forestry

Symbiotic Relationship:

Much research in agroforestry systems is concerned with increasing the biological input of nutrients to trees and to the crops grown concurrently or consecutively or consecutively with them and with determining how and in what quantities these nutrients become available. The exchange of nutrients among the plants of the agroforestry system results largely from the activity of appropriate soil microorganisms. Associative or symbiotic microorganisms are responsible for nitrogen input and for the availability of other minerals, especially phosphorous, in the ecosystem. Other bacteria make available the nutrients of dead and decaying plants for uptake by the root systems of crop species. Symbiotic microorganisms like Rhizobium and Frankia have potential roles of nitrogen fixation, plant growth regulation, phosphate solubilization and concerned with nutrient transformations of decaying plant material.

The important nitrogen-fixing symbioses are:-1) those between many legume trees species and Rhizobium or Bradyrhizobium and 2) those between Frankia and woody species within the eight non leguminous plant families (so called acinorhizal plants that are nodulated by the nitrogen-fixing actinomycete. For temperature and warm temperate conditions, the most important of the Frankia associations are with Alnus (Betulaceae) or Elaeagnus and Hippophae (Elaeagnaceae), and in the tropics and subtropics, with members of the Casuarinaceae. The most promising candidates for agroforestry are the Casuarina and Allocasuarina genera.

Allelopatic Relationship:

Interference occurs when one plant species fails to germinate, growth more slowly, shows symptoms of damage, or does not survive in the presence of another plant species. Interference can result from competition, allelopathy, or other indirect influences. Competition is the phenomenon by which one plant removes limited resources (such as light, water, or nutrients) from the environment, thereby reducing the survival or growth of a neighboring plant. Allelopathy is the phenomenon in which a plant or microorganisms releases a natural product into the environment that subsequently reduces or enhances the survival or growth of neighboring plants.

Agroforestry intentionally combines woody perennials with agricultural crops or pasture plants in variety of spatial or temporal arrangements, thus the choice of species combinations may dramatically influencing the productivity and ultimate success of some agroforestry systems. The challenge in plant interference work is identifying which of trees various factors because the associated plant response. Allelochemicals originating in foliage teachings, root products, or mulches of crops or woody plants may result in reduced productivity or death of companion plants.
The concept of allopathy is at least 2,000 years old, though the team was not coined until 1973(Wills, 1985). During the past 30 years there has been a significant effort to understand the role of allelopathy in ecological processes.
Because agroforestry is a relatively new field, little work has been conducted on species compatibility (Wood, 1988). Some species currently used in agroforestry systems reportedly have allelopathic interference from Eucalyptus foliage leaching and volatiles and plant residues has been described. Also, residual mulches of Luceaena lecucocephala reportedly have allelopathic properties.

Alleopathy and intercropping: Allelopathy interference can result form natural products in intercrop foliage leaching, root products, and volatiles. There are four ways in which these chemicals are released into the environment.
1) Leaching: - Leaching is the removal of substance from plants by aqueous solutions such as rain, dew, and fog. Radioisotope labeling of plant tissue before leaching has shown that large quantities of both inorganic elements and may classes of organic natural products are leached from plant tissue.

2) Root Exudation: - Root exudation is the release of substances into the surrounding medium by healthy, intact plant roots. A variety of natural with leaves, the amounts of organic materials are much smaller (Rovira, 1969). Boulterand colleagues (1986) found that greater amounts of amounts of amino acids were exuded into sand by pea roots than into solution culture. Similarly, exudation in soils can be expected to bary with soil physical and chemical properties. Root exudation usually is increased greatly by wilting conditions and root damage.

3. Volatization: - Volatization is the release of natural products into the atmosphere. A variety of plants either secretes or excretes metabolic products into special structures such as incomes and glands into intercellular spaces and canals or onto leaf surfaces. In hot dry weather, natural products with high vapor pressure released into the atmosphere where they may be associated directly by plants or adsorbed onto soil surface.

Allelopathic interference can result from natural products released from mulches of plant residue. To improve nitrogen of crop plants plant residue mulches particularly of nitrogen-fixing species are commonly used in agroforestry systems. These plant residues may in fact result in allelopathic interference and decreases crop production.
Examples of Allelochemicals: Phenols, Benzoic acid, Aldehydes, Acetophenon, Cinnamic acid, Quinonenes, Flavonoids, tannins, Gentistic acid, etc.