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What is the use of several trees grown in the forest? A single tree by the way side is far better where under people can rest (Vrikshayurveda).

The towns and villages are made more healthy, more beautiful and more efficient by well planned streets with avenue planting. There is no doubt that, an impressive avenue gives any property the special distinction that it desires. In the past, Emperor Ashoka and Mughal rulers paid attention to roadside planting. Avenue trees primarily provide shade and act as windbreaks, weather conditioners-by purifying and humidifying the air, check radiation of heat and obstruct glare, limit noise and enhance the beauty of the street where they are planted. Roadside planting has two aspects; beauty and utility and both these aspects should be combined. This requires a thorough knowledge of plant material and imagination in planning.

Urbanization coupled with industrial development creates problems of environment like air pollution, water pollution, noise pollution and civic ugliness. Of late, there is a greater awareness of environmental issues and increasing understanding of several attributes of vegetation. This led to an intensification of activity in the field of landscape horticulture to combat pollution and create an aesthetic environment for recreation and healthy living. Planning and execution of landscape projects for different urban locations should be done to meet both aesthetic and ecological requirements.

Trees are perennial in nature and are valuable elements in a landscape. Trees provide shade, privacy and beauty but a tree in a wrong place may create problem in years to come. To avoid this, care should be taken while selecting trees for landscape. The ultimate size and shape of the mature tree, how much annual maintenance it requires, location and purpose are major considerations while selecting a tree. Matured tree height and tree diameter should fit it into the space when tree is planted. Shape of canopy, colour of flower and season of flowering are also to be considered.

Moniliformis means ‘like a string of beads’ signifying the arrangement of flowers on the spike. The tree is native of tropical Australia. It is valued for afforestation and beautification. It grows in warm climate under both dry and moist atmosphere. It is a medium-sized, thornless, quick growing, evergreen tree of 13 to 15 m height.

Leaves: True leaves are present only in early seedling stage. The apparent leaves are the modified midrib called phyllode, leathery, sickle-shaped, tapered at each end, up to 18 cm long and 6 cm broad; nerves parallel.

Bonsai first appeared in China over a thousand years ago on a very basic scale, known as pun-sai, where it was the practice of growing single specimen trees in pots. These early specimens displayed sparse foliage and rugged gnarled trunks which often looked like animals, dragons and birds.

With Japan’s adoption of many cultural trademarks of China, bonsai was also introduced to Japan during the Kamakura period (1185-1333) by means of Zen Buddhism, which at that time was rapidly spreading around Asia. Once bonsai was introduced into Japan, the art was refined to an extent not yet approached in China. Over time, bonsai, which was confined to the Buddhist monks and their monasteries, were introduced as a symbol of prestige and honour. It became an important part of Japanese life by being displayed on specially designed shelves.  For the Japanese, bonsai represents a fusion of strong ancient beliefs with the Eastern philosophies of the harmony between man, the soul and nature.

Renewable energy sources, are the sources of power that quickly replenish and which can be used again and again. For this reason they are sometimes called energy resources. The main sources of renewable energy and their advantages and disadvantages are :

Solar
Energy from sunlight is captured in solar panels and converted into electricity.  Potentially it is an infinite source of energy supply, but the solar panels are costly.

Vedic literature indicates that the energy in the universe gets transmitted into the human body through star galaxies. The star galaxies of our universe are divided into 27 parts each signified by a particular star (nakshatra). Every human being is assigned to a particular star by virtue of his birth time.  Each star is associated with a tree. The tree associated with his star gives the universal energy to the human being. Thus astrology gives us a chance to identify the tree which is auspicious to us, being associated with our birth star. When one plants the tree associated with his birth star near his dwelling, it will bring good luck and prosperity to him.

Fruits and vegetables are very important food commodities. India has become the largest producer of fruits and the second largest producer of vegetables. Though the production of fruits and vegetables make us believe in our strength for self-sufficiency, a significant qualitative and quantitative loss occur in the produce from harvest till consumption.  For self sufficiency and also for processing, export and to meet additional requirements, a lot of emphasis need to be given to reduce post harvest losses, besides increasing  production and productivity of horticultural produces. Processing plays an important role in  conservation and effective utilisation of these perishable commodities. However, only less than 2% of total production of fruits and vegetables are processed.  The new industrial policy has placed this sector in the list of high priority areas.

