A Novel Way To Disperse Seeds

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Although widespread among fungi, lichens, liverworts, and mosses, seed dispersal mechanisms operated by rain are not common among flowering plants (Brodie, 1952⇓; Van der Pijl, 1982⇓). Some desert plants and herbs from temperate regions apparently use rain as a seed dispersal mechanism (Brodie, 1951⇓; Savile, 1953⇓; Friedman, Gunderman, and Ellis, 1978⇓). Generally speaking, two mechanisms are involved in seed dispersal by rains: the splash-cup and the springboard (sensu Brodie, 1955⇓). In the former, raindrops are caught by cup-shaped capsules, and the seeds they contain are thrown away with the splashing water. In the springboard or catapult mechanism, the fruit or persistent calyx tube (or gemmae in the case of Kalanchoe tubiflora; Brodie, 1955⇓) is attached to the stem by a resilient pedicel, which is bent downwards by the pressure of a falling raindrop. When the pressure is released, the pedicel springs back into its normal position shooting the seeds away from the plant.

In this paper we describe a new seed dispersal mechanism operated by rain in a Neotropical rainforest herb, Bertolonia mosenii Cogniaux (Melastomataceae). We experimentally demonstrate that rain is necessary to release the seeds from the capsules through what we call the “squirt-corner” seed dispersal mechanism. In addition, dispersal distances of seeds were measured under laboratory conditions. Fruiting phenology was also monitored and compared to the seasonal distribution of rains. As far as we know, the squirt corner is a novel mode of seed dispersal for the family Melastomataceae and for flowering plants occurring in Neotropical forests.

STUDY SITE AND PLANT SPECIES

The study was carried out in lowland Atlantic rainforest of Parque Estadual Intervales (24°14′ S, 48°04′ W), a 49 000-ha reserve located in São Paulo State, southeast Brazil. The site (Saibadela Research Station, 70 m above sea level) is one of the wettest places within Brazilian Atlantic forests, receiving ∼4000 mm of rain annually (Morellato et al., 2000⇓). Although rains are well distributed throughout the year and in no month is there <100 mm, rains are more intense and frequent from September to March (Fig. 1), a period that also corresponds to the hottest season (Morellato et al., 2000⇓). Old-growth forest predominates in the study site, with an open understory and a canopy 25–30 m high (Morellato et al., 2000⇓).

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Fig. 1. Comparison between the reproductive phenology of Bertolonia mosenii and rainfall pattern at Parque Estadual Intervales: bar = monthly rainfall and lines = number of flowers, mature fruits, and immature fruits produced (N = 20 plants)

The genus Bertolonia Raddi comprises 17 species almost restricted to the Brazilian Atlantic forest (with the exception of B. venezuelensis, which occurs in the high plains of the Venezuelan Amazon; Baumgratz, 1991⇓). Bertolonia species are small herbs (sometimes creeping) and have long been used as ornamentals (Bailey, 1947⇓). Their fruits are so distinctive from other Melastomataceae that they attracted the attention of early botanists (De Candolle, 1828⇓; De Chamisso, 1834⇓). The fruit is prominently triquetous (Figs. 2–3); the triangular, dry capsule dehisces in three valves with persistent ovary apex that are only seen from a polar angle (Figs. 4–5; Baumgratz, 1991⇓). Such an unusual fruit received its own category (bertolonid fruit) in the classification system of Baumgratz (1989)⇓ for fruits of Brazilian melastomes. Wind was thought to be the dispersal agent for the seeds of Bertolonia (Baumgratz, 1991⇓).

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Figs. 2–6. Bertolonia mosenii reproductive plant (Figs. 2–3), and schematic polar view of a mature fruit (4–6). 2. Plant frontal view illustrates the fruits arranged sequentially along the infructescence stem. 3. Polar view of the infructescence shows that one fruit does not obstruct the passage of a falling raindrop to the fruit immediately below it; note at left green, immature fruits, and at right, light brown, mature fruits. 4. Capsule of a mature fruit with seeds covered by the ovary apex. 5. Same as Fig. 4, with the capsule walls pushed backward, exposing the small seeds under the ovary apex. 6. Same as Fig. 5, with the ovary apex removed, showing the seeds arranged along two placental axes and one placental axis without seeds

Bertolonia mosenii is a forest-dwelling, small herb (5–30 cm height) occurring from Minas Gerais to Santa Catarina States in Brazil (Baumgratz, 1991⇓). It is a common species at the study site, where it usually forms patches on the forest floor. It also grows on rocks, along riverbanks, and on fallen and standing (live or not) tree trunks. Each plant bears 1–24 pediceled capsules (N = 37; x [±SD] = 7.9 ± 4.9) whose triangular, raindrop-capturing surface measures 4.0–6.2 mm on its side (N = 29, x [±SD] = 4.7 ± 0.5; Figs. 2–4). The fruits are upright positioned and arranged sequentially along the infructescence stem in a manner that avoids the superposition among them (Fig. 2). As a consequence, no fruit obstructs the passage of a falling raindrop to the fruit immediately below it (Fig. 3). Seeds are tiny, measuring from 0.3 to 0.7 mm in length (Baumgratz, 1991⇓), and are covered by the ovary apex in the mature fruit (Figs. 4–5). Each fruit may contain >700 seeds (see below) arranged along the three placental axes (Fig. 6). Besides sexual reproduction, B. mosenii can also propagate via vegetative growth.

