Description of the Pattern and Timing of Flowering

Flowering plants of H. mantegazzianum have a distinct architecture. The inflorescences are compound umbels of four orders. The main flowering shoot develops as a leafy stem that terminates in a primary (first-order) umbel, also called 'terminal'. Lateral shoots, which are produced on the stem, terminate in secondary (second-order) umbels and can be found in a satellite position, surrounding the primary umbel (hereafter also called 'satellites'), or in a branch position below them on the stem (hereafter also called 'branches'). Third-order umbels may arise on shoots branching from secondary shoots (in both satellite and branch position) and fourth-order umbels on shoots branching from tertiary shoots (Fig. 4.2). Under favourable conditions, strong plants can produce several other shoots, which arise from the base of the flowering stem at ground level (further referred to as 'basal branches'). The character of the terminal umbels of these basal branches varies and in terms of umbel size, fruit size, fecundity and proportion of male flowers is intermediate between typical first-and second-order umbels.

Heracleum Mantegazzianum Videos

Fig. 4.2. (A) Schematic representation of the ordering of umbels and their position within the hierarchical inflorescence system of H. mantegazzianum. Umbel orders: primary ■, secondary ■ , tertiary ■ and quaternary «. (B) Each compound umbel consists of umbellets that bear a large number of small, closely packed flowers. Taken from Perglovâ et al. (2006), published with permission from the Czech Botanical Society.

Fig. 4.2. (A) Schematic representation of the ordering of umbels and their position within the hierarchical inflorescence system of H. mantegazzianum. Umbel orders: primary ■, secondary ■ , tertiary ■ and quaternary «. (B) Each compound umbel consists of umbellets that bear a large number of small, closely packed flowers. Taken from Perglovâ et al. (2006), published with permission from the Czech Botanical Society.

Each compound umbel consists of umbellets (umbellules), simple umbels that bear a large number of small, closely packed flowers. Flowers are either hermaphrodite or male (staminate); the latter usually contain reduced stylopo-dia (Bell, 1971) and no, or a shrunken, style. The proportion of male flowers increases in higher-order umbels, while the terminal umbel usually contains only hermaphrodite flowers. If present, male flowers are located in the central part of umbellets. Within the same umbel, the proportion of male flowers seems to be the same in all umbellets (Perglova et al., 2006), despite the increasing percentage of male flowers towards the centre of the umbels reported for Zizia aurea (Michaux) Fernald and Thaspium barbinode (Michaux) Nutt. (Bell, 1971). Fourth-order umbels usually consist only of male flowers (Perglova et al., 2006).

Male sterility was observed in the experimental garden in Pruhonice. A plant transplanted from a natural stand at the seedling stage and grown in a garden bed bore only physiologically female flowers with stamens, filaments of which remained unrolled, and the anthers remained closed and did not dehisce. Pistils were fully functional and the fruits were set after fertilization. The same phenomenon is described for wild Daucus carota L. plants by Braak and Kho (1958).

Within-flower and within-umbel phenology

A study of flowering phenology conducted on plants growing in the experimental garden in Pruhonice, Czech Republic (Perglova et al., 2006) revealed that in an individual flower, flowering starts by the sequential expansion and dehiscence of the five stamens, which takes usually 1 and sometimes 2 days. Anthers are ready to shed pollen almost immediately after expansion is completed. Within an umbellet, the outer flowers are first to flower and flowering continues to the centre, where pollen is shed 3 (in the case of the terminal umbels) to 6 (secondary umbels) days later (Fig. 4.3A). In umbellets located in the centre of an umbel, the onset of flowering can be 1 day later than in peripheral umbellets.

In contrast, stigma receptivity is well synchronized throughout the whole umbel and lasts 1-2, maximum 3 days. Receptivity can be recognized visually - the stigmas are fully elongated with a fresh glistening appearance at the tip of the initially dome-shape style, which spreads and becomes bulbous

Fig. 4.3. (A) The flowers of an umbellet open centripetally over a period of several days. (B) When the stigmas are receptive the styles are fully elongated, separate and with a fresh glistening appearance at the tip. Photo: I. Perglova.
Fig. 4.4. The overlap in anther dehiscence and stigma receptivity in flowers of the same umbellet. Photo: P. Pysek.

(Fig. 4.3B). Flowers in the centre of umbellets can be male and do not have a female phase.

Between anther dehiscence and stigma receptivity, there is a neutral phase of variable length, depending on the position of a flower in an umbellet and position of an umbellet in the umbel. In the outer flowers of peripheral umbel-lets, the neutral phase may last up to 6 days because they are the first flowers within an umbel to shed pollen. The neutral phase of the outer flowers of central umbellets is usually 1 day shorter. Neutral phase of central flowers, both in peripheral and central umbellets, lasts 2 days at most or there is no neutral phase or even an overlap in anther dehiscence of these flowers, which are often male, with the receptivity of other flowers in the same umbel (Fig. 4.4).

