Scholarly article on topic 'Insect pollination and seed set in four ephemeral plant species from Namaqualand'

Insect pollination and seed set in four ephemeral plant species from Namaqualand Academic research paper on "Biological sciences"

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South African Journal of Botany
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Keywords
{Annuals / autogamy / geitonogamy / Namaqualand / xenogamy}

Abstract of research paper on Biological sciences, author of scientific article — C. Ueckermann, M.W. van Rooyen

The hypothesis that the only species that can successfully invade the abandoned fields in Namaqualand are those that have the ability to set seed in the absence of insect pollinators was investigated for four species known to produce mass displays on abandoned fields, i.e Arctotis fastuosa, Dimorphotheca sinuata. Tripteris hyoseroides and Ursinia cakilefolia. The study indicated that all four species showed a strong dependence on insects for cross-pollination to set seed successfully Self-incompatibility systems were operative in all four species and the avoidance of self-pollination was further enhanced by the presence of protandry The above-mentioned hypothesis can, therefore, not be accepted.

Academic research paper on topic "Insect pollination and seed set in four ephemeral plant species from Namaqualand"

S. Air. J. Bot. 2000. 66( I) 28-30

Insect pollination and seed set in four ephemeral plant species from Namaqualand

C. Ueckermann and M.W. van Rooyen*

Department of Botany, University of Pretoria, Pretoria, 0002 Republic of South Africa

Received ! J July 1999, revised 5 Sovemher 1999

The hypothesis fhat the only species that can successfully invade the abandoned fields in Namaqualand are those that have the ability to set seed in the absence of insect pollinators was investigated for four species known to produce mass displays on abandoned fields, i.e Arctotis fastuosa, Dimorphotheca sinuata, Tnpteris hyoseroides and Ursmia cakilefolia, The study indicated that all four species showed a strong dependence on insects for cross-poliination to set seed successfully Seff-incompatibility systems were operative in all four species and the avoidance of self-pollination was further enhanced by the presence ofprotandry The above-mentioned hypothesis can, therefore, not be accepted.

Keywords" Annuafs, autogamy, geitonogamy, Namaqualand, xenogamy.

'To whom correspondence should be addressed

Introduction

Reproduction by means of seeds is a critical part of an annual plant's life cycle (Bazzaz & Ackerly 1992). Viable populations of annuals are maintained by successive replacement by seeds and in most plants seed set requires pollination.

Optimal life-history models predict that short-lived plants, which arc best adapted to life in highly disturbed habitats, should be self-pollinated (autogamy) (Symonides 1988). Indeed, the majority of the most successful non-cultivated colonizers are self-pollinated annual species. Autogamy, therefore, seems a feasible strategy for the annuals of Namaqualand.

The apparent lack of insect pollinators on the inflorescences of species growing on ploughed fields {Smuts & Bond 1995) seemed to support the notion of self-pollination, or alternatively wind-pollination (anemophily), in these species and led to the hypothesis that the only species which could successfully invade the abandoned fields were those that had the ability to set seed in the absence of insect pollinators (Smuts & Bond 1995). However, considering the tremendous investment of energy in floral advertisement during the mass flowering period in Namaqualand it seems improbable that these plants should rely on self-pollination or wind-pollination for successful seed set.

The aim of this study was, (a) to test the hypothesis of Smuts and Bond (1995) on four Asteraceae species commonly found on abandoned fields throughout most of Namaqualand and (b) to investigate the importance of cross pollination for successful seed production.

Materials and Methods

All four selected species. Arctotis fastuosa Jacq.. Dimorphotheca si/mala DC., Triptens hyoseroides DC. and Ursuua cakilefoha DC., are known to produce mass displays of spring flowers on abandoned Ileitis (Roscli et a! 1997: Van Rooyen 1999).

Plants were grown from achenes (hereafter termed seeds) collected previously in either The Goegap Nature Reserve, near Springbok. or the Namaqua National Park (formerly known as the Skilpad Wild Mower Reserve), near Kamieskroon. Plants were grown out of doors at the University of Pretoria in 1 000 cm3 plastic pots filled with quartz sand. A single plant was grown per pot and five replicates were used for all treatments. All plants were watered daily with lap water and weekly with Amon and Hoagland's complete nutrient solution (Hewitt 1952).

