Germination provides many potentially unrecognized sources of variation in the regeneration niche. In this study we relate germination requirements and seed size for 16 species of pioneer trees to microclimatic conditions present in gaps in semi-deciduous rain forest in Panama. We found that, whereas increased duration of direct irradiance can be an effective indicator of the presence of a canopy gap across all scales of canopy openness, diel fluctuations in soil temperature effectively discriminate both understory sites and small gaps (25 m2) from larger gaps. Germination response was significantly related to seed size. Small-seeded species (seed mass 2 mg) germinated equally in light and darkness (wit...

Germination ecology of neotropical pioneers: interacting effects of environmental conditions and seed size

There has been little written on the penetration of light through soil. In this review, we attempt to collate most of the work that has been published on this topic in order to stimulate further research and to clarify the often-confused literature.

Light penetration can be measured directly with, for example, a spectroradiometer, or indirectly by using germination of light-sensitive seeds or the presence of growing algae as bioindicators. Although the penetration of light through soil is greatly affected by factors such as soil moisture content and particle size and colour, it generally appears that physiologically and ecologically significant amounts of light rarely penetrate more than 4–5 mm through the soil, and may often penetrate much less than this. Any penetration beyond 10 mm would generally not be significant, especially as most soils are covered with litter, algae or lichens, or are shaded from direct sunlight. However, for some light-stimulated geo-tropic responses of roots, which can be sensitive to very low fluences, the penetration of light to greater depths could well be significant. The role of light in soil in directing root growth is also discussed.

The penetration of light through soil

Seed Research-Past,Present and Future.- Use of Transgenic Plants for Studies of Seed-Specific Gene Expression.- Impact of Simulated Drought Stress on Protein Body Cycle in Radicles of Developing and Germinating Cotton Seeds.- Synthetic Seed for Clonal Production of Crop Plants.- Synthesis of Acid-Soluble,ABA-Inducible Proteins in Wheat Embryos from Dormant Grain.- Hormones, Genetic Mutants and Seed Development.- Membrane Behavior in Drought and Its Physiological Significance.- The Basis of Recalcitrant Seed Behaviour: Cell Biology of the Homoiohydrous Seed Condition.- Phytin Synthesis and Deposition.- Calcium-Regulated Metabolism in Seed Germination.- Transport and Targeting of Proteins to Protein Storage Vacuoles (Protein Bodies) in Developing Seeds.- Biochemical Adaptations to Anoxia in Rice and Echinochloa Seeds.- Some Aspects of Metabolic Regulation of Seed Germination and Dormancy.- Phytochrome and Sensitization in Germination Control.- The Role of Light and Nitrate in Seed Germination.- Water-Impermeable Seed Coverings as Barriers to Germination.- Control of Germination and Early Development in Parasitic Angiosperms.- Factors Eliciting the Germination of Photoblastic Kalanchoe Seeds.- Factors Influencing the Efficacy of Dormancy-Breaking Chemicals.- Physiological Mechanisms Involved in Seed Priming.- Germination Research Towards the Nineties: A Summary and Prognosis.

Recent advances in the development and germination of seeds

SUMMARY
The tropical forest pioneer trees Cecropia obtusifolia and Piper auritum germinate and become established in large light gaps of the forest canopy in the rain forest of south-eastern Mexico. Germination of the seeds of both species is under photocontrol and is triggered when the red: far-red ratio (R: FR) of the incident light increases due to a reduction of the green canopy density. Exposure to simulated light canopies retarded and reduced germination. The light environment inside the forest inhibits germination totally. Experiments with alternate R and FR light treatments indicate the need for long periods of exposure to R light for germination, and demonstrate a strong reversibility of the R light stimulation by FR light in both species. This property of the seeds may be related to the detection of light gap size and its differentiation from the normal sunflecks of the forest.

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Phytochrome control of seed germination in the tropical rain forest pioneer trees cecropia obtusifolia and piper auritum and its ecological significance

The phototransformations of phytochrome involve several intermediates. The initial photoreaction of the chromophore is followed by a series of dark relaxation states of the chromophore and apoprotein. The nature of these intermediates is outlined and they are discussed in relationship to our knowledge of the tetrapyrrole chromophore.

Phototransformations of phytochrome.

This chapter discusses phytochrome control of seed germination in relation to the light environment. Positive responses to both short and long periods of light show an action peak in the red region of the spectrum, corresponding to the absorption peak of the Pr form of phytochrome. Reversal of the red effect by far-red light confirms that phytochrome is the photoreceptor. This was first demonstrated in seeds of Lactuca sativa . Seeds of some species, such as Limonium vulgare , are stimulated only by a long period of red light, suggesting that there is a requirement for Pfr action over a prolonged period. In species, such as Amaranthus caudatus , where seed germination is inhibited by prolonged exposure to light, far-red and blue are the most effective parts of the spectrum. Seeds exposed to prolonged white light (high Pfr/P) germinate on transfer to darkness, whereas seeds exposed to prolonged far-red light (low Pfr/P) do not germinate on transfer to darkness unless given an irradiation with red light. Many inhibitory effects of low-intensity far-red light can be interpreted in terms of the maintenance of a low level of Pfr.

B. Frankland

Papers

Phytochrome control of seed germination in relation to the light environment