Germination Characteristics Related to the Position on the Plant

Heracleum mantegazzianum bears umbels at various positions and orders (for a scheme of the plant architecture see Moravcova et al., 2005: Fig. 1; and Perglova et al., Chapter 4, this volume) and characteristics of seeds are affected by where on the plant they are produced. The position of a seed or fruit on a mother plant has been shown to affect seed mass, morphology, germination and dormancy characteristics (for a survey see Baskin and Baskin, 1998; Gutterman, 2000). For Apiaceae, seed mass and/or germination has been shown to depend on umbel position in many species (e.g. Ojala, 1985; Thomas et al., 1978, 1979; Hendrix, 1984a; Hendrix and Trapp, 1992). Compared to the other members of its family, H. mantegazzianum is not unusual with respect to seed position on the mother plant. Moravcova et al. (2005) studied the influence of fruit position on the mother plant on fruit mass, germination percentage and rate of germination in H. mantegazzianum. The data were collected in the Slavkovsky les Protected Area in the west of the Czech Republic (for details on this region, see Perglova et al., Chapter 4, this volume). Seeds were collected from the terminal (i.e. primary) umbel, secondary umbels in satellite positions and secondary umbels in branch positions, separately from the centre and margin of each sampled umbel (see Fig. 4.1). The overall mean mass of a single fruit was 13.1 mg (Table 5.1) and corresponded to the range 4.6-23.2 mg given by Tiley et al. (1996) for H. mantegazzianum. Fruits from terminal inflorescences were heavier than those from satellites and branches, and those produced in the centre of an umbel were heavier than those from the margin (Moravcova et al., 2005). Fruits from terminals weighed on average 15.9 mg, whereas those from satellites and branches weighed 11.7 mg, and the fruits from the centre were significantly heavier than those from the margins, being 13.3 and 12.9 mg, respectively (Table 5.1). Overall, the determination of fruit mass in H. mantegazzianum follows the same rules as in other Apiaceae (Hendrix, 1984a, b; Thompson, 1984; Hendrix and Sun, 1989), but the variation is several orders of magnitude lower than in some other species.

The mean percentage germination at 8-10°C (after 2 months of cold stratification at 2-4°C) found by Moravcova et al. (2005) was 91%, and varied among the seven sites studied, but was not affected by fruit position on a plant.

Table 5.1. Fruit mass in H. mantegazzianum depends on the position of fruit on a mother plant. Data are means ± sd (mg) and n (sample size) for fruit sampled from combinations of umbel types on mother plant (terminal, satellite, branch) and positions within an umbel (centre or margin) from eight plants in each of seven sites in the Slavkovsky les region, West Bohemia, Czech Republic. There were five replicates of each treatment. Data from Moravcova et al. (2005).

Table 5.1. Fruit mass in H. mantegazzianum depends on the position of fruit on a mother plant. Data are means ± sd (mg) and n (sample size) for fruit sampled from combinations of umbel types on mother plant (terminal, satellite, branch) and positions within an umbel (centre or margin) from eight plants in each of seven sites in the Slavkovsky les region, West Bohemia, Czech Republic. There were five replicates of each treatment. Data from Moravcova et al. (2005).

Fruit position within umbel

Umbel type (fruit position on the plant)

Terminal

n

Satellite n

Branch n

Mean n

Centre

16.15 ±2.75

270

11.93 ±3.47 270

11.65 ±3.12 265

13.25 ±3.74 805

Margin

15.60 ±2.59

265

11.52 ±3.44 275

11.66 ± 3.3 270

12.90 ±3.66 810

Mean

15.88 ±2.67

535

11.73 ± 3.46 545

11.66 ± 3.12 535

13.08 ± 3.701615

Such a high percentage germination seems to be usual within the context of the Apiaceae family, where seeds usually germinate readily once dormancy is broken (Baskin and Baskin, 1990, 1991; Baskin et al., 1995); a value of 94% was reported for Anthriscus sylvestris (Baskin et al., 2000). In the study of Moravcova et al. (2005), cold and dark conditions mimicked closely the natural situation under the soil surface during spring. Moreover, the high percentage germination recorded in the laboratory corresponds with that obtained in a garden burial experiment, where about 90% of seed stored in the soil germinated over the first winter (Fig. 5.5 and Moravcova et al., 2006). Given its fecundity (see Perglova et al., Chapter 4, this volume), H. mantegazzianum exerts enormous pressure of highly germinable propagules in invaded sites (see Pysek et al., Chapter 19, this volume).