To measure the need for insect pollinators, plants of each species were selected randomly for the different treatments as soon as inflorescence buds appeared. Control plants «ere left uncovered outside. To exclude all insects, live plants of each species were covered with

a line insect-excluding gauze and left adjacent to the control plants. After seed maturation equal numbers oI seed heads were harvested from plants within each treatment and the number of tilled seeds counted.

Artificial pollination experiments were conducted on plants in an insect-free environment in a phyiotron. Temperature was controlled at 20°C during daytime (08:00-18:00) and 15DC during night (18:00-08:00). The effect of four treatments on seed production was investigated: (i) the control with no interference represented autogamy; (ii) pollination within one inflorescence, referred to as geitonogamy 1: (iii) pollination between two inflorescences on the same plant, geitonogamy 2: and (iv) pollination between two inflorescences on two different plants, xenogamy . Seeds were collected and counted as described above-

In the above definitions the inflorescence was used as flowering unit. Flowers were not emasculated because it resulted in harming other flowers in the flower head. Geitonogamy I could, therefore, also contain seeds derived h\ autogamy, while (he geitonogamous 2 as well as the xenogamous treatments at the same time also induced geitonogamy 1 and autogamy.

Several ratios were calculated:

a. Need for insect pollinators (IP) is given by:

IP = [1 - (seed set in absence of insects/seed sel in presence of insects)] x 100.

b. Index of self-incompatibility (ISI) calculated by Dafni (1992) as:

ISI = seed set from artificial pollination/seed set from cross-pollination. was altered to:

ISI =seed set in geitonogamy 1/seed set in the presence of insects.

A value > 1 indicates self-compatibility: > 0.2 - < 1 indicates partly self-incompatible; < 0.2 indicates mostly self-incompatible: and 0 indicates complete selt-incompatibilih,

c. Chances of self-pollination (SP) are indicated by the IP-value, or if the results of the artificial pollination experiments are used it is calculated as:

seed set in autogamy/seed set by cross-pollination

The results were analysed by means of a one way analysts of variance (ANOVA) to determine statistical differences at a = 0,05.

Results and Discussion

All four species produced significantly more seeds in the presence of insects than in their absence (Table 1). The need for insect pollinators (IP) was > 95% for all species (Table 2).

The few seeds that were produced in the absence of insects were often deformed and probably not viable, leading to an over-

S. Afr. J. Bot. 2000. 66(1)

Table 1 Seed production of four annual species from Namaqualand in the absence and presence of insects

Species Number of seeds per flmverltead

Absence of insects Presence of insects

Arciolis !astnosa 2 17?

Ihnwrpholhecit xuruatu 3 fi7

Tripteris Innxeroides 0 11

I I sunt! i ukilefotia 0 54

estimation of self-pollination. In the case of Arciolis fastuosa viable seeds are bulky and golden brown or black, whereas those produced in the absence of insects were much smaller and grey. In the absence nf insects Dimorphothcca sinuata did not produce disc floret seeds and the few ray floret seeds that were produced remained green and were abnormally enlarged. Tri pier is hyose-roides and t 'rsinia cakilefolia were unable to produce any seeds when insects were excluded.

No significant differences could be demonstrated in seed production of plants covered with gauze and left outside and those of the autogamous treatment in tiie phytotron (Table I and Figure I ).

Results of the different pollination treatments (Figure I) indicate that all four species produced significantly more seeds in the xenogamous treatment than in any of the other treatments. Few seeds were produced in the autogamous and geitonogamous

treatments. Although the difference was not significant, al! four species produced more seeds when cross-pollinated within an inflorescence (geitonogamy I) than between inflorescences on the same plant (geitonogamy 2). Genetically. howe\er. geitonogamy l, geitonogamy 2 and autogamy are equivalent.

Under natural conditions all four species seemed to avoid self-fertilization. The commonest way in which this can be achieved in plants is by self-incompatibility systems. These involve the ability of plants to distinguish between their own pollen and that of another plant, allowing only that from other plants to fertilize the ovules (De Nettancourt ¡977: Kearns & Inmiye 1993; Proctor el a! I996). ISl-values (Table 2) indicated that Arciolis fastttosa. Dimopphoiheca s'uuuUa and f rsiniti cakilefo-lia were mostly self-incompatible, while Tripteris hyoseroiiks was partly self-incompatible. The form of self-incompatibility that is characteristic of the Asteraceae is known us sporophytic self-incompatibility. I n this system the site of recomiition is the stigma surface and the material that is recognised are the proteins in the outer coat of the pollen grain i.e. sporophytic material tProctor el al. 19%).

