Elsevier

Scientia Horticulturae

Volume 98, Issue 4, 29 September 2003, Pages 423-431
Scientia Horticulturae

Promotion of Globularia sarcophylla flowering by Uniconazol, an inhibitor of gibberellin biosynthesis

https://doi.org/10.1016/S0304-4238(03)00042-6Get rights and content

Abstract

Globularia sarcophylla, originating from the Canary Islands, was recently introduced as a new cut flower in Israel. Two major problems have prevented its commercialization: the late-summer blooming and the low quality of its flowering shoots. In the present work we studied the factors affecting G. sarcophylla flowering. We found that long-day (LD) conditions slightly promote flowering but artificial photoperiodic illumination does not enable the manipulation of flowering time. On the other hand, treatment with the gibberellin (GA)-biosynthesis inhibitor, Uniconazol, had a dramatic promotive effect on flowering time. Application of Uniconazol in autumn or winter induced flowering in winter or early spring, respectively. The inhibitor did not advance flowering during the summer or in the phytotron under high temperatures. When plants were grown in the spring/summer under heavy shading, they did not flower unless they were treated with Uniconazol. Uniconazol treatment also improved flower quality by reducing the length of inflorescence pedicles. This effect was found in all seasons. Based on our results, we raise the hypothesis that Uniconazol treatment induces flowering by diverting assimilates to the apex. Inhibition of GA biosynthesis under conditions limiting photosynthetic activity reduces vegetative growth and increases the availability of assimilates to the apex, leading to flower initiation. However, under high irradiance and/or high temperatures, when the levels of assimilates are sufficient to induce natural flowering, the inhibitor has no further effect.

Introduction

Globularia, native to North Africa, the Cape Verde Islands and the Canary Islands, belongs to the Globulariacea. The family consists of three genera, but only Globularia has been cultivated as a garden ornamental (Bailey, 1939). G. sarcophylla is a herbaceous plant, a sub-shrub with alternate, undivided leaves and tiny flowers carried in heads (capitula). The young petals are blue but turn white as they mature. This change starts from the head’s periphery to the center and thus maturating flower heads appear white with a blue center. Several years ago, G. sarcophylla was introduced to Israel from the Canary Islands. Early observations revealed variability in the population, and screening for clones suitable for cut flowers, led to the selection of cv. ‘Blue Eyes’. This clone has a long flowering stem that produces many flower heads. When grown in Israel, outdoor or in greenhouses, G. sarcophylla plants exhibit a flash of flowering at the beginning of the summer. This summer blooming reduces the value of commercial production of the plant for cut flowers since the main export season in Israel is winter and early spring. Furthermore, the quality of the flowers naturally developing during the summer is poor due to excess elongation of inflorescence pedicles. Preliminary experiments (Weiss, unpublished data) indicated that long-day (LD) conditions or vernalization treatment to young plants could not induce flowering during the winter.

Gibberellin (GA) plays a major role in the regulation of flower initiation in many plant species. In numerous species, GA promotes flowering (Metzger, 1995), but in some others, it inhibits flowering. Inhibition of flower initiation by GA has been reported for herbaceous plants such as Fuchsia hybrida (Sachs and Bretz, 1961), Pisum sativum (Reid et al., 1977) and Pharbitis nil (King et al., 2000), as well as for woody species such as citrus (Monselise and Goldschmidt, 1982) and Ixora (Shillo and Tsook, 1989). In some of these cases, treatment with GA-biosynthesis inhibitors induced flowering (Shillo and Tsook, 1989, Halevy, 1990). In the present study, we investigated the effect of the GA-biosynthesis inhibitor Uniconazol (E-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl-1-pentan-3-ol) on G. sarcophylla flowering.

Section snippets

Plant material

G. sarcophylla cv. ‘Blue Eyes’ plants were produced locally from terminal (10 cm long) cuttings. Plant were grown under two sets of experimental conditions—greenhouse and phytotron. Plants were grown under various day-length conditions (see below) and 1 year after planting, they were pruned. One month after pruning the plants were sprayed to drench with 40 mg l−1 (unless otherwise specified) Uniconazol (Sumitomo Chemical, Hyogo-Ken, Japan), repeating the treatment four times, once every 2 weeks.

Effect of Uniconazol on Globularia flowering

To study the possible effect of a growth retardant on G. sarcophylla flowering, plants were grown in the greenhouse under ND and LD conditions and treated with Uniconazol as described in Section 2, starting at the end of January.

The entire flowering period of treated plants was earlier than that of control plants (Fig. 1). Two months after the first Uniconazol treatment, the sprayed plants started to bloom (March) under both LD and ND conditions (Table 1). Control, non-treated plants started to

Discussion

Growing cut flowers in warm countries such as Israel can be advantageous in that winter production is possible without extra heating and/or photosynthetic illumination. Many naturally summer-blooming plants have been introduced to Israel and to other southern, warm countries and are now grown as winter crops (Halevy, 1995). To achieve this, however, a procedure to force flowering during the winter is required. In recent years we have tried, to introduce G. sarcophylla as new cut flower in

Acknowledgements

This research was supported by the Israeli Ministry of Agriculture and by the Association of Israeli Flower Growers. This work was supported also by the Pearlstein Fund for research in floriculture at the Hebrew University, and we thank the donors for their help.

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