2 Macroecology and evolution of seeds

2.1 Ecological correlates and evolutionary history of post-dispersal embryo growth and embryo growth rate in angiosperms

Vandelook, F.1, Saatkamp, A.2, Pritchard, H.W.3, Rosbakh, S.4, Mattana, E.3, Fernández-Pascual, E.5, Dickie, J.3, Carta, A.6

1Meise Botanic Garden, Belgium; 2Aix Marseille University IMBE, France; 3Royal Botanic Gardens, Kew, UK; 4University of Regensburg, Germany; 5University of Oviedo, Spain; 6University of Pisa, Italy

Several angiosperm lineages disperse seeds with small embryos and copious endosperm or perisperm. Nutrient reserves are transferred to the embryo either before germination, during post-dispersal embryo growth; or following germination, with the embryo acting as haustorium. Until now post-dispersal embryo growth has been viewed mainly as a dormancy mechanism, termed ‘morphological dormancy’. To re-assess the ecological role and evolutionary history of post-dispersal embryo growth and embryo growth rate, we performed a literature search for studies documenting embryo growth and analyzed relations with other plant traits, climate and habitat variables. We compiled a list of over 150 studies presenting data on embryo growth for over 250 angiosperm species. Wherever possible, for each species we collected data on the extent of embryo growth, embryo growth rate, seed mass and germination timing. In addition, for all species we collected data on adult plant and seed functional traits (e.g., seed mass) and the climate and habitat conditions the species grow in. When we related the total growth of the embryo prior to germination to the time required to complete embryo growth only a weak positive linear relationship was observed, suggesting a poor relationship between embryo growth and germination timing. Post-dispersal embryo growth predominantly occurs in species growing in moist and shaded habitats and has been studied mostly in temperate climates. Plausible explanations for the variation in post-dispersal embryo growth are not limited to germination timing and include predation escape, efficient nutrient supply, seedling growth and avoidance of parental conflicts.

2.2 Macroevolutionary correlation of seed traits in flowering plants: current advances and future prospects

Carta, A.1, Vandelook, F.2

1University of Pisa, Italy; 2Meise Botanic Garden, Belgium

Seeds show important variation as plant regenerative units among species, but their macroevolutionary co-variations with other plant characteristics are still poorly understood. Since the evolution of single seed traits cannot be understood without considering their relation with other traits, assessing and understanding co-variations is crucial. For example, whilst a positive association of seed mass with genome size (GS) has already been documented, a broad-scale quantification of their evolutionary correlation and adaptive selection has only recently been conducted. GS is highly variable across angiosperms, but there is strong evidence that it is correlated with both cell division and tissue growth rate. This correlation has important implications for plant functions, including seed germination regulating-processes. Recent evidence also highlights the importance to consider not only seed size but also internal seed morphological traits, like embryo and endosperm sizes. Using exemplar case studies we explored the co-variations of seed mass, embryo, and genome sizes over a dataset containing several hundred angiosperm species. We first estimated whether these traits are phylogenetically clumped; then using multivariate modeling we evaluated what evolutionary pressures may drive the observed patterns. Specifically, we highlighted the need to test for an asymmetry in the correlated evolution acting on seed traits and genome sizes due to life form and macroclimate. We believe that the availability of high quality data and modern analytical tools provide a new macroevolutionary framework for a deeper understanding of seed traits integration and the functional roles of reproductive traits.

2.3 The global seed germination spectrum of alpine plants

Fernández-Pascual, E.1, Carta, A.2, Mondoni, A.3, Cavieres, L.4, Rosbakh, S.5, Venn, S.6, Satyanti, A.7, Guja, L.8,9, Briceño, V.F.10, Vandelook, F.11, Mattana, E.12, Saatkamp, A.13, Bu, H.14, Sommerville, K.15, Poschlod, P.5, Liu, K.14, Nicotra, A.10, Jiménez-Alfaro, B.1

1University of Oviedo, Spain; 2University of Pisa, Italy; 3University of Pavia, Italy; 4University of Concepción, Chile; 5University of Regensburg, Germany; 6Deakin University, Australia; 7Department of Agriculture, Water and the Environment, Australia; 8Center for Australian National Biodiversity Research, CSIRO, Australia; 9National Seed Bank, Australian National Botanic Gardens, Australia; 10The Australian National University, Australia; 11Meise Botanic Garden, Belgium; 12Royal Botanic Gardens, Kew, UK; 13Aix Marseille University IMBE, France; 14Lanzhou University, China; 15The Australian Plant Bank, Royal Botanic Gardens and Domain Trust, Australia

