Seed coat development in explosively dispersed seeds of Cardamine hirsuta

Background and Aims Seeds are dispersed by explosive coiling of the fruit valves in Cardamine hirsuta. This rapid coiling launches the small seeds on ballistic trajectories to spread over a 2 m radius around the parent plant. The seed surface interacts with both the coiling fruit valve during launch and subsequently with the air during flight. We aim to identify features of the seed surface that may contribute to these interactions by characterizing seed coat differentiation. Methods Differentiation of the outermost seed coat layers from the outer integuments of the ovule involves dramatic cellular changes that we characterize in detail at the light and electron microscopical level including immunofluorescence and immunogold labelling. Key Results We found that the two outer integument (oi) layers of the seed coat contributed differently to the topography of the seed surface in the explosively dispersed seeds of C. hirsuta vs. the related species Arabidopsis thaliana where seed dispersal is non-explosive. The surface of A. thaliana seeds is shaped by the columella and the anticlinal cell walls of the epidermal oi2 layer. In contrast, the surface of C. hirsuta seeds is shaped by a network of prominent ridges formed by the anticlinal walls of asymmetrically thickened cells of the sub-epidermal oi1 layer, especially at the seed margin. Both the oi2 and oi1 cell layers in C. hirsuta seeds are characterized by specialized, pectin-rich cell walls that are deposited asymmetrically in the cell. Conclusions The two outermost seed coat layers in C. hirsuta have distinct properties: the sub-epidermal oi1 layer determines the topography of the seed surface, while the epidermal oi2 layer accumulates mucilage. These properties are influenced by polar deposition of distinct pectin polysaccharides in the cell wall. Although the ridged seed surface formed by oi1 cell walls is associated with ballistic dispersal in C. hirsuta, it is not restricted to explosively dispersed seeds in the Brassicaceae.

Fruit and seed structural characteristics and seed dispersal in Mercurialis annua L. (Euphorbiaceae)

The fruit of <em>Mercurialis annua</em> L. is a two-seeded capsule with a caruncle, a small appendage which arises from the outer integument in the micropylar area. The inner integument has a thick layer of Malpighian-like cells, interrupted at the micropyle by a remnant of nucellus, the walls of which contain suberin. There is no cuticle covering the caruncle and its cells have lipids and protein bodies as reserves. Seed dispersal occurs by a combination of autochory followed by myrmecochory. The explosive ejection of seeds is caused by dehydration-induced torsion of the mechanical layer of the fruit walls. This phenomenon is due to the different orientation of the cells and a folding over of the mechanical layer in the chalazal area. The seeds were found to be thrown distances from 1 to 130 cm (mean 41.1 cm; modal peak 10-20 cm). Ballistic dispersal is influenced by the weight of the seed. The seeds were collected by ants in a mean time of 24.4 minutes the maximum and mean distances of dispersal being 14 and 3.4 m, respectively. Of the seeds removed, 95.2 % were recovered on excavation of the ant nest. The seeds in the nest were intact but without the caruncles. The mean distance of dispersal by diplochory was &lt; 5 m. These features are discussed in relation to possible benefits to the plant species in terms of adaptive advantages of seeds.

Seed dynamics of two fire-dependent Geranium species in the boreal forest of southeastern Sweden

We analyzed critical life-history variables for two rare fire-dependent annual Geranium species in southern Sweden, which are today threatened because of effective fire suppression. At recently burned sites with abundant recruitment, seedlings occurred only where the humus layer had been completely removed by smoldering fire. Emergence depths ranged 1–6 cm in the mineral soil. Soil sampling at four sites revealed that in unburned soil Geranium seeds were located only in the mineral soil. Surprisingly, residual seeds were still present where fire had burned away the humus layer. An experiment showed that both species deposit seeds relatively evenly within a radius of 5–6 m, through ballistic dispersal. Repeated sampling in the field over a 2-year period after seed dispersal at one site indicated a low rate of seed depletion, corroborated by an indoor incubation of seeds. Our results show that successful management of these species depend on deep-burning prescribed fire, which can only result after severe drought. On the other hand, the seed bank is extremely long-lived, as viable seeds were present at a site last burned 200 years ago. This well-protected seed bank will likely buffer against both ill-timed fires and occasional failure in the recruiting seedling populations.

