This article is modified from Perez-Harguindeguy et. al. (2013). The “New handbook for standardised measurement of plant functional traits worldwide” is a product of and is hosted by Nucleo Diversus (with additional Spanish translation). For more on this and on its context as part of the entire trait handbook visit its primary site Nucleo DiverSus at http://www.nucleodiversus.org/?lang=en
Hans ter Steege, Frank Sterck
Seedling functional types refer to morphology of seedlings in relation to cotyledon function and position. It is a categorical trait that can be used to characterise plant regenerative strategies. The distribution of seedling traits across families is still rather poorly known, although the importance of seedling traits in systematics was recognised quite early. This trait has been created on the basis of woody species (trees and shrubs) and has been mainly used in tropical forests. Recently, Garwood (1996) established the following five seedling categories based on cotyledon characters, with presumed ecological significance:
(1) cryptocotylar hypogeal with reserve storage cotyledons (CHR),
(2) cryptocotylar epigeal with reserve storage cotyledons (CER),
(3) phanerocotylar epigeal with foliaceous cotyledons (PEF),
(4) phanerocotylar epigeal with reserve storage cotyledons (PER) and
(5) phanerocotylar hypogeal with reserve storage cotyledons (PHR).
These categories result from the combination of different possibilities in relation to cotyledon exposure (phanerocotylar or cryptocotylar), position (epigeal or hypogeal) and function (foliaceous or reserve storage).
cotyledon exposure – phanerocotylar if the seed coat opens and two cotyledons emerge from seed, cryptocotylar if cotyledons remain within the seed coat;
cotyledon position – epigeal when hypocotyl develops at least 2 cm above soil surface, hypogeal when hypocotyl develops on the soil surface; or
cotyledon function – reserve cotyledon are fleshy, foliaceous cotyledons (also called paracotyledons) are primarily photosynthetic.
Although eight combinations are potentially possible, cryptocotylar foliaceous seedling types are biologically not possible, and phanerocotylar hypogeal foliaceous seedlings have not yet been reported. Seedling functional types are correlated with other plant traits such as seed size; e.g. large seed sizes are related to reserve storage seedling types, whereas small seed sizes are related to foliaceous and photosynthetic cotyledons. Because the above-mentioned types have been particularly identified for tropical forests, the occurrence and proportion of each type in other ecosystems should be tested.
What and how to collect
Because this is not a plastic trait, we recommend germinating sufficient number of seeds to obtain about five seedlings per species. Seeds without evidence of, for example, pathogen damage and predation must be selected among the collected ones to run the experiments.
Seedling morphology (cotyledon exposure, position and function) should be described when at least five individuals have developed at least three leaves each. Species are then assigned to the seedling morphological categories indicated above (and in Fig. 1):
Fig. 1. Seedling functional types. Five seedling functional types as described in Seed mass (leaves in white, cotyledons in grey, seeds in black). Seedling functional morphology: PHR = phanerocotylar hypogeal with storage cotyledons; PER = phanerocotylar epigeal with storage cotyledons; PEF=phanerocotylar epigeal with foliaceous cotyledons; CHR = cryptocotylar hypogeal with storage cotyledons; and CER = cryptocotylar epigeal with storage cotyledons.
Notes and troubleshooting tips
(1) Chlorophyll in fleshy cotyledons – In many cases fleshy cotyledons do contain chlorophyll; however, they are still considered reserve organs (e.g. Aspidosperma spp.).
References on theory, significance and large datasets:
De Vogel EF (1980) Seedlings of dicotyledons. Centre for Agricultural Publishing and Documentation: Wageningen, The Netherlands
Garwood NC (1996) Functional morphology of tropical tree seedlings. In The ecology of tropical forest tree seedlings. Ed. MD Swaine, pp. 59-129. Parthenon: New York.
Hladik A, Miquel S (1990) Seedling types and plant establishment in an African rain forest. In Reproductive ecology of tropical plants. Eds KS Bawa, M Hadley, pp. 261-282. UNESCO/Parthenon: Carnforth, UK.
Ibarra-Manriquez G, Martínez Ramos M, Oyama K (2001) Seedling functional types in a lowland rain forest in México. American Journal of Botany 88, 1801-1812. doi:10.2307/3558356
Kitajima K (1996) Cotyledon functional morphology, patterns of seed reserve utilization and regeneration niches of tropical tree seedlings. In The ecology of tropical forest tree seedlings. Ed. MD Swaine, pp. 193-210. UNESCO: Paris.
Leck MA, Parker VT, Simpson RL (2008) Seedling ecology and evolution. Cambridge University Press: Cambridge, UK
Ng FSP (1978) Strategies of establishment in Malayan forest trees. In Tropical trees as living systems. Eds PB Tomlinson, MH Zimmerman, pp. 129-162. Cambridge University Press: Cambridge,UK
Wright IJ, Clifford HT, Kidson R, Reed ML, Rice BL, Westoby M (2000) A survey of seed and seedling characters in 1744 Australian dicotyledon species: cross-species trait correlations and correlated trait-shifts within evolutionary lineages. Biological Journal of the Linnean Society. Linnean Society of London 69, 521-547. doi:10.1111/j.1095-8312.2000.tb01222.x
Zanne AE, Chapman CA, Kitajima K (2005) Evolutionary and ecological correlates of early seedling morphology in east African trees and shrubs. American Journal of Botany 92, 972-978. doi:10.3732/ajb.92.6.972