stem elongation of petunia (Petunia x hybrida) seedlings. We grew seedlings for 28 days in controlled-environment chambers after two true leaves had expanded, with or without EOD-FR, at 40 % or 80 % RH. We found a significant EOD-FR x RH interaction in stem length, with a smaller increase in stem length due to EOD-FR at lower RH. Flower development, which was evaluated using the node position at first flowering, was promoted by EOD-FR. However, low RH suppressed flower development both with and without EOD-FR, thereby counteracting the promotion of flower development by EOD-FR. Thus, EOD-FR treatment at low RH could not prevent excessive stem elongation while promoting flowering. Clarifying the eco-physiological reasons why low RH counteracts the FR-induced promotion of flowering associated with the shade-avoidance response may be necessary to improve flowering control with bedding plant transplants under a controlled environment.

Automated summary

Far-red light at the end of the day (EOD-FR) can promote flowering of long-day plants, but excessive stem elongation may occur. The study found that relative humidity (RH) plays an important role in suppressing EOD-FR-induced stem elongation. Low RH inhibits flower development, both with and without EOD-FR, counteracting the promotion of flowering by EOD-FR.