Botanical Journal of the Linnean Society ( 1993), 111: 129- 138

Pollination biology of some species of genus Plantago L.

NAMRATA SHARMA, PUSHPA KOUL AND AWTAR KRISHAN KOUL

Department of Biosciences, Uniuersity of Jammu, Jammu-180004, J&K, India

Received October 1991, reuised and acceped for publication August 1992

SHARMA, N., KOUL, P. & KOUL, A.K. 1993. Pollination biology of some species of genus Plantago L. The sample of six species of genus Plantago dealt with in this communication reveals a wide variation in pollination system. The predominantly inbreeding species. P. patagonica, P . drurnmondii and P. ovata, ensure self-pollination either through bud pollination or by synchronizing anther dehiscence with stigma receptivity. In the outcrossed species, wind and insects bring about pollen transfer.

ADDITIONAL KEY WORDS:-Anemophily - entomophily - Plantaginaceae.

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . 129 Material and methods . . . . . . . . . . . . . . . . 129 Observations . . . . . . . . . . . . . . . . . . . 130 Discussion . . . . . . . . . . . . . . . . . . . 135 Acknowledgements . . . . . . . . . . . . . . . . . 137 References . . . . . . . . . . . . . . . . . . . 137

INTRODUCTION

The 282 species constituting the genus Plantago exhibit a great deal of variation in breeding system. The outcrossing rates vary from 0 to 100% (Wolff, Friso & Van Damme, 1988). While some species are exclusively in- or outcrossing, others practise both, but in varying proportion. The variation is caused by the diversity in pollination mechanism of these species. This paper, based on observations on six species of the genus, attempts to highlight this component of the variation. The sample of six species covers the whole spectrum of breeding system variation in the genus (Sharma, Koul & Koul, 1992b). All of them are distributed widely in a variety of habitats (Primack, 1979; Sharma, 1984). Except for Plantago ouata, which is cultivated for its seed husk, all other species are wild.

MATERIAL AND METHODS

The species investigated are: Plantago ouata Forsk., P. patagonica Jacq., P. drummondii Decne., P. major L. , P . lagopus L . and P. lanceolata L. Plants of all

129 002+4074/93/020129+ 10 $08.00/0 0 1993 The Linnean Society of London

130 N. SHARMA E T AL.

TABLE 1 . Number of spikes per plant and length of spike and scape at different developmental stages

Number of Length of spike/scape at ( N = 30) Increase in Spikes per plant length of

Initiation of Initiation of End of spike/scape Species P+ Number 0 phase (cm) anthesis (cm) anthesis (cm)

Plantago lanceolata 36 43 k 3.023 1.87/22.05 2.54/50.98 6.97/57.1 5.1 /35.05

P. lagopus 35 72.8F9.1 1.4/18.08 1.68/35.11 5.49148.6 4.09/30.52 P . major 34 10.04+ 1.31 8.14/13.17 10.47/13.49 19.97/16.4 11.83/3.23 P. ouata A t 38 31.9k1.5 2.07/18.9 2.2/19.5 4.24/22.0 2.17/3.11

P . drrrrnmondii 29 21.4k 1.5 ~ 9.83/13.08 13.2/14.55 3.37/1.47 P . patagonica 31 49.5f8.5 ~ 2.78/15.2 6.93p1.7 4.15/6.55

B _ _ - ~ 2.2/20.2 3.9/22.3 1.7/2.1

*.N = Sample size. ?Plants with protogynous flowers. $Mean S.E.

these species were raised in beds in the University Botanic Garden, Jammu, where they have been maintained for the past 10 years. The plant and floral morphologies were studied from live plants.

Pollen output per flower was calculated by estimating the number of pollen grains per anther and multiplying the figure by four. This number, multiplied by the number of flowers per spike and number of spikes per plant, provided the estimate of total pollen output per plant. Pollen grain dimensions were calculated from freshly prepared acetocarmine mounts of pollen, using ocular and stage micrometer.

In order to detect wind pollination, glass slides (7.5 x 2.5 cm) smeared with Mayer’s albumen were suspended from T-shaped wooden stands fixed around individual plants of each species at distances of 0.5-2 m. After 24 h exposure, the slides were checked under the microscope for the number of pollen grains trapped.

Stigmas of male-sterile individuals of P. lagopus were also screened for pollen load and the pollen grain size distribution recorded.

Comparison of the relative efficiencies of selfing, anemophily and entomophily was made by bagging spikes of a few individuals of P. Lagopus and P. LanceoLata with butter paper and mosquito net. The seed set on bagged inflorescences was compared with the seed set on open pollinated spikes.

The numbers of insect visitors to plants of different species were recorded at regular time intervals. A few specimens of each pollinator were trapped and checked for pollen load on different body parts. The behaviour of insects on spikes was observed during peak hours of insect activity.