These results (Moravcova et al., 2005) have practical implications as mechanical control often focuses on cutting terminal umbels or whole stems at flowering time (see Pysek et al., Chapter 7, this volume). Regeneration then occurs via higher-order umbels that produce fruit with the same capability to germinate as those produced on the terminal and low-order umbels. Similarly, the ability to produce a standard fruit in terms of weight, even on umbels of higher orders, contributes to successful regeneration after the loss of flowering tissues due to control efforts (see Pysek et al., Chapter 7, this volume).

The percentage of seeds of H. mantegazzianum which germinate does not depend on where on the plant the fruit is produced, but the rate at which they germinate does (Moravcova et al., 2005). Large seeds germinated faster than small seeds; germination rate increased with increasing fruit mass and this pattern was consistent for all plants at each site (Fig. 5.2). Since seeds from terminals were heavier than those from branches, the former germinated sooner than the latter. Heavy seeds germinated faster than light seeds, but the difference was only obvious at the beginning of the experiment (Moravcova et al., 2005). Faster germination of heavier seed adds to the ecological advantage resulting from their size; heavier seeds produce bigger seedlings (Harper, 1977) and this was also reported for other species within Apiaceae (Thomas et al., 1979; Thomas, 1996).

Fig. 5.2. The rate of germination of H. mantegazzianum seed increases with increasing fruit mass and the relationship is valid for all three umbel types (terminal, satellite, branch). Data are for a randomly chosen plant (n = 715 seeds). Terminal: rate = 1/exp(-3.345 + 3.852mass)(1/245); satellite: rate = 1/exp(-4.296+8.171mass)(1/245); branch = 1/exp(-3.763 + 6.441mass)(1/245). x2 = 1301.0; df = 5; P < 0.001. The y-axis is reversed so the seeds that germinate first appear above those that germinate last for a given fruit mass. For most of the ranges in fruit mass, the germination rate of a particular fruit mass is higher for satellites and branches than terminals, but the average germination rate for terminals is higher than for satellites and branches because fruit from terminal umbels is, on average, much heavier than that from the other two umbel types. Hence seeds produced on terminals germinate faster. Seeds were stratified for 2 months at 2-4°C and then germinated at 8-10°C. Germination was recorded weekly for 6 months. From Moravcova et al. (2005).

Average weight of 25 fruits (g)

Fig. 5.2. The rate of germination of H. mantegazzianum seed increases with increasing fruit mass and the relationship is valid for all three umbel types (terminal, satellite, branch). Data are for a randomly chosen plant (n = 715 seeds). Terminal: rate = 1/exp(-3.345 + 3.852mass)(1/245); satellite: rate = 1/exp(-4.296+8.171mass)(1/245); branch = 1/exp(-3.763 + 6.441mass)(1/245). x2 = 1301.0; df = 5; P < 0.001. The y-axis is reversed so the seeds that germinate first appear above those that germinate last for a given fruit mass. For most of the ranges in fruit mass, the germination rate of a particular fruit mass is higher for satellites and branches than terminals, but the average germination rate for terminals is higher than for satellites and branches because fruit from terminal umbels is, on average, much heavier than that from the other two umbel types. Hence seeds produced on terminals germinate faster. Seeds were stratified for 2 months at 2-4°C and then germinated at 8-10°C. Germination was recorded weekly for 6 months. From Moravcova et al. (2005).

Neither the characteristics of individual umbels (duration of flowering, size) nor those of whole plants (fecundity, age, height, basal diameter) had an effect on germination characteristics. The only significant relationship found was a negative one between fruit mass and plant height (Moravcova et al., 2005).

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