Such overlaps only occur in some umbels and only a small proportion of the late dehiscing anthers are usually involved (Perglova et al., 2006). Consequently, stigmas are not covered by a mass of pollen from the same umbel but geitonogamous selfing can occur.

Within-plant phenology

Umbels of different orders flower in sequence. The terminal (primary) umbel is the first to flower, followed by secondary, and later tertiary and quaternary umbels on satellites and branches. By way of an example, Fig. 4.5 shows the course of flowering of different umbels and the flowering phases of one plant growing in the experimental garden. In some umbels of higher orders (mainly quaternary), the female phase and fruit development do not occur after the male phase, because those umbels contain only male flowers and thus wither after anther dehiscence. At the umbel level, male phase is defined as a phase in which at least some flowers dehisce anthers while other flowers can be

Plant Phenology

Fig. 4.5. Phenological pattern of a single plant. The sequence of ■ male, x neutral and ■ female phases throughout the vegetation period is shown for each umbel. Overlap in anther dehiscence and female receptivity within an umbel is indicated by ■. Numbers I, II, III, IV refer to umbel order, letters s, b to the location of tertiary umbels on branches (satellite or branch position).

Fig. 4.5. Phenological pattern of a single plant. The sequence of ■ male, x neutral and ■ female phases throughout the vegetation period is shown for each umbel. Overlap in anther dehiscence and female receptivity within an umbel is indicated by ■. Numbers I, II, III, IV refer to umbel order, letters s, b to the location of tertiary umbels on branches (satellite or branch position).

already in the neutral phase. The neutral phase of an umbel is thus defined as the stage before stigma receptivity when none of the flowers contain dehiscing anthers. However, neutral phases are uncommon. Male and female phases of umbels on the same plant can overlap and a study of the flowering phenology of 100 plants at ten localities in the Slavkovsky les area revealed that such overlaps are common; at least a short overlap between some umbels was observed in 99% of plants (Perglova et al., 2006).

Evidence for overlaps is also provided by Stewart and Grace's (1984) study of hybridization between H. mantegazzianum and H. sphondylium. They report complete protandry within an umbel and an overlap between female and male phases of primary and secondary umbels in only two plants out of the nine studied. However, the plants were transplanted to a greenhouse in the spring of the same year in which they flowered. The results therefore might have been affected by the plants being smaller and possibly not producing tertiary umbels, which reduce the possibilities of overlaps between male and female phases. Stewart and Grace (1984) did not include plants growing under natural conditions in their study.

An opportunity for geitonogamous selfing usually increases if a plant produces basal branches, because these often exhibit asynchronous flowering (umbels do not flower at the same time as other umbels of the same order). Basal branches are usually vigorous and branched, bearing umbels of higher orders. However, several plants observed at the Slavkovsky les study sites produced late umbels on short basal shoots (shoot length up to about 20 cm, umbel diameter 10-25 cm), which consisted of physiologically female flowers with shrunken anthers (I. Perglova, Prilhonice, 2003, personal observation). At that time, pollen from quaternary umbels (usually containing only male flowers) was still available; such plants exploited the last opportunity and resources to produce fruit and did not invest in the production of pollen, which would have been wasted, as there were no or very few receptive stigmas.

Timing of flowering at the plant and population level

In plants destined to flower, the first signs of the development of a flowering stem become visible in early June in the Slavkovsky les (Perglova et al., 2006), late May in Giessen in Germany (Otte and Franke, 1998) and approximately 1 month earlier in the UK (Tiley et al., 1996). A rapid stem elongation culminates in the opening of a terminal bud, which contains terminal and satellite umbels. Then the terminal umbel expands, opens its flowers and starts to flower.

In the Slavkovsky les protected area in 2002, flowering began within a period of 1 week (from 20 to 27 June) at all ten localities, despite differences in exposure and altitude (Table 4.1). The peak of flowering, expressed as the average date on which the primary and secondary umbels of 30 randomly selected plants at each locality flowered, was between 27 June and 6 July. The duration of flowering of an individual plant (time from beginning of male phase in the terminal umbel to end of female phase in the last umbel on the plant) in the Slavkovsky; les was on average 36 days (range of averages for individual localities: 31-41 days). The maximum observed duration of flowering was 60 days and duration increased with the number of umbels on a plant. The terminal umbel flowered on average for 10 days and its fruits were ripe on average 44 days after the beginning of flowering (Perglova et al., 2006). In the second half of August, the majority of all fruits were ripe and started to be released.

0 0

Post a comment