Long-day induction of flowering in Lolium temulentum involves sequential increases in specific gibberellins at the shoot apex

Abstract

One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectrometry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lolium temulentum. Two separate roles of GAs in flowering are indicated. First, within 8 h of an inductive LD, i.e. at the time of floral evocation, the GA(5) content of the shoot apex doubled to about 120 ng g(-1) dry weight. The concentration of applied GA(5) required for floral induction of excised apices (R.W. King, C. Blundell, L.T. Evans [1993] Aust J Plant Physiol 20: 337-348) was similar to that in the shoot apex. Leaf-applied [(2)H(4)] GA(5) was transported intact from the leaf to the shoot apex, flowering being proportional to the amount of GA(5) imported. Thus, GA(5) could be part of the LD stimulus for floral evocation of L. temulentum or, alternatively, its increase at the shoot apex could follow import of a primary floral stimulus. Later, during inflorescence differentiation and especially after exposure to additional LD, a second GA action was apparent. The content of GA(1) and GA(4) in the apex increased greatly, whereas GA(5) decreased by up to 75%. GA(4) applied during inflorescence differentiation strongly promoted flowering and stem elongation, whereas it was ineffective for earlier floral evocation although it caused stem growth at all times of application. Thus, we conclude that GA(1) and GA(4) are secondary, late-acting LD stimuli for inflorescence differentiation in L. temulentum.

Figure 1

Diurnal changes in three separate experiments in the GA₅ and GA₁₉ content (ng g⁻¹ dry weight [dwt]) of the shoot apex of L. temulentum of vegetative plants in SD (white symbols) and for plants exposed to 1 or 2 florally inductive LD (black symbols). The LD low-irradiance day length extension is shown as a stippled bar, darkness as a black bar, and daylight as a white bar.

Figure 2

Changes during inflorescence development in the content of various GAs in the shoot apex of L. temulentum exposed to 2 LD (Expt. Lt 447). Harvests were at the start (8:30 am) of each daily 8-h sunlight exposure. a, GA content is shown as ng g⁻¹ dry weight; b, GA content is shown as ng per apex, together with apex dry weights (×).

Figure 3

Effect of increasing numbers of LD on shoot apex length (mm) for L. temulentum (Lt449). Measurements continued until 21 d after the first exposure to up to 5 LD. The error bars were smaller than the symbols. For each daylength treatment, apex dry weight (μg) was determined at 21 d on batches of 20 apices. Plants held in SDs remained vegetative and apex dry weight and length increased relatively little over 3 weeks. These dry weights along with values at earlier harvests were used in deriving a relationship between apex length and weight (see “Materials and Methods”).

Figure 4

Effect on L. temulentum shoot apex GA content of exposure to a single LD or to up to 5 LD (Expt Lt 443). Apices were harvested at 8:30 am each day for up to 10 d after the single LD or at 8:30 am on the day of ending the exposure to 1, 2, 3, or 5 LD. Where multiple LD cycles were imposed prior to harvest (filled symbol), their number is shown next to the symbol.

Figure 5

Effect on flowering of a single application to the leaf of 25 μg of GA₄ or GA₅. The earliest treatment was 6 h after the start of the main light period prior to the single LD extension. Flowering was determined at 21 d and the values are the mean and se.

Figure 6

Timing of GA₅ export from a treated leaf to the shoot apex as shown either by increase in the floral response of the shoot apex with increasing time until removal of the GA-treated leaf blade, or by arrival at the shoot apex of [²H₄] GA₅ following its application to the leaf. In both treatments, a dose of 25 μg of GA was applied to the leaf blade 6 h after the start of the main light period prior to the single LD extension. Values for floral apex length are the means and se. Plants not treated with GA₅ flowered to the same extent as those with the treated leaf removed at the start. There was only a trace of [²H₄] GA₅ detected at the apex after 24 h, but it is shown in brackets because there was uncertainty about quantitation of the gas chromatography (GC)-MS peak for the [²H₂] GA₅ internal standard.