Most plant species, even those with self-incompatibility systems, have some ability to self-fertilize. These 'leaks' may have an advantage if insect pollinators are not available and serve as a back-up if cross-fertilization fails.

Self-incompatibility should be treated as separate from the chances of self-pollination. Despite being the least self-incompatible, the chances of self-pollination (SP) in Tripteris

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Figure 1 Seed production of four annual species from Namaqualand under different pollination treatments in an insect free environment: ¡. Arctotis fastuosa: ii, Dimorphotheca sinuata: iii. Tripteris hyoseroides and iv. Ursinia cakilefolia. Bars with !he same letter do not differ significantly from each other at a = 0.05.

S. Air. J. Bot. 2000,66(1)

Table 2 The need for insect pollinators (IP), index of self-incompatibility (IS!) and chances of self-pollination (SP) of four annual species of Namaqualand

Species Need for pollinators IP (%) Self-incompatibility ISI Chances of self-pollination SP

Intotis fastuosa 99 0.05 (mostly self-incompatible) 0,(101

Diiimrphotheca sinuala 95 0.08 (mostly self-incompatible) 0.010

Tnpteris hyoseroides 100 0,22 (partly self-incompatible) 0.000

t'rsinia cakilefo'Ul 100 0.16 (mostly self-incompatible) 0.200

hyoseroides are zero (Table 2). In the case of L'rsinia cakilefolia two unmanipulated inflorescences in the phytotron produced a few seemingly normal seeds and was the only species in which the SP-value indicated that the chances of self-pollination was appreciable. It was also the only species in which the xenoga-mous treatment yielded significantly less seeds than the treatment where insects were present (Table I and Figure Id).

Although self-incompatibility prevents self-fertilization it does not prevent self-pollination. The easiest way to avoid self-pollination is by the separation of anthers and stigma in space or time. Physiological self-incompatibility systems are often associated with protandry, a form of temporal separation where the anthers mature first and shed their pollen before the stigmas become receptive, Protandry is common among insect-pollinated plants and is characteristic of most Asteraceae (Lloyd & Webb 1986; Bertin & Newman 1993).

Floral structure should be used with caution to infer the type of pollination. In general, flower characteristics of self-, wind- and cross-pollinating species differ in several ways. Most habitually self-pollinating species have smaller flowers, usually also fewer flowers with little or no nectar, fewer pollen grains and ovules, and a lower pollen to ovule ratio than cross-pollinating species. Since attraction of insects is not necessary for self-pollination the energy expended on the production of large, showy flowers with rewards for pollinators are selected against. Wind pollination is a passive, wasteful, non-directional mechanism and its success depends on climatic conditions (Frankel & Galun 1977; Faegri & Van der Pijl 1979). Anemophilous plants have simple flower structures, petals arc insignificant or absent, they use no energy on attractants. but produce great quantities of small, smooth, dry pollen (Iwanami el aI 1988; Kaufman 1989; Proctor el al. 1996). Plants pollinated by animals generally have traits opposite to those listed for wind-pollinated plants.

The showy flowers or inflorescences of Namaqualand annuals (especially those of the Asteraceae) have sticky pollen and do not fit the self- or wind-pollination syndromes. The spring peak of flowering in Namaqualand produces a surplus of flowers relative to the pollinators, resulting in high competition between flowers for pollinators. To ensure successful pollination this necessitates a large investment in advertisement (colour and/or scent) and reward (pollen and/or nectar) (Cohen & Shmida 1993; Dafni & O'Toole 1994). The preponderance of large, open flowers and pseudanthia which are accessible to a wide range of insects has led to an anthophilous insect fauna which is dominated by generalists (Struck 1994a, 1994b).

Conclusions

The hypothesis that the only species that can successfully invade the abandoned fields are those that have the ability to set seed in the absence of insect pollinators cannot be accepted. Self-incompatibility systems were operative in all four species examined in this study. The avoidance of self-pollination was further enhanced by the presence of protandry. Experimental evidence indicated a strong dependence on pollinators for cross-pollination to set seeds successfully.

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