The size and growth rate of a seedling is critical for its competitiveness and survival (Harper, 1977; Schwinning & Weiner, 1998; Walters & Reich, 2000). Initial seedling growth is completely dependent on reserves stored in the endosperm or cotyledons. Although knowledge of the developmental and biochemical events in germinating seed plants is substantial (Ashton, 1976; Bewley & Black, 1994), there is little quantitative knowledge about the importance of reserve- and current assimilation-derived substrates for root and shoot growth of seed plants during the successive phases of seedling growth.
Identification and quantification of the contributions of reserves and current assimilates to growth requires differential labelling of the two substrate sources with a known, constant isotopic composition (Geiger & Shieh, 1988; Schnyder, 1992; Deléens et al., 1994). For studies of the transition from heterotrophic to autotrophic growth, this means that either the current assimilate must be labelled continuously over the entire experimental period (long-term, steady-state labelling), or that the seed reserves must be homogeneously labelled by growing the mother plants in an atmosphere with altered, constant carbon (C) isotope composition of CO2. To date, such studies have been performed with maize (Zea mays; Deléens et al., 1984) and walnut (Juglans regia; Maillard et al., 1994a,b), two species with hypogeal germination, in which the cotyledons do not develop any (walnut) or hardly any (maize) photosynthetic activity. To our knowledge, there are no reports of reserve- and current assimilation-derived C and N (nitrogen) allocation during germination and seedling growth of species with epigeal germination.
The work with walnut and maize indicated that early shoot and root growth were entirely dependent on reserves, but that the reserve-dependence of roots was much longer than that of shoots. This was particularly true for walnut where an approx. 3-wk delay between the onset of photosynthesis and first detection of autotrophic C in roots was observed (Maillard et al., 1994b). In addition, the work with maize (Deléens & Brulfert, 1983) and walnut (Maillard et al., 1994a) indicated that transition from reserve-dependence to current assimilation supply was faster for the respiratory activity of plants than for growth. Thus, it appears, that the transition from heterotrophic to autotrophic substrate supply is asynchronous for different functions. Different kinetics in the transition from reserve to current assimilation dependence suggest that the substrate pools serving root and shoot growth and respiration are at least partially separated. Such a separation could be biochemical – the different functions using chemically distinct substrates (e.g. carbohydrates or amino acids) – or physical, but is not well characterized and understood. Also, it is unknown if this transition pattern is the same in species with epigeal germination, in which the transition is initiated by the photosynthetic activity of the cotyledons.
A related question, which has not been addressed experimentally, is whether the transition from reserve to current assimilation supply to different functions is influenced by environmental conditions, such as the concentration of CO2. In studies with cotyledons of sunflower, Pfeiffer & Kutschera (1996) observed a faster lipid mobilization in light than in the dark. This was interpreted in terms of increased substrate demand by light-induced cotyledon expansion. Eastmond et al. (2000) found that addition of sucrose to seeds of Arabidopsis thaliana, germinating on agar plates caused a delay of storage lipid breakdown, suggesting that the breakdown process is subject to metabolic control.
To shed light on these questions, we analysed (1) the timing and rate of seed reserve-C and -N mobilization, and of their allocation to root and shoot tissue, (2) the time-course and rate of autotrophic C and N acquisition and allocation and (3) how these processes are modified by contrasting CO2 concentrations in sunflower, a species with epigeal germination. Plants were germinated and grown in controlled environment with CO2 concentrations of either 200 or 1000 µmol mol−1 and sampled at intervals over the first 15 d after imbibition (DAI) of seeds. Steady-state 13CO2/12CO2 and 15NO3−/14NO3− labelling was used to distinguish and quantify heterotrophic and autotrophic C and N (net) fluxes.