Effect of high temperature on germination of four legumes from a forest-grassland mosaic in Southern Brazil

Abstract

Passage of a fire can break dormancy of the seeds of many plant species in fire-prone ecosystems. This response to fire is especially well known for the Fabaceae family, but has been poorly studied in Southern Brazil. We collected seeds of four Fabaceae species present in grasslands-forest mosaics of Southern Brazil: Mimosa bimucronata, Desmodium barbatum, Sesbania virgata and Collaea stenophylla. Seeds were exposed to different heat treatments (exposure to 60° and 80 °C for 5 minutes, to 100 °C for 2 minutes, control without heat treatment); not all species were tested in all treatments. After the treatment, the seeds were kept in a germination chamber with a light period of 12/12 hours and temperature of 25 °C. Germinated seeds were counted every 2 days. The results were analyzed by randomization testing. Germination of D. barbatum and S. virgata was increased after exposure to high temperatures (80° and 60 °C, respectively), while M. bimucronata showed reduced germination after temperatures of 80 °C and C. stenophylla no response. This study is the first for Southern Brazil to show a positive response of germination for grassland species exposed to high temperatures, simulating fire effects. As the study presented distinct responses of species to the heat treatment, it seems important to conduct more works with other species from the family, in order to be able to detect more general patterns. Resumo: A passagem do fogo pode quebrar a dormência de sementes de muitas espécies em ecossistemas suscetíveis ao fogo. Essa resposta ao fogo é especialmente bem conhecida para espécies da família Fabaceae, mas tem sido pouco estudado no Sul do Brasil. Coletamos sementes de quatro espécies de Fabaceae presentes em mosaicos de campo-florestas do Sul do Brasil: Mimosa bimucronata, Desmodium barbatum, Sesbania virgata e Collaea stenophylla. As sementes foram expostas a diferentes tratamentos de calor (60 °C e 80 °C, ambos por 5 minutos, e 100° C por 2 minutos, mais controle sem tratamento de calor). Nem todas as espécies foram submetidas a todos os tratamentos. Após o tratamento, as sementes foram mantidas em câmara de germinação com fotoperíodo 12/12 horas e temperatura de 25 °C. A porcentagem de sementes germinadas foi contada a cada 2 dias. Os resultados foram analisados por teste de aleatorização. A taxa de germinação de D. barbatum e S. virgata aumentou quando expostas a altas temperaturas (80 °C e 60 °C, respectivamente), enquanto M. bimucronata mostrou redução e C. stenophylla nenhuma resposta aos tratamentos. Esse estudo foi o primeiro para o Sul do Brasil a mostrar uma resposta positiva na germinação de espécies campestres expostas a altas temperaturas, simulando os efeitos do fogo. Como o estudo apresentou respostas distintas, é importante conduzir trabalhos com outras espécies da família a fim de detectar padrões gerais. Palavras-chave : Campos Sulinos, ecofisiologia, Fabaceae, fogo, vegetação campestre, choque térmico.

To resist or to germinate? The effect of fire on legume seeds in Brazilian subtropical grasslands

“…Earlier studies found little evidence that germination could be enhanced by seed exposure to high temperatures (Overbeck et al 2006;Fidelis et al 2010b). However, recent studies (Silveira & Overbeck 2013) have demonstrated that heat exposure at 80ºC led to increased germination by a grassland legume, Desmodium barbatum.…”

“…In fact, Bradstock & Auld (1995) registered temperatures from 0-3cm soil depth ranging from 60ºC to 120ºC. Moreover, we used temperatures similar to the ones used by Overbeck et al (2006) and Silveira & Overbeck (2013) in studies on seed germination of Brazilian subtropical grasslands. Therefore, we chose the range from 60ºC (which is usually measured in the upper layers of soils during fire) to 150ºC (extreme temperatures) to test our hypothesis.…”

“…In the case of our study, 78% of the remaining seeds were damaged by fungi and thus, it is not possible to affirm if they would have germinated in the absence of the fungi. Silveira & Overbeck (2013) showed that a grassland species, Desmodium barbatum, increased the germination when exposed to 80ºC, but it is not clear if the remaining seeds were dormant or dead, since the authors did not mention a viability test by the end of experiments. Most studies addressing the effects of high temperatures on the germination of Fabaceae species found peak germination between 70 -100ºC (Martin et al 1975;Auld & O'Connel 1991;Williams et al 2003).…”

Environmental control of seed dormancy and germination of Mimosa calodendron (Fabaceae): implications for ecological restoration of a highly threatened environment

“…Exposing seeds of some Mimosa species to high temperatures greatly stimulated germination (Chauhan and Johnson 2009). Conversely, germination of M. bimucronata seeds decreased after a few minutes of pretreatment at 80 and 100°C (Silveira and Overbeck 2013). Therefore, conditions for seed coat break seem to vary among Mimosa species.…”

Effects of heat shock on seed germination of Turkish red pine (Pinus brutia)

“…For example, Reyes and Trabaud (2009) Núñez and Calvo (2000) also observed that high temperature did not increase the germination in Pinus halepensis and Pinus sylvestris. Silveira and Overbeck (2013) tested the seeds of Fabaceae species and the seeds were exposed to different heat treatments (exposure to 60° and 80 °C for 5 minutes, to 100 °C for two minutes). The effect of heat-shock treatments (60, 80, 100, 120 and 140 °C for five min) on the germination of 21 herbaceous species in Central Anatolian steppe was evaluated and it is suggested that the seeds of plant species in Central Anatolian steppes are resistant to low-intensity surface fires (Tavsanoglu et al 2015).…”