OBSERVATIONS

Plants of all the six species investigated are stemless or short-stemmed herbs with flowers borne in spikes at the end of long scapes. The number of spikes per plant, as well as the length of spike and scape differ among the species (Table 1). Flowers arranged compactly in spikes are tetramerous, actinomorphic, hermaphrodite and hypogynous.

POLLINATION BIOLOGY OF P L A N T A G O 131

TABLE 2. Floral features of six Plantago species

Length of Length of Length of Length of floral tube anther lobe filaments* stigma?

Protogyny Species (9-6) N (mm) N (mm) JV (mm) (mm)

Plantago

P . lagopus

P. major

P . ovata A

lanceolata

B P. drummondii P . patagonica

+ 31 3.51 f0.03 27 3.08f0.03 32 6.61+0.04 24 6.38f0.07

+ 34 3.64f0.38 25 2.67f0.02 26 5.80f0.06 31 5.76+0.1

+ 29 1.85f0.03 39 1.19+0.04 37 3.09f0.04 73 2.37f0.05

+ 29 3.27f0.02 24 2.19+0.04 33 4.34f0.03 37 5.240.08

(4-5 days)

(4-5 days)

(2-3 days)

(1-2 days) ~ 30 3.30f0.04 45 2.04+0.03 41 4.43f0.06 51 3.50f0.07 ~ 34 1.93f0.02 67 1.03+0.02 36 2.13+0.02 39 1.45f0.04 - 30 3.52f0.02 32 0.62+0.05 38 0.96&0.01 41 1.36+0.05

*Length above the floral tube (degree of anther exsertion) in all except P . patagonica. ?On the day of flower opening.

Except for P. patagonica, P . drummondii and some plants of P. ovata, flowers of all other species are protogynous; the degree of protogyny varies (Table 2) . In P. drummondii, and some plants of P. ovata, anther dehiscence and stigma receptivity synchronize with each other and with the opening of flower. In P. patagonica, stigma receptivity and anther dehiscence overlap but both precede flower opening by 2-3 days, resulting in bud pollination as well as fertilization.

The anther and stigma sizes also vary (Table 2) . In all species (except in P. patagonica, in which they remain concealed) the stamens are raised 2-6 mm above the corolla tube (Table 2). Adnation of anthers to filaments is versatile but in P. patagonica it is basal, the stigma is plumose, dry and highly papillate.

In large populations of P. ovata, P . lagopus and P. lanceolata, a few individuals are male sterile, carrying highly shrivelled anthers, which may or may not be shed.

The flowers borne on a spike bloom in acropetal succession. The spike and scape continue to elongate from the time the stigmas of basal flowers start protruding (female phase in protogynous plants), up to the time when all flowers in the spike have bloomed. Measurements made at the onset of the female phase, and the initiation and completion of anthesis are tabulated in order to calculate the actual increase in size (Table 1).

In all the species except for P. patagonica, in which pollination is accomplished inside the bud, pollen is shed after the flowers open and the stamens are exserted. In P. patagonica, the anthers dehisce 2-3 days prior to the opening of the flowers and the pollen lands directly on the stigma.

The number of pollen grains differentiating per anther varies considerably: the highest count is in P. lanceolata and the lowest in P. patagonica (Table 3) . However, at the level of whole plant, the highest pollen output is in P. lagopus. The percentage viability is high in all the six species. Pollen grains are dry, smooth-walled and three-celled. The pollen grains produced in an anther are uniform in size in all species, except P. lagopus and P. lanceolata. In the latter two species, they vary in size from 13.3 to 26.7 pm and 13.3 to 23.3 pm, respectively. The percentage frequency of each size group typelanther is tabulated (Table 4).

132 N. SHARMA ET AL.

TABLE 3. Data on pollen output, pollen size and viability

Mean Mean Pollen Pollen pollen pollen output output viability diameter

Species per flower per plant (%) (Pm)

Plantago lanceolata 47 632 245 791 440 83.70 20.04 P . lagopus 35 840 388 774495 85.67 18.12 P . major 1 1 504 18 943 672 71.63 17.23 P . ovata 13 028 21 217661 91.67 23.54 P. drummondii 5 424 8 081 688 85.20 18.57 P . patagonica 174 206,593 89.90 30.27

TABLE 4. Percentage frequency of pollen grains of different sizes

Diameter of Frequency (yo) in Sample different pollen No. types (Pm) P . lagopus P . lanceolata

26.72 5.7 ~

23.38 26.42 25.74 20.04 38.34 53.16 16.73 26.94 18.99 13.36 2.59 2.11

TABLE 5. Pollen load on exposed slides

Average pollen load per slide placed at a distance of

Species 0.5 m I m 1.5 m 2 m

Plantago lanceolata 2383 1785 1130 49 1 P . lagopus 266 1 2502 1267 95 1 P. major 422 124 57 14 P . ovata 61 1 145 30 3 P . drummondii 5 ~

~ ~

The relative efficiency of wind pollination in all chasmogamous species was determined by hanging glass slides smeared with Mayer’s albumen on T-shaped stands, fixed at distance of 0.5-2 m from the pollen source. The slides, exposed for 24 h, were screened for the pollen load under the microscope.

