Seeds are one of the most important organs in spermatophytes. The seed coat is thought to be an important physical barrier between the embryo and the external environment. Many plants invest a considerable amount of biomass in the seed coat; however, the relative biomass allocated to the seed coat, the seed coat ratio (SCR), varies widely among plant species, and the relative importance of the factors contributing to such large interspecific variation in the SCR has not yet been quantitatively assessed. 

In a study published in Functional Ecology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and Anhui University provided the first quantification of the relative explanations of six factors that have been commonly considered to be associated with the interspecific variation in SCR, highlighting the important role of seed desiccation response and seed dormancy in explaining interspecific variation in the SCR.  

The researchers compiled a global database of the seed coat ratio, a proxy for seed physical defense, for 1362 species. They tested the association of the six factors, including seed mass, seed desiccation response (desiccation-sensitive vs. desiccation-tolerant), seed dormancy (nondormant, physically dormant, or other dormant types), growth form (herbaceous vs. woody), fruit type (dry vs. fleshy), and climate (19 bioclimatic variables representing temperature and precipitation). 

Their global-scale study successfully disentangled the relative explanations of the six most frequently discussed factors associated with interspecific variation in the SCR. Seed desiccation response was shown to be the most important factor associated with the interspecific SCR variation, suggesting that natural selection imposed by drought stress may be much more important than any other abiotic or biotic factor in determining the SCR.  

Furthermore, seed dormancy showed a modest contribution to interspecific variation in SCR, indicating that biotic and abiotic stresses that are related to seed dormancy may have an indirect effect on SCR variation. However, growth form, climate, fruit type and seed mass were irrelevant in explaining SCR variation. 

"Our results shed some light on the ecological patterns underlying the physical defense mechanisms and allocation strategies of plant seeds," said WU Lamei of XTBG.