Is Seed Sensitivity to Desiccation a Matter of Size?
To analyse the physical properties, morphology and germination physiology of large seeds, and to synthesise a relationship between these attributes and their habitats in relation to desiccation tolerance.
Dissected seed of the palm Jubaea chilensis illustrating a rudimentary embryo in the endosperm.
Plants adopt different strategies for seed survival and germination to exploit prevailing conditions in the plants natural habitats at the time of seed shed to promote seedling recruitment. Seed mass plays a significant role in these strategies, and for many trees; shown in a study analysing 1,037 species, this may involve the production of large seeds with delayed germination and a greater capacity to exploit open spaces through tolerance to temporarily dryer environments (Norden et al, 2009). With a thicker seed coat they will survive better predation and adverse conditions in the leaf litter, but this does not always correlate with a fast rate of germination. Many larger seeds, especially tree species, are desiccation sensitive, and subject to their native habitat, can account for 3 to 47% of the flora. This creates a problem for ex situ conservation, where reduced water content prior to freezing is a prerequisite for long-term storage. Seed coat ratio has been used as part of a predictive model to differentiate desiccation tolerant and sensitive species. In a study on >100 Panamanian tree species we predicted seed desiccation sensitivity at a probability of c. 0.90 when the seeds is >1g and has a seed coat ratio of <0.25 (Daws et al, 2006). However, this model does not predict for sensitivity in the Arecaceae (palms), where following a successful screen of 125 species, the overwhelming determinate of sensitivity appears related to the prevailing climate at seed dispersal.
The management of water content in relation to tissue survival presents an opportunity to understand both how tolerance to seed desiccation has evolved, and informs new approaches for their conservation. Research at Kew on the relationships between parameters such as seed mass, mean time to germinate, water content, oil content and morphology are being used to both understand the seed biology of these species, and their behaviour in nature, and also to develop new methods for seed conservation. The screening of a wide range of palm taxa do not reveal easily resolved correlations for whole seeds, although comparison of their rudimentary embryos suggest that relative size and morphology may be expressions of desiccation sensitivity or tolerance. The relationship between seed oil and water content has been screened in Quercus species, in particular in the sub-genus Cyclobalanopsis; a taxa of high economic value in tropical and sub-tropical Asia. Results show that differences in seed mass are due to variations in oil content, rather than water alone, the composition of which may affect longevity in storage.
The relationship between seed mass, germination rate and desiccation tolerance is complex, and understanding the importance of physical restraints across gross seed size ranges, has illustrated that more intricate aspects of seed morphology may influence desiccation sensitivity, especially that of the embryo, which in palms are simple in structure and often small relative to the endosperm, while in many tree species, they fill the entire seed.
Output from this project is through peer-reviewed papers, a PhD study and the training of undergraduates.
Project partners and collaborators
Kunming Institute of Botany
Xishuangbanna Tropical Botanical Garden
University of Aberdeen
South Illinois University
University of Connecticut
Key papers published since 2006:
Xia, K., Daws, M.I., Hay, F.R., Chen, W.-Y, Zhou, Z.-K. and Pritchard, H.W. (2011). A comparative study of desiccation responses of seeds of Asian Evergreen Oaks, Quercus subgenus Cyclobalanopsis and Quercus subgenus Quercus. South African Journal of Botany. doi:10.1016/j.sajb.2011.05.001 (IF=1.106).
Xia, K., Seal, C.E., Chen, W.Y., Zhou, Z.-K. and Pritchard, H.W. (2010) Fruit oil contents of the genus Quercus (Fagaceae): A comparative study on acorns of subgenus Quercus and the Asian subgenus Cyclobalanopsis. Seed Science & Technology 38: 136-145 (IF=0.605).
Norden, N., Daws, M.I., Antoine, C., Gonzalez, M.A., Garwood, N.C. and Chave, J. (2009). The relationship between seed mass and mean time to germination for 1037 tree species across five tropical forests. Functional Ecology 23: 203-210 (IF= 4.645)
Daws, M.I., Crabtree, L.M., Dalling, J.W., Mullins, C.E. and Burslem, D.F.R.P. (2008). Germination responses to water potential in neotropical pioneers suggest large-seeded species take more risks. Annals of Botany 102: 945-951 (IF=3.388).
Daws, M. I., Garwood, N.C. and Pritchard, H.W. (2006). Prediction of desiccation sensitivity in seeds of woody species: A probabilistic model based on two seed traits and 104 species. Annals of Botany 97: 667-674 (IF=3.388).
Conferences and workshops attended:
Daws, M.I., Wood, C.B., Marks, T.R. & Pritchard, H.W. (2007) Desiccation sensitivity in the Arecaceae: Correlates and frequency. South African Journal of Botany 73(3): 483. Abstract from the 5th International Workshop on Desiccation Tolerance & Sensitivity of Seeds & Vegetative Plant Tissue held at the Nest Hotel, Drakensberg/Ukihlamba, KwaZulu-Natal, South Africa from 14-21 January 2007.
Daws, M.I., Beardmore, T.L. & Pritchard, H.W. (2007) Are there any recalcitrant seeded Acer L. species? Evidence based on seed mass and physiological characterisation. In: Recent advances in seed phsiology and technology. Proceedings of the IUFRO Tree Seeds Meeting, July 2006, Frederiction, Canada. pp. 16-29.