Red and blue light effects on morphology and flowering of Petunia × hybrida

•A combination of far-red-deficient film and red light resulted in compact petunia plants without much delay in flowering time.•The effect of red light was less affected by the background light level than blue light.•Blue light can replace far-red light and/or high irradiance to promote flowering of petunia.

To investigate the effects of R and B in environments with different FR levels, Petunia × hybrida ‘Tidal wave’ were grown in a greenhouse during early spring (March) and late spring (May) under plastic film transmitting (+FR) and not transmitting far-red light (−FR) in combination with R and B light emitting diodes (LED) as supplementary light placed underneath the films. The main objective of the study was to test the effect of R (600–700 nm, peak 660) and B light (400–500 nm, peak 450) on morphology and flowering in a FR-deficient greenhouse environment compared with an environment with FR light in different seasons. R light reduced shoot elongation and resulted in more compact plants in both seasons and the combination of –FR and R light resulted in the most compact plants without much delay in flowering. By contrast, the effect of B light was dependent on the natural background light level and the presence of FR light. In early spring when the natural irradiance was low (1.35 mol m−2 h−1), B light promoted stem elongation, caused a more upright shoot orientation, increased plant height and promoted flowering compared to control and R light, whereas in late spring, when the natural irradiance was higher (2.33 mol m−2 h−1), the effect of B light in a FR-deficient environment was not significantly different from R light in the response to stem elongation and flowering.