Abstract
Quercus seedlings have hypogeal cotyledons and tap roots, both of which act as storage organs. The importance of the storage function in the two organs may change as the seedling develops. Therefore, changes in carbohydrate reserves in cotyledons and roots of Q. crispula grown under 40 % and 3 % of full light from shoot emergence to the completion of the first leaf flush were monitored. In addition, a shoot-clipping treatment was performed to examine the relative contribution of the cotyledons and tap roots to resprouting. Cotyledons maintained large amounts of nonstructural carbohydrates during shoot development, and carbohydrates were still present in the cotyledons during the final phase of leaf flush. In addition, a notable increase in the amount of carbohydrates was observed in tap roots before leaf flush at both light levels. Since root development occurred before leaf flush, even in plants grown under 3 % light, the carbohydrate found in them presumably originated from seed reserves and was translocated to roots as storage reserves. When shoots were clipped at the leaf flushing stage, the amount of carbohydrate decreased only in the cotyledons after resprouting, suggesting that cotyledons act as the main storage organs during shoot development stages. However, it could be advantageous as a 'risk avoidance strategy' for the seedlings to store reserves in both cotyledons and roots, since cotyledons may be removed by predators during shoot development.
Highlights
Quercus species have well-developed tap roots at the seedling stage (Abrams, 1996; Larsen and Johnson, 1998), and this feature ®rst appears in the germination-establishment stages (Jones, 1959; Crow, 1988)
In addition to helping withstand drought (Rao, 1988; Callaway, 1992; Canham et al, 1996), storage has been regarded as an important function of the tap roots in Quercus species (Jones, 1959; Matsubara and Hiroki, 1985; Crow, 1988; Larsen and Johnson, 1998)
Storage functions in tap roots have been shown in seedlings older than 1 year, and saplings (Kruger and Reich, 1993; Sakai et al, 1997; Canham et al, 1999), but there have been few studies concerning the importance of the tap roots as storage organs during germination to establishment stages
Summary
Quercus species have well-developed tap roots at the seedling stage (Abrams, 1996; Larsen and Johnson, 1998), and this feature ®rst appears in the germination-establishment stages (Jones, 1959; Crow, 1988). In addition to helping withstand drought (Rao, 1988; Callaway, 1992; Canham et al, 1996), storage has been regarded as an important function of the tap roots in Quercus species (Jones, 1959; Matsubara and Hiroki, 1985; Crow, 1988; Larsen and Johnson, 1998). To evaluate the importance of the biomass remaining in the cotyledons several studies have been carried out, in which cotyledons were removed when seedlings had almost developed functional shoots Some of these studies have suggested that the residual cotyledon biomass may be bene®cial for the growth and survival of seedlings growing under stressful conditions (Bon®l, 1998; Dalling and Harms, 1999).
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