HIGH-TEMPERAURE ADAPTATION IN TROPICAL PLANT SEEDS
Center for Integrative Conservation, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Menglun, Mengla 666303, Yunnan, China
The tropics are characterized by high temperatures, which may be an important stressor to tropical plant seeds. Air temperatures have important effects on developing seeds; while after shedding, seeds are mostly affected by soil surface temperature. Taking Xishuangbanna in southwest China, an edge area of tropical Asia, as example, there soil surface maximum temperature often excess 60°C on open ground in sunny days, with a extreme value of 71.4°C documented by weather observation station there. However, active cellular membrane is usually destroyed by high temperature around 60°C, how seeds survive so high temperature in tropics?
Temperature changes greatly depending habitats, meanwhile high temperature tolerance in seeds varies from species to species. It was found that seeds native to tropical rainforest are usually high-temperature sensitivity, including recalcitrant seeds, which can not be dried so neither can tolerate high temperature above 60°C, such as Hopea hainanensis and Baccaurea ramiflora, and seeds of some rare and endangered species, which also are high-temperature intolerant even if desiccation tolerant, such as Pellacalyx yunnanensis and Tacca chantrieri. Fortunately, no soil surface maximum temperature more than 30°C has been detected in intact tropical rainforest there. This category of seeds adapts the avoidance strategy to high temperature, obviously. Another category of seeds, including those produced by weeds, invasive plants and pioneer species, adapts the tolerance strategy to high temperature. They exhibit much stronger tolerance to high temperature in air-dried state, such as Amaranthus spinosus, Piper aduncum, and Tithonia diversifolia.
High-temperature tolerance in seeds may contribute to plant distribution in the tropics. Nowadays, globular warming, deforestation, rainforest fragmentation and other human activities are enlarging hot habitats and making hot habitats hotter. This decreased habitats suitable for climax species in tropical rainforests, made them rare and endangered; on the other hand, increased habitats suitable for non-climax species, paved way for plant invasion.