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.