Structure

Structural unit of edible portion of most fruits and vegetables is the parenchyma cells and makes up bulk of cells. A parenchyma cell is rather thin walled and may be polygonal or cubical in shape.  Walls of parenchyma cells in young plants are composed almost entirely of fibrils of cellulose. In parenchyma cells of plants actively metabolising protoplast portion represents only about 5% of total cell volume. This protoplast is rather film like and is pressed against cell wall by large water filled central vacuole.  Protoplast has inner and outer semi permeable membrane layers between which is confined cytoplasm and its nucleus. 

Maturity at harvest is the most important determinant of storage life and final fruit quality. Maturity indices are determined for many fruit, vegetable and floral crops. Harvesting crops at proper maturity allows handlers to begin their work with the best possible quality produce. Produce harvested too early may lack flavour and may not ripen properly, while produce harvested too late may be fibrous or over ripe.
 
Almost all fruits reach their best eating quality when allowed to ripen on the tree or plant (exceptions e.g., pears, avocados, and bananas). In general, fruits become sweeter, more colourful, and softer as they mature. A few fruits are usually picked mature but unripe so that they withstand post harvest handling system when shipped long distances.  Most of  currently used maturity evaluation is based on a compromise between those methods that would ensure the best eating quality and those that provide flexibility in transportation and marketing.  Optimum harvesting stage depends upon type of fruit or vegetable and their final use. Such a division is usually based on their classification according to physiology, environmental conditions, ripening bahaviour and nutritional aspect. Some of the classifications, important for their harvesting and storage are briefed in  subsequent sections.

Various systems were developed to take produce from field to consumer and selection of a particular method depends on several factors, including perishability and value of crop.  One method is to harvest the crop and take it to a packhouse for various unit operations like grading, cleaning, post-grading treatments, and packing for transport and marketing. When deciding upon where to locate a packinghouse, access to field and market point, adequate space for vehicles to enter and leave  packinghouse and ease of access to labor will all be considered.
 
In the simplest packinghouse, produce is delivered in picking containers, immediately after harvest, directly to packers.  Packers then sort, grade, size and pack the produce directly into appropriate transport containers. In this case, each worker must be aware of produce defects, grade and size requirements, and packing methods.

Storage is one of the most important aspects of post harvest handling of fruits and vegetables. The main objective of storage is to extend storage life of fruits and vegetables and increase their period of availability. A substantial quantity of fruits go waste in India due to lack of proper storage facilities. Storage of fruits and vegetables is practiced for various reasons. Proper post harvest handling of fresh horticultural produce helps in extending  period of availability of wide range of  fruits and vegetables round the year, prevention of seasonal market gluts, orderly marketing, better return to grower and  preservation of  nutritional qualities. The   long  term  storage  of  fruits  and  vegetables is expensive and requires high level of technical  knowledge of crop. However, an orderly marketing reduces storage period considerably. 

Processing of fruits  and vegetables into various products which could be preserved for a long time and add to value of product using proper cost effective technology will be a viable tool for improving   economic status of  farmer as well as our country. Modern methods of fruit and vegetable preservation are  broadly classified as follows:

Drying is the oldest known method of preserving food. Drying or dehydration is removal of majority of water contained in fruit or vegetable. Although preservation is the primary reason for dehydration, it also lowers cost of packaging, storing and transportation by reducing both weight and volume of final product. In addition, dried foods add variety to our diets. In the process of drying, sufficient moisture is removed to protect product  from  spoilage. Processing should be done in such a way that food value, natural flavour and characteristic cooking quality of fresh material are retained after drying.