Observations on the slides revealed considerable differences. Whereas the slides fixed around plants of P . drummondii carried few or no pollen grains of this species, those hung around the plants of other species carried a considerable number (Table 5 ) .

Certain male-sterile plants growing at varying distances from fertile plants of P. lagopus were studied for the pollen grains trapped by their long stigmas. The dimensions of intact pollen grains taken from these stigmas were measured (Table 6). The pollen grains taken from slides hung around plants of P. lugopus and P. lanceolata were counted and measured (Table 7).

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POLLINATION BIOLOGY OF PLANTAGO

TABLE 8. Seed set in open pollinated and bagged spikes

Seed set* in

135

spikes open bagged spikes

Sample pollinated with butter bagged no. Species spikes Paper with net

1 P. lagopus 78.2 3.72 56.1 k2 .49 74.33k2.27 2 P . lanceolata 78.7 k 2 . 8 0 69.28k2.72

*.N = 20.

when flowering starts, no insect was seen visiting the plants. Around mid-March as more and more spikes entered anthesis, individuals of Apzs dorsata and some dipteran flies started visiting the plants. The frequency of visits was low and insect activity was limited to the late morning. Plants of P. lagopus and P. lanceolata were visited by insects throughout the flowering season, but the relative frequency of different types of visitors during the period varied. Apis dorsata was the most frequent visitor to spikes during March. During the following two months, the numbers of this bee gradually diminished. The other two pollinators, ApisJEorea and dipteran flies, are more frequent in April and May. The insect visits commenced at about 08.00 hours when dehiscence of anthers had started and continued up to 16.00 hours, with the peak activity between 10.00 and 12.00 hours. On completion of their visits to the spikes, the insects carried considerable pollen load on their bodies.

The insects alight at the base of spike and quickly crawl upwards. They crawl around flowers whose anthers are extruded and they fly away. At times, they move upwards towards flowers whose stigmas but not their anthers are exserted. The nature of insect activity on spikes varies. Apis dorsata moves very swiftly and covers an average of 16 spikes per min. ApisJEorea is relatively sluggish, visiting only half as many spikes within as much time. More sluggish are the dipteran visitors which cover two spikes per minute.

In order to estimate the proportion of selfing, anemophily and entomophily, a few spikes each of P. lagopus and P. lanceolata were bagged with butter paper and mosquito net bags. The seed set on these spikes was calculated (Table 8). Plantago lanceolata did not set any seed on selfing, revealing that the species is self- incompatible. Bagged spikes of P. lagopus set fewer seeds than cross-pollinated spikes.

DISCUSSION

All the species studied, except P. lanceolata, are self-compatible. Plantago patagonica, which has cleistogamous flowers, is a habitual inbreeder. In P. drummondii and some plants of P. ovata synchrony in the receptivity of stigma and dehiscence of anthers helps in the accomplishment of self-pollination. The incidence of cross-pollination in these and other species varied. Wind and insects are the major pollinating agents in these and some other species of Plantago (Sagar & Harper, 1964; Stelleman & Meeuse, 1976; Primack, 1978; Sharma, Koul & Koul, 1990).

The syndrome of anemophily consists of unisexual flowers, insignificant perianth, non-availability of attractants, profuse pollen production, presence of

136 N. SHARMA E'T AL

flowers well above the foliage, emergence of flowers before the leaves, highly elongated stigmatic surface and reduction in number of ovules (Faegri & Van der Pijl, 1979). Plants of P. ovata, P. major, P. lagopus and P. lancealata have many of these features: the flowers are borne in spikes at the end of highly elongated scapes which hold them well above the leaves that form rosettes along the soil surface. Since the velocity of wind increases with height, it is advantageous for pollen dispersal to have the pollen source located well above the ground (Whitehead, 1969).

The number of spikes per plant is quite high in all the six species studied. In the protogynous taxa within the sample, the scape continues to elongate, from the time of initiation of female phase to the completion of anther dehiscence (Table 1) . Protogyny and indeterminate growth of spike are recognized adaptations for anemophily (Bos et al., 1985). In the wind-pollinated plants, pollen generally flows downwards. Functioning of the spikes located at lower heights as pollen recipients and of those above as pollen donors makes it easy for the plumose stigma to trap pollen. In Plantago spathulata and Elocharis fusilla (Cyperaceae) also, the scape length increases during the period intervening between emergence of the stigma and pollination (Lloyd & Webb, 1986).