Density- and distance-dependent seedling survival in a ballistically dispersed subtropical tree species Philenoptera sutherlandii

We examine the density- and distance-dependent seedling survival of Philenoptera sutherlandii, a common pod-bearing and dehiscent legume (Fabaceae) in Ongoye Forest, South Africa. Short-range ballistic dispersal causes seed to fall beneath the parent tree, where density- or distance-dependent mortality effects are expected to be concentrated. One hundred and eighty marked seedlings were monitored in a 0.5-ha plot containing 30 adults. Our survival data do not support the escape hypothesis. Predation levels declined with increasing seedling density (positive density-dependent survival), but seedling survival after 15 mo was not distance-dependent. Nevertheless, a unimodal (hump-shaped) recruitment curve, typically associated with decreasing seedling density and increasing seedling survival with distance, was observed. In the context of ballistic dispersal, this recruitment curve may indicate a hump-shaped dispersal kernel with predator satiation at high seedling densities near a parent tree. This recruitment curve likely arises because generalized insect seedling predators while attracted to the adult trees also tend to forage farther away. Short dispersal distances, in turn generate the high densities needed to satiate seed and seedling predators. Predator satiation results in long-term survival rates in P. sutherlandii similar to more widely dispersed and less common tree species.

Secondary seed dispersal of Ricinus communis Linnaeus (Euphorbiaceae) by ants in secondary growth vegetation in Minas Gerais

In this study, I tested the efficacy of ants as secondary seed dispersers of Ricinus communis in southeastern Brazil. In a natural population of 143 individuals, I determined the ballistic dispersal distance for 62 seeds and 100 additional seeds were experimentally offered to ants in groups of ten seeds along a transect of 50 m. Fifty-three seeds were removed by ants, mainly by the leafcutter Atta sexdens (90.4%). The dispersal distance by ants was high, compared to the global average (4.38 m ± 0.74 m vs. 0.96 m), but was lower than the ballistic distance (7.27 m ± 0.13 m). Ants increased the total dispersal distance (8.66 m ± 0.60 m), but the main benefit for the plant was the directed dispersal, with seed deposition on the enriched soil of ant nests.

Potential ballistic dispersal of Cytisus scoparius (Fabaceae) seeds

Cytisus scoparius (L.) Link (Fabaceae) has a ballistic type of primary seed-dispersal mechanism in which its legumes dehisce suddenly when they dry. This paper presents an experimental approach to the measurement and modeling of distances reached by seeds under optimum dispersal conditions. Branchlets of the species carrying mature legumes were collected and attached to 1.20-m-high platforms on a flat roof terrace. For 2 weeks, daily measurements were made of distances attained by seeds ejected from legumes in the previous 24 h. Seeds were found at a distance of 2343 ± 113 mm (mean ± s.e., n = 245), with 10.2% of recorded distances greater than 5 m and some about 7 m. The mixture model fitted to the distances traveled by seeds allows the mathematical isolation of two underlying processes, the ballistic projection of seeds by dehiscent legumes (nearly 49% of seeds, reaching 3686 ± 1797 mm) and the barochorous dispersal of the remaining seeds in the platform environs (1254 ± 1254 mm). Modeling shows that seeds dispersed ballistically reach locations at some distance from the shrub crown, with low densities of sibling seeds—a potential advantage for the establishment of new individuals.

Fruiting phenology and seed dispersal syndromes in caatinga, a tropical dry forest in the northeast of Brazil

This study describes the fruiting phenology of woody plants and their dispersal syndromes in caatinga (semi-arid region in the northeast of Brazil). The fruiting phenology of 42 species with different dispersal modes and life-forms was followed over a period of 1 y. Animal dispersal was the most commonly observed dispersal mode (36%), followed by anemochory (33%), ballistic dispersal (19%) and barochory (12%). Overall, a greater number of species fruited during the rainy season. Zoochorous species were the most representative in the rainy season, whereas anemochorous species predominated during the dry season. Five different life-forms were observed, and the occurrence of dispersal modes was discussed for each of them. In the caatinga plant community studied the patterns of life-forms, fruiting phenology and seed dispersal syndromes were similar to other tropical seasonal ecosystems.Este trabalho teve como objetivo principal descrever como se comportam as espécies de caatinga (região semi-árida do Nordeste brasileiro) com relação ao modo de dispersão durante as estações seca e chuvosa. Foram observados também os modos de dispersão apresentados em cada forma de vida. A fenofase de frutificação foi acompanhada em 10 indivíduos de 42 espécies durante um intervalo de um ano. A zoocoria representou o modo de dispersão mais observado (36%), seguido da anemocoria (33%), balística (19%) e barocoria (12%). Um maior número de espécies frutificou durante a estação de chuvas, sendo a zoocoria mais representativa nesta estação, enquanto que a anemocoria foi mais representativa na época de estiagem. Foram observadas várias formas de vida, discutindo-se o modo de dispersão em cada uma delas. Constatou-se que as espécies de caatinga estudadas encontram-se dentro dos padrões gerais de época de dispersão dos diásporos e representatividade das síndromes de dispersão observados em outros ecossistemas com sazonalidade.