We present a quantitative synthesis of the seed germination spectrum of a coherent global biome: temperate alpine habitats above the treeline. We created a collaborative database of 9,799 primary records from laboratory germination experiments, contributed by 12 research groups specialized on the topic. The database included data from four continents and 661 species. To analyse the database, we used Bayesian meta-analysis of primary data, estimating the influence of six environmental cues (scarification, stratification, GA3, average temperatures, alternating temperatures and light) on germination proportion, mean germination time and germination synchrony; accounting for possible effects of seed morphology (mass, embryo:seed ratio), phylogeny and between-study variation. In general, alpine plants (1) have physiological seed dormancy and thus need cold stratification to release dormancy, (2) germinate better at warm temperatures (20-25ºC) and (3) show positive germination responses to light and alternating temperatures. Specialist species of the alpine belt have a more pronounced warm-cued germination and a stronger response to cold stratification than generalist species that occur both in the alpine belt and below the treeline. Germination responses to the environment are constrained by seed mass, embryo size and phylogeny; with smaller and more endospermic seeds being more responsive to warmth, light and alternating temperatures. Globally, overwintering and warm-cued germination are key drivers of germination timing in alpine habitats. The interaction between germination physiology and seed morphological traits further reflects pressures to avoid frost or drought stress. Our synthesis indicates the convergence, at the global level, of the seed germination spectrum of alpine species.

2.4 Two life-forms - two patterns of seed morphology in Orchidaceae

Valdelvira, G.1, Gamarra, R.1, Ortúñez, E.1, de la Fuente, P.1

1Autonomous University of Madrid, Spain

Dust-seeds are one of the most common characters in the family Orchidaceae. In general, the seeds have a thin coat of dead cells and a pluricellular embryo surrounded by an amount of free air space. Morphological studies of seeds have been mainly developed in terrestrial taxa of the Northern hemisphere, focusing on qualitative and quantitative characters. Qualitative traits were referred to seed shape, morphology of the testa cells and their anticlinal and periclinal walls, ornamentation in the walls and the presence of testa extensions and verrucosities. Due to seed traits are considered more conservative than others, they have been used in taxonomic, phylogenetic and phytogeographic studies. The vast majority of orchids are terrestrial or epiphytic. Terrestrial species can be found throughout the distribution range of the family, but epiphytic orchids are mostly restricted to tropical and subtropical areas. Few studies have related the seed morphology with the habit. To address whether terrestrial and epiphytic orchids consistently differ in seed morphological characters, we analyse 14 traits in a comparative framework. We report a detailed morphological survey with the aim to identify the most diagnostic characters of seed morphology related to the habit. This survey includes a large representation of genera from the tribes Neottieae (exclusively represented by terrestrial orchids) and Vandeae (mostly represented by epiphytic orchids). Co-varying with the orchid habit, the seed coat showed differences in the orientation of the cells along the longitudinal axis, the width of periclinal walls, and the presence of waxes or testa extensions. Two main patterns have been observed according to the two main life-forms in Orchidaceae.

2.5 Orchid seed morphometrics as a predictor of germination behaviour and some ecological implications

Oikonomidis, S.1, Thanos, C.A.1

1National and Kapodistrian University of Athens, Greece

Although several members of Orchidaceae can be germinated easily on sterile nutrient media, there are numerous species where propagation has been completely unsuccessful. On the other hand, seed morphology of orchids has been previously associated with their dispersal ability but relations to germinability have not been generally addressed. We have gathered seed morphometry and germination data for 203 orchids species world-wide from the available international literature, complemented with unpublished results of our lab. On the basis of: 1) final germination percentage and 2) pre-treatment duration, two major groups of germination behavior (defiant and compliant) are identified. Seed morphometric and germination data along with corresponding data of several ecologically important parameters (habitat shadiness, mycoheterotrophy level, growth form, climatic zone, subfamily) are correlated and statistically analyzed. A strong relationship between the ratio of embryo length to testa length (E:T) and germination behavior is detected. E:T values tend to be lower for species with defiant germination, which generally abound in shaded habitats in contrast to compliant germinating species, which prefer open habitats. Based on the mycoheterotrophy continuum, we suggest that the relative reduction of the embryo size (as a fraction of the entire seed length) seems to be linked with several lineages adapted to shaded habitats and their increased dependence on fungal symbiosis.