Flowers are compactly arranged in spikes; they are small, scentless and without any nectar. Anthesis initiates with the expansion of petal lobes at the tip and emergence of stamens. The long filaments carry anthers well above the floral tube (Table 2). The anthers are large and full of pollen. The stigma is plumose and covered with prominent papillae which widen the stigma surface for the windborne pollen to land upon (Primack, 1978; Faegri & Van der Pijl, 1979; Burd & Allen, 1988). Pollen production is profuse and pollen viability high in the predominantly outcrossed species of Plantago investigated here. Pollen diameter ranges between 17 and 24 pm, except for P. patagonica, in which pollen measures 30 pm. Pollen dimensions in typical wind-pollinated plants fall in the range of 20-30pm (Faegri & Van der Pijl, 1979).

The quantity of airborne pollen of a species seems to be a function of three factors: total pollen production, mean pollen diameter and degree of anther exsertion (Tables 2 and 3) . In P. lagopus and P. lanceolata, the pollen production and degree of anther exsertion are greatest, as is the quantity of their pollen in the atmosphere. Plantago drummondii produces the least quantity of pollen; the degree of anther exsertion of its anthers is, incidentally, least. Pollen production in P. ovata is higher than that of P. major. At shorter distances, the correlation between total pollen producton and the number trapped also holds true for these two species, but as the distance from pollen source increases, the quantity of windborne P. ovata pollen reduces more rapidly than that of P. major pollen, which is probably due to the difference in the pollen diameter of the two species.

Plantago lagopus and P. lanceolata carry pollen grains of different sizes in their pollen sacs (Table 4). The percentage frequency of the pollen grains in the size range 16.73-23.38pm is greatest.

Data on pollen grains taken from hanging slides (Table 7) and stigmas of male-sterile plants (Table 6) also reveal the predominance of pollen in this size range. The relative frequency of pollen grains of different sizes on hanging slides varies with the distance from pollen source. Pollen grains of small size (16.73 pm) increase in number as the distance from the pollen source increases, although their percentage production per anther is lower than the other two predominant types (20.04 pm and 23.38 pm).

POLLINATION BIOLOGY OF P L A N T A G O 137

Dimorphy in pollen size is an adaptation which facilitates wind pollination. According to Whitehead (1969), small pollen grains disperse well over long distances but are not easily trapped by the stigma. I n contrast, the large-sized pollen grains have a high terminal velocity which facilitates their safe landing. In this way dimorphy fulfils the dual need of dispersal over long distances and easy trapping by the stigma.

Although wind plays a major role in bringing about pollen transfer in species of Plantago, insects visit spikes of some taxa. Faegri & Van der Pijl (1979) have cited P. media and P. lanceolata as examples of taxa which combine ento- and anemophily. In fact, most of the reports on entomophily in the genus Plantago concern P. lanceolata (Clifford, 1962; Stelleman & Meeuse, 1976). Of the six species studied here, P. ovata, P. lagopus and P. lanceolata are visited by Apis dorsata, Apis jorea and dipteran flies. All these three insects fall within the natural ranges of these plants. I n the absence of nectar in the flowers of these species, pollen is the only reward that visiting insects collect in return for their visit. Out of the three species, the degree of entomophily is least in P. ovata. Moreover, insect pollination is effected only during a specific part of the flowering season. In this species, the tendency for self-pollination is most prominent (Sharma, Koul & Koul, 1992a). I n contrast, P. lanceolata and P. lagopus are predominantly outcrossed; the degree of anemo- as well as entomophily in these species is high. The elimination of insect visits to those plants, however, does not cause any major adverse effect on seed set.

ACKNOWLEDGEMENTS

N. Sharma is thankful to Council of Scientific and Industrial Research, New Delhi and P. Koul to University Grants Commission, New Delhi for financial assistance.

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Burd M, Allen TFH. 1988. Sexual allocation strategy in wind pollinated plants. Evolution 4 2 403-407. W o r d HT. 1962. Insect pollination of Plantago lanceolata. Nature 193: 196. Faegri K, Van der Pijl L. 1979. The principles of pollination ecology. New York: Pergamon Press. Lloyd DC, Webb CJ. 1986. The avoidance of interference between the presentation of pollen and stigma in

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Whitehead DR. 1969. Wind pollination in the Angiosperms. Evolutionary and environmental considerations.

Wolff K, Friso B, Van Damme JMM. 1988. Outcrossing rates and male sterility in natural populations of Evolution 23: 28-35.

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