The duration of seed-reserve dependency is an important aspect of the ecology of seedling establishment (Fenner 1987; Kitajima & Fenner 2000). Initially, seedlings are completely dependent on seed reserves for all resources except water, but they gradually become dependent on the external supply of resources acquired by shoots (light energy fixed in carbon-based compounds) and roots (mineral nutrients) (Krigel 1967; Fenner 1986). Whether dependency on external sources develops earlier for above- or below-ground resources affects the relative importance of light vs soil nutrients during seedling establishment. Among mineral elements, nitrogen is generally the first to become insufficient in supply from seed reserves alone (Fenner 1986; Fenner & Lee 1989; Hanley & Fenner 1997).
The duration of strict seed dependency for a given resource may vary among species in relation to four characteristics: seed size (total seed mass); seed quality (concentration of the focal resource); major function of cotyledons (whether cotyledons serve as photosynthetic or storage organs of seed reserves after germination); and inherent rate of seedling growth and development. These traits are correlated with each other according to the species’ life-history strategies. Shade-tolerant species tend to have large seeds, storage cotyledons, and inherently slow relative growth rate (RGR) via low specific leaf area (SLA) and leaf area ratio (LAR) (Kitajima 1996a; Leishman et al. 2000; Veneklaas & Poorter 1998; Walters & Reich 1999, 2000). In contrast, light-demanding species tend to have small seeds, photosynthetic cotyledons, and inherently fast RGR via high SLA and LAR. Although large-seeded species with storage cotyledons are often assumed to have longer periods of seed-reserve dependency, the differences in duration of seed-reserve dependency between these contrasting regeneration syndromes have not been quantified explicitly. No previous studies have determined whether energy or mineral reserve in seeds is the first to become insufficient to meet seedlings’ demands, and whether the results differ between species with different regeneration syndromes.
For quantitative comparisons of periods of seed-reserve dependency among species, a new method was developed to determine when the transition from heterotrophy to autotrophy occurs for a given resource. When seedlings are grown from seeds with or without the supply of a particular resource, the growth rate should be identical as long as seedlings depend completely on seed reserves. However, as seed reserves alone become insufficient in meeting the seedlings’ demand, growth of seedlings without an external supply of the focal resource becomes significantly slower than growth of seedlings with a sufficient supply of all necessary resources (Fig. 19·1 of Kitajima 1996b). The point at which this occurs indicates the end of strict seed-reserve dependency. This can be quantified by determining when functional growth curves constructed from frequent harvests become significantly different due to a difference in external resource supply.
Here, this method was applied to three tropical species in the Bignoniaceae differing in morphological and ecological characteristics, in order to ask the following questions: (i) does this transition occur earlier for nitrogen or energy? (ii) do periods of dependency differ among species differing in degrees of shade tolerance and cotyledon functions (photosynthetic vs storage)? The main prediction for the second objective was that species with photosynthetic cotyledons begin to utilize light energy earlier than species with storage cotyledons. We also explored whether seed nitrogen concentration was correlated with the dependency periods for these resources.