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 three cotyledon characters of presumed ecological significance (position, texture and exposure) (Fig. 9):
(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, see below within the present Protocol). 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 e.g. 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 in the present Section (and in Fig. 8):
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.
Special cases or extras
(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: Ng (1978); De Vogel (1980); Hladik and Miquel (1990); Garwood (1996); Kitajima (1996); Wright et al. (2000); Ibarra-Manriquez et al. (2001); Zanne et al. (2005); Leck et al. (2008).