MATERIALS AND METHODS

Plant reproductive phenology was monitored from April 1999 to April 2000 in 20 marked plants. These plants were censused monthly for the appearance of flowers and fruits, and the number of flowers (flower buds plus open flowers), immature fruits, and mature fruits were counted.

To evaluate the role of raindrops in dispersing the seeds, 32 additional plants were marked in January 2000 immediately prior to fruit maturation (Fig. 1). These plants were randomly assigned to two groups, treatment and control, with 16 plants each. Plants in the treatment group were covered with plastic shelters positioned 10–20 cm above the infructescences. The shelters, made of translucent plastic and totally open on the sides, prevented raindrops from contacting the fruits. Control plants were left uncovered. In April 2000, following the fruiting period, five fruits of each plant in both groups were harvested and the number of seeds was counted.

Seed dispersal distances were recorded by dropping water from a dropper on mature fruits in the laboratory. To facilitate locating seeds following simulated raindrops, we placed a white cloth underneath the plants. Drops of water were allowed to fall from heights of 1 m (in 17 plants) and 2 m (in another 10 plants) on the fruits. The seed displacement distances were then measured.

RESULTS AND DISCUSSION

Bertolonia mosenii reproduction was very synchronous and restricted to the wettest season (Fig. 1). Flower production was concentrated in December, followed by the production of immature fruits in January. Mature fruits were produced mainly from February to March, the wettest months at the study site. Less than 5% of the plants were observed bearing fruits in April (Fig. 1). Fruit maturation during the wettest months may insure seed dispersal by rain. Highly synchronized reproductive phenology of short duration has been reported for other Neotropical Melastomataceae (Mori and Pipoly, 1984⇓).

Treatment and control plants differed greatly in the number of seeds their fruits contained (mean ± SD = 412.4 ± 186.1 seeds/fruit and 52.4 ± 70.7 seeds/fruit, respectively, N = 80 fruits in both cases; Mann-Whitney tests: U = 269.9, P < 0.001), suggesting that raindrops effectively release seeds from fruits. Additionally, control plants had a much greater variation in mean number of seeds per fruit than treatment plants (coefficients of variation [CV] = 135.0% and 45.1%, respectively). Such variation probably reflects individual differences in seed dispersal success, which depend upon the arrangement of the canopy and subcanopy foliage above the plants. In fact, for plants positioned under certain herbs in the Marantaceae, whose broad leaves intercept raindrops, we observed germinating seeds sprouting directly from capsules, thus indicating unsuccessful dispersal.

The average distance of seed dispersal was 8.5 cm (SD = 6.2 cm, range = 1.3–29.5 cm, N = 97) when water drops fell from 1 m height and 12.7 cm (SD = 8.9 cm, range = 1.9–35.3 cm, N = 100) from 2 m height above the fruit. Dispersal distances from this experiment likely represent only a minimum estimate, as under natural conditions raindrops may fall from the forest canopy (20–30 m), likely producing a much wider range of dispersal distances.

In Bertolonia mosenii, the seeds are covered by the persistent ovary apex (Figs. 4–5). When a raindrop strikes the mature fruit, the water droplet forces the seeds outward to the angles (corners) of the triangular capsule and the seeds are released. Unlike the splash-cup mechanism, where the seeds exposed on the bottom of the cup are released by the splashing action of the raindrop (Brodie, 1951⇓), in B. mosenii, seeds are “squirted” through the fruit corners. Seed shape also differs between splash-cup fruits and Bertolonia. In the former, the seeds are lenticular, while the Bertolonia seeds are obovate to clavate, favoring its displacement through the fruit corners. Differences in fruit and seed structure, as well as mechanisms for releasing seeds, warrant a new denomination for Bertolonia: squirt-corner seed dispersal mechanism.

Given that all Bertolonia species possess triangular fruits similar to those of B. mosenii (Baumgratz, 1991⇓), we suggest that raindrops are the seed dispersal agent initiating the squirt-corner mechanism for all members of the genus. Dispersal by raindrops should also be investigated for other melastomes in the genus Salpinga (S. margaritacea and S. longifolia), which also possess bertolonid fruits (Baumgratz, 1989⇓; Barroso et al., 1999⇓) and may share a similar seed dispersal mechanism.