Physiology of Young Seedlings

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In both gymnosperm and angiosperm seedlings, the cotyledons play a paramount role in seedling growth and development. The cotyledons are important in storage of foods and mineral nutrients, in photosynthesis, and in transfer to the growing axis of substances needed for growth.

Hypogeous Seedlings

In oaks, walnut, buckeye, and rubber, the cotyledons remain below ground and serve primarily as storage organs. Foods stored in the seed sustain the growing embryo until the leaves expand to provide a photosynthetic system and roots develop to absorb water and minerals, thereby making the young plant physiologically self-sufficient.

The carbohydrate sources of some hypogeous species often vary with environmental conditions. For example, white oak seedlings in a shaded understory exhibited only one annual flush of shoot growth in the spring (Reich et al, 1980). Hence, all current photosynthate used in growth was provided by one set of leaves. In contrast, white oak seedlings grown under favorable environmental conditions exhibited four annual flushes of shoot growth that alternated with periods of root growth. During development of northern red oak seedlings, carbohydrate sources shifted from cotyledon reserves to photosynthesis of leaves of three sequential growth flushes (Hanson et al, 1988a,b). The leaves of the first flush were the major source of total photosynthate, through the second flush of growth. The first-flush leaves contributed 80 to 100% of total photosynthate until the second-flush leaves were fully expanded. The second-flush leaves then became the major carbohydrate source until the third-flush leaves matured. The proportional reduction in the photosynthetic contribution of the first- and second-flush leaves as the seedlings developed was attributed largely to an increase in leaf area and, to a lesser extent, to a change in the rate of photosynthesis of new and existing leaves.

Epigeous Seedlings

The physiology of epigeously germinating seedlings varies somewhat among species. The epigeous cotyledons of some species store appreciable amounts of carbohydrates. Those of other species, such as pines, accumulate only small amounts, but they become photosynthetically active shortly after they emerge from the ground.

Following seed germination, a pine seedling is a system of competing carbohydrate sinks. Seedling development is an integrated continuum, with the site of synthesis of carbohydrates shifting during ontogeny from cotyledons to primary needles to secondary needles. There is a close dependency of growth of one class of foliar appendages on the capacity of the preceding class to synthesize compounds needed for growth. Hence, development of primary needles requires metabolites from cotyledons, and development of secondary needles depends on metabolites from primary needles. The young seedling in the cotyledon stage appears to be operating at threshold levels of growth requirements and is especially sensitive to environmental stresses. There is a strong influence of shoot environment early in ontogeny on initiation of all but the early formed primary needle primordia and on expansion of all primary needles, including those formed early (Sasaki and Kozlowski, 1968a, 1969, 1970). Initiation of a few primary needles depends on availability of current photosynthate of cotyledons. Low temperature or low light intensities during the cotyledon stage prevent initiation of most of the normal complement of primary needles of red pine (Tables 2.6 and 2.7). However, when seedlings are placed in a favorable environment, following prolonged exposure to low temperature or low light intensity, primordia of primary needles form readily and subsequently expand (Fig. 2.16).

The physiology of epigeous broad-leaved seedlings varies somewhat among species, depending on cotyledon function and patterns of leaf production. Species with seeds that lack endosperm (exalbuminous seeds) have cotyledons adapted for both storage and photosynthesis; examples are beech and black locust. Cotyledons of species with endosperm act not only as storage organs, but also as transfer organs in absorbing reserves from the endosperm and transferring them to growing axes. Endosperm was not essential for limited early growth of green ash seedlings (Marshall and Kozlowski, 1976b). However, cotyledons in contact with the endosperm exhibited higher rates of elongation and dry weight increase as well as slower depletion of proteins and lipids than cotyledons that were separated from the endosperm. The rate of growth of intact seedlings was much greater than in seedlings grown from excised embryos (Table 2.8).

The early development of epigeous broad-leaved seedlings occurs in

Table 2.6 Effect of Temperature on Development of Cotyledons and Primary Needles of Red Pine«4

Temperature (°C)

Table 2.6 Effect of Temperature on Development of Cotyledons and Primary Needles of Red Pine«4

Temperature (°C)

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