The technique of preserving fruit by the use of low temperature dates from the eighteenth century with the first use of ice house in 1750. Later in 1800s commercial cold storages have come up for the storage of perishables like meat, dairy and fruit products.  The main change took place in the cold store in recent years has been the introduction of multi temperature chambers with humidity control for specific products. Cold preservation can be either refrigeration or freezing. Household refrigerators usually run at 4-7^ºC. Chilling or cold storage usually uses a slightly lower temperature on the basis of the food to be refrigerated. In freezing a temperature of -18^ºC or below is used. Chilling will preserve perishable foods for days or weeks and freezing will preserve foods for months or even years.

Fruit beverages

Fruit beverages are highly refreshing, thirst quenching, appetizing, easily digestible and nutritionally far superior to synthetic drinks. The fruit beverages includes natural and sweetened juices, squash, syrup, cordial, nectar, Ready to Serve (RTS) beverages, fruit juice concentrate and citrus juice barley water. Fruits most commonly used to prepare beverages are oranges, lime, pineapple, grapes, cashew fruit, mango, jack fruit, apple etc. To prepare fruit beverages select fresh, fully ripe, tart, juicy and seasonal fruits.

Fermentation was known to mankind from time immemorial. But, development of biochemical principles of  fermentation  was originated by  Lavoisier in France in 1789 by way of analysing chemical composition of  sugar and its fermentation products such as ethanol, carbon dioxide and  acetic acid. Many products are produced by fermentation process. In fermentation, controlled action of selected microorganisms is used to alter  texture of foods, preserve foods by production of acids or alcohol or to produce fine flavours and aromas which increase value of raw materials.

Minimally processed fruits and vegetables are products that have the attributes of convenience and fresh like quality. Their forms vary widely, depending on nature of unprocessed commodity and how it is normally consumed. In many cases, minimally processed product is raw and cells of tissues are alive, but these characteristics are  not regarded as essential. It is important that products have fresh-like character, regardless of   state of cells.  Minimally processed products for both retail and food service applications have increasingly appeared in market place recently.  In coming years, it is commonly perceived that this industry will have unprecedented growth.

Packaging is an integral part of fruit and vegetable processing. It has  major influence on storage life and on marketability of fresh as well as processed products. Packaging fulfills several functions including containment, facilitating transportation, protection of fruit/vegetable from further damage, protection of environment from contents of package (for example, if the contents are dirty), marketing, product advertising, and stock control.

Transportation and distribution of fruits and vegetables are important stages   of post-harvest loss. In India,  transportation of perishable commodities is in the most precarious stage. For local market, the produce is brought either by bullock cart or tractor trollies. The long distance transportation is mainly by rails and trucks, which is very costly. The basic reason for preference to road transportation is that it takes short transit period.

Quality is an increasingly important factor in the production and marketing of biological products. Consumers are becoming more discerning as their affluence increases, and would be suppliers of products must meet these demands if they are to maintain or increase market share. The term ‘quality’ is one of the most defined terms in use in food industry today. Quality may be defined as ‘The totality of features and characteristics of a product that bear on its ability to satisfy a given need’. The first part, ‘The totality of features and characteristics of a product…..’ concerns objective factors related to the product.  The second part, ‘…… to satisfy a given need’, concerns subjective factors related to user or consumer of goods.

Mango occupies a pre-eminent place amongst the fruit crops grown in India and is acknowledged as the ‘King of Fruits’. Mango is the pride of the garden and one of the choicest fruits known to mankind. Mango possesses unique nutritional and medicinal qualities apart from being a rich source of vitamins A and C. Young and unripe fruits are utilized for culinary purposes, preparing pickles, chutneys because of their acidic taste as well as for extraction of tannin and other astringent products. Ripe fruits are a delicacy for the table and are utilized in preparing squash, juices, nectar, jam, cereal flakes, custard powder, baby food, mango leather and toffee. Canned mango slices and pulp are indeed very popular. The inside kernel of the stone being rich in carbohydrates, calcium and fat, can be used as a source of food for certain industrial purposes.

Citrus is the world’s leading tree fruit crop, comprising of mandarins, sweet oranges, grape fruit, limes and lemons. It is one of the most important fruit crops grown under varying agroclimatic conditions in India except in the high hilly