Growth & Development

Most Australian shrubs have “sleepy seeds” and seed dormancy types might be driven by rainfall seasonality

Have you ever found it tricky to get some seeds to germinate? You might have wondered if it was the water, or the temperature, or the soil, or perhaps the seeds themselves that are the problem?

Seeds are smart. Thanks to the wonders of evolution and molecular processes, seeds aim to germinate at the right time and at the right place. However, some seeds can remain dormant for years and not germinate despite favourable environmental conditions. Producing dormant seeds can be quite costly to a plant but it is a good survival strategy in unstable, disturbed habitats where germination cues might be too erratic. 

Drs Justin Collette and Mark Ooi from the University of New South Wales investigated the relationship between three seed dormancy classes of almost 4,000 shrub species across temperate Australia and different environmental conditions. This is the first large-scale study showing that most shrubs produce dormant seeds in Australia. The temperature and rainfall seasonality seem to explain the distribution of the different seed dormancy types. The aim of Ooi’s research group is to better understand seed ecology and improve the conservation outcomes for planned burns.

There are five main types of seed dormancy. It can be physiological (water can enter the seed but chemical changes initiate the germination), physical (water cannot enter the seed), morphological (under-developed embryo delays the germination), morphophysiological or a combination of physiological and physical.

Examples of Australian perennial shrubs with different seed dormancy categories. From the top right: Banksia coccinea (non-dormancy, threatened), Darwinia apuculata (physical dormancy, threatened), Cryptandra amara (physical dormancy, not threatened) and Exocarpos cupressiformis (morphophysiological dormancy, not threatened). Sources: Cygnis insignis/​​Jean and Fred/John Tann (Wikimedia Commons)

Colette and Ooi compiled perennial shrub records from the Australasian Virtual Herbarium and limited their study to the temperate regions of Australia. The dormancy classification was based on previous research publications and germination trials at the Australian Botanic Garden. The researchers checked the conservation status of each species and based on climatic factors (e.g., rainfall, temperature, seasonality), shrub occurrence, dormancy, the scientists tested the drivers of dormancy amongst 3990 species from 281 genera. 

The majority of 4,000 Australian shrubs species produce dormant seeds. Source: Collette and Ooi, 2021

Most of the shrub species (79.4%) produced dormant seeds across Australia and overall, dormancy was more likely in cooler areas. Rainfall seasonality could have explained the distribution of shrubs with physiological and physical dormancy. 

Physiological dormancy was more prevalent in areas with aseasonal rainfall whilst physical dormancy was likely in areas with highly seasonal rainfall. Physiological dormancy is said to be the most common dormancy type and in a fire-prone habitat, breaking this type of dormancy requires temperature and fire-related (e.g., smoke) cues. 

The two most common seed dormancy types in Australian shrubs are physical (PY) and physiological (PD), whilst the morphophysiological (MPD) type is less common. Source: Collette and Ooi, 2021

Threatened species were more likely to produce dormant seeds, especially physiological dormant ones, and occurred in aseasonal rainfall environments. These findings suggest that physiologically dormant species are at a disadvantage (more threatened) in fire-prone regions with aseasonal rainfall.

“[O]ne possible explanation for decreased suitability of aseasonal rainfall climates for the persistence of PD [physiologically dormant] species is delayed emergence,” wrote Collette and Ooi. 

“If the seasonal obligation for germination does not match with the seasonal timing of a fire, germination and therefore seedling emergence can be delayed by up to a year. For other types of dormancy, with no seasonal temperature obligation, germination can occur immediately post-fire, as long as there is rainfall present.” 

This large-scale study found a few patterns between seed dormancy type and environmental conditions for Australian shrubs. Whilst ecologists usually focus on seed size, shape and mass when studying plant communities and their functions, here, Collette and Ooi highlight that seed germination traits need more attention in the future. 

“Understanding how all dormancy classes interact with the environment is the first step in predicting the outcomes of climate change and giving fire-managers the best chance of achieving positive management outcomes.”

RESEARCH ARTICLE

Collette, J.C., Ooi, M.K.J., 2021. Distribution of seed dormancy classes across a fire-prone continent: effects of rainfall seasonality and temperature. Annals of Botany. https://doi.org/10.1093/aob/mcaa203

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