Effects of plant growth retardant and gibberellin on flower coloration patterns of Petunia × hybrida

Petunia (Petunia × hybrida) plants are treated with plant growth retardant (PGR) to inhibit stem growth. However, the effects of PGR differ depending on the PGR type, direction of use, and plant species or cultivars. In red petunias, PGR treatment inhibits coloration. Here, we investigated the effects of gibberellin (GA) and four PGRs, namely, daminozide (SADH), trinexapac-ethyl (TNE), paclobutrazol (PBZ), and prohexadione-calcium, on four Petunia cultivars with different coloration patterns: ‘Baccara Red’, ‘Red morn’, ‘Red picotee’ and ‘Star Red and White’). The SADH, TNE, and PBZ treatments effectively inhibited stem growth in all four cultivars. The SADH treatment affected the corolla coloration of the ‘Star Red and White’ (star type color pattern) and ‘Red Picotee’ (picotee type color pattern), however, did not affect ‘Baccara Red’ and ‘Red Morn’ (gradation coloration pattern) cultivars. These results suggest that the effects of SADH treatment on corolla coloration differ depending on the cultivar. We elucidated that the alteration in coloration by SADH in ‘Star Red and White’ and ‘Red Picotee’ was caused by GA activity suppression. However, the corolla coloration of ‘Red Morn’ was not affected by SADH or GA. These findings indicated that variations in GA activity may result in different coloration patterns. This study demonstrated that treatment with four types of PGRs inhibited stem elongation and affected flower coloration patterns in petunia. Additionally, GA may play a role in the formation of petunia bicolor patterns.