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Interactions between Climate and Plant Species Function and Distribution for Recalcitrant Seeded Taxa

Impact of developmental heat sum across Europe (from south to north; left to right) on Aesculus hippocastanum seed traits e.g. size

This project is one of a series in the theme 'Climate and Reproductive Biology'.

Seeds can be divided into two broad groups, orthodox or recalcitrant, based on their response to desiccation. Orthodox seeds are capable of tolerating desiccation to low (generally < 7%) moisture contents with little effect on viability. In contrast, recalcitrant seeds are killed by drying the seed/embryo to water contents as high as 20–30%. Because desiccation-sensitive seeds cannot be dried and also progress towards germination when stored fully hydrated, they are difficult to store for anything other than the short term. Consequently, their use in reforestation programs and ex situ conservation is problematic. Therefore, tools for predicting seed storage classification would be of use when handling seeds of species for which seed storage behavior is unknown.

Seed desiccation sensitivity is potentially a high-risk regeneration strategy for plants because a prolonged dry spell at the time of seed shedding could result in the death of an entire annual cohort of seeds. This presumably explains the typically large size of desiccation-sensitive seeds, their high frequency in aseasonal tropical forests, and their convex shape, all of which will reduce the likelihood of seed drying. As a result, there is a general perception that, in the tropics, desiccation-sensitive species do not occur in arid or savannah habitats. However, such species do occur naturally in the tropical drylands although little is known about their regeneration strategies.

The possible ecological advantages and disadvantages of seed desiccation sensitivity have received little study and are currently unclear. It has been proposed that desiccation tolerance may be the ancestral state in seeds, with desiccation sensitivity being a derived trait. This implies that the loss of desiccation tolerance provides a fitness advantage relative to desiccation-tolerant species. Whilst desiccation sensitivity could be viewed as a neutral trait in aseasonal wet forests this may not be the case in drier habitats. Consequently we are exploring the traits and adaptations of recalcitrant seeds in dry and seasonally dry environments. Currently we have found from analyzing primary data for >300 species from two distinct habitats that recalcitrant seeds 1) have a large seed mass to reduce the rate of seed drying; 2) germinate rapidly to reduce the likelihood of seed dehydration; and 3) are shed at the time of maximum annual rainfall. However, even with these adaptations, seed desiccation sensitivity appears to present a constraint to species distributions patterns since their occurrence frequently seems to be restricted to wet micro-sites.

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