The presence of leaf litter of different depths within a tropical forest creates many different microsites for plant establishment. The amount and distribution of leaf litter within a forest can influence patterns of plant establishment. In this study, we de- termined the spatial variability in leaf litter in the forest understory, and investigated how different litter depths (bare, 1, 6, and 12 cm) affected the establishment of several tropical tree species in both growth house (sun and shade) and field (gap and understory) experiments in the semideciduous tropical forest of Barro Colorado Island, Panama. The tree species used in this study (Aspidospermum cruenta, Ceiba pentandra, Cordia alliodora, Gustavia superba, Luehea seemannii, Ochroma pyrimidale) were chosen to represent a range of seed masses and a gradient in the light requirement for establishment of the species. The spatial distribution of leaf litter was not correlated between adjacent sampling points within the forest understory, suggesting that the establishment environment for seedlings, with respect to litter, is highly variable at scales of 1-20 m. The presence of litter affected five of the six species, but the nature and the magnitude of the effect were species specific. The smaller seeded shade-intolerant species had fewer seedlings establishing under leaf litter than on bare ground. The species ranged from strongly negatively affected (Luehea) to moderately negatively affected (Cordia, Ochroma) to affected only by extreme amounts of litter (Ceiba). The presence of litter influenced Gustavia, one of the larger seeded shade-tolerant species, but did not affect Aspidospermum, the other larger seeded species. The effect of litter on Gustavia depended on the light environment. Gustavia had more seedlings establishing under litter in the sun, but the presence of litter had no effect in the shade. Differences among the smaller seeded shade-intolerant species in the amount they were negatively influenced by litter were not correlated with seed mass. Data from our field study were consistent with our growth house results for the shade- intolerant species. Additional data from the field study indicated that these species with similar habitat requirements differed in the developmental stage at which they were affected by the presence of litter. Luehea had fewer seeds germinating under litter while the other two species, Ochroma and Cordia, were affected only after germination. Interspecific comparisons done for each light level and litter depth indicated that the presence of litter caused reversals in the relative ranking of species success. For example, Gustavia preferentially established under relatively deep litter depths in the sun where Luehea could not establish. In conclusion, the presence of litter can potentially increase seedling diversity within the forest by creating heterogeneity in the establishment envi- ronment and by causing reversals in species' rankings.

The Effect of Leaf Litter on Early Seedling Establishment in a Tropical Forest

Summary The importance, characteristics, positive and negative impacts, and future role of weeds as an integral part of the natural and agroecosystems are evaluated and discussed. Interference between plants in nature and the importance of differentiating between competition and allelopathy are interpreted. Allelopathy as one component of weed/crop interference, allelochemicals from weed species and their possible mechanism of action are listed and discussed. Weed species with inhibitory action against cultivated crops, other weed species, and plant pathogens, as well as self-inhibitory (autopathic) species are reviewed. Stimulatory or inhibitory allelopathic effects of different crop plants, trapping and catching species, and the potential of allelopathic weeds in inhibiting or stimulating certain parasitic weed species are discussed and evaluated. Allelopathy as a mechanism and future strategy for agricultural pest control and farm management and the potential use and development of some allelochemicals...

Weed allelopathy, its ecological impacts and future prospects: a review

1 Cellular Aspects of Secretory Activity in Plants.- 1.1 Significance of Secretory Processes for the Cell.- 1.2 Compartmentation of Metabolites and Mechanisms of Their Secretion.- 1.3 Secretion into the Free Space of the Cell.- 1.4 Secretion into the Vacuole.- 1.5 Idioblasts.- 2 Intratissular Secretion.- 2.1 Air-Bearing System of Plants.- 2.2 Internal Gases.- 2.2.1 Carbon Dioxide.- 2.2.2 Ethylene.- 2.2.3 Other Volatile Compounds.- 2.2.4 Transport of Internal Gases.- 2.3 Intratissular Secretory Structures.- 2.3.1 Secretion of Resins.- 2.3.2 Secretion of Latex.- 2.3.3 Secretion of Gum and Essential Oils.- 3 External Secretion.- 3.1 Guttation.- 3.2 Salt Glands and Secretion of Inorganic Salts.- 3.3 Secretion of Nectar.- 3.4 Secretion of Polysaccharides.- 3.5 Secretion of Proteins.- 3.6 Secretion of Essential Oils.- 3.7 Secretion of Resins.- 3.8 Secretion of Phenols.- 3.9 Secretion of Alkaloids.- 3.10 Secretion of Acetylcholine and Amines by Stinging Trichomes.- 4 Gas Excretion.- 4.1 The Pathways of Gas Release.- 4.2 Volatile Excretions as Complexes of Substances.- 4.3 Components of Gaseous Excreta.- 4.3.1 Short-Chain Hydrocarbons.- 4.3.2 Isoprene and Terpenoids.- 4.3.3 Aldehydes and Ketones.- 4.3.4 Low-Molecular Alcohols.- 4.3.5 Volatile Nitrogen-Containing Substances.- 4.3.6 Carbon Monoxide and Hydrogen.- 4.4 The Significance of Gas Excretion.- 5 Leaching.- 5.1 The Cell Wall as a Phase of Leaching.- 5.2 Leaching of Salts.- 5.3 Leaching of Organic Compounds.- 5.4 Dependence of Leaching on External Factors, Phase of Development, and Anatomy of Plants.- 5.5 Physiological Meaning of Leaching.- 6 The Elimination of Substances in Response to Extreme Factors.- 6.1 Injuries to Membranes Under Stresses.- 6.2 Metabolites Released Under Stress.- 6.2.1 Ethylene.- 6.2.2 Ethane and Other Simple Hydrocarbons.- 6.2.3 Terpenoids.- 6.2.4 Alcohols.- 6.2.5 Aldehydes and Ketones.- 6.2.6 Hydrogen Cyanide.- 6.2.7 Phenols.- 6.2.8 Alkaloids.- 6.2.9 Polyacetylenes, Thiophenes, and Traumatic Acids.- 6.2.10 Other Nitrogen- and Sulfur-Containing Compounds.- 6.2.11 Phytoalexins.- 7 Biological Effects of Plant Excreta.- 7.1 Growth Processes and Cell Destruction.- 7.1.1 Division and Elongation of Cells.- 7.1.2 Pollen Germination.- 7.1.3 Destructive Changes in Cells.- 7.2 Cellular Membranes as Targets for Action of Plant Excreta.- 7.3 Energetic Reactions.- 7.4 Metabolic Processes.- 7.5 Problems and Perspectives in the Use of Plant Excreta.- 7.5.1 Plant Resistance to Pathogens.- 7.5.2 Chemical Interactions: Plant-Insect and Plant-Plant..- 7.5.3 Use in Medicine.- Conclusion.- References.- Index of Latin Names.

The Excretory Function of Higher Plants

Factors affecting the dominance of Silybum marianum L. (Asteraceae) in its specific habitats.

The allelopathic potential of red pine (Pinus densiflora), pitch pine (Pinus rigida) and black pine (Pinus thunbergii) has attracted attention of Korean botanists for the last several years. It has been a point of interest why the understorey species are sparse (Lee and Monsi, 1963) and species growing there were similar with the other pine understoreys. It was hypothesized that the similarity of floristic composition of pine stands was caused by some regulating mechanism controlled by pine through the release of certain toxic substances (allelochemicals) in the soil. When the greenhouse soil was mixed with pine leaves, the growth of the plants was suppressed. However, the toxicity of the soil was gradually reduced, and ultimately diminished after several years. This further strengthened the idea of the possible release of allelochemicals by pine. Therefore, it deemed necessary to verify experimentally whether pine is indeed producing allelochemicals which, in turn, affect neighbouring plants. For this, several experiments were performed involving various species growing inside and outside the pine forests. The results obtained from various laboratory and field studies with respect to the allelopathic effects of pine and details of isolation of allelochemicals from pine materials are reported here.

S.C. Datta

Papers

Allelopathic Potential of Polygonum orientale L. in Relation to Germination and Seedling Growth of Weeds

Allelopathic Potential of Clerodendrum viscosum Vent. in Relation to Germination and Seedling Growth of Weeds