Seedling dependence on seed reserves

A germinating seedling will be dependent initially on seed reserves for all its energy and nutrient requirements. The rapid development of the root will allow the seedling to become anchored and to start taking up water and nutrients. Foliaceous cotyledons can begin to photosynthesise and reduce the seedling's reliance on the seed reserves for fixed carbon. Seeds vary in the form of energy reserves they contain. Starch is common, but some seeds are rich in oils and other lipids. Lipid is a more concentrated store of energy than starch. In theory, a lipid-rich seed should produce a seedling 30% heavier than the seed (Kitajima 1996). A starchy seed of the same mass should only achieve a seedling mass 8% less than the seed mass. However, in reality plants are biochemically less efficient at utilising lipid than starch, so the advantage of lipids as seed stores is not as great as predicted, although they are still space-saving in comparison to carbohydrates. One might predict therefore that very small seeds should tend to be oilier than large ones, in order to store more energy in the tiny packet. However, wide-ranging surveys have found that big seeds tend to be oilier than small ones (Levin 1974). This is possibly to keep very energy-rich seeds within the size limits imposed by effective dispersal (Kitajima 1996).

Mineral nutrient concentrations of seeds tend to decline with seed size, both within a species (Grubb & Burslem 1998) and between species (Grubb 1996; Grubb & Coomes 1997), but not fast enough to prevent total nutrient content of seeds increasing with seed size. Grubb (1996) found that large-seeded lauraceous species dispersed by scatter-hoarding rodents in the rain forests of Queensland had higher N concentrations than the general regression of concentration against seed size would predict. He hypothesised a selection pressure in favour of higher seed N in such species as a reward for the mammalian seed dispersers.

Height (m)

Figure 5.8 Leaf size as a function of tree height for Cecropia obtusifolia individuals at Los Tuxtlas, Veracruz, Mexico. After Alvarez-Buylla & Martinez-Ramos (1992).

Height (m)

Figure 5.8 Leaf size as a function of tree height for Cecropia obtusifolia individuals at Los Tuxtlas, Veracruz, Mexico. After Alvarez-Buylla & Martinez-Ramos (1992).

Kitajima (1996) performed an ingenious experiment to estimate the duration of seedling dependence on seed reserves for three tree species of the Bignoniaceae. Seedlings were grown at high and low (2%) light and high and low nitrogen availability. When the growth of the low-resource treatment seedlings fell below that of the high-resource treatment it was assumed that the seed reserves were no longer the sole supplier of resources to the seedling. For all three species, the seed store of energy ran out before that of nitrogen. In the shade, the nitrogen store of the seed lasted up to 40 days. Tabebuia rosea, with the smallest seeds and leafy cotyledons, was reliant on seed reserves for the shortest period. It is not clear whether this was because the reserves ran out more quickly, or because photosynthesis began sooner than in the other two species. The mass-based assimilation rate of seedling cotyledons was strongly negatively related to cotyledon thickness (Kitajima 1992). Thick, largely storage, cotyledons photosynthesised little, but it may have been sufficient to balance the respiration of the storage cells, meaning that the cotyledons were not a net drain on the reserves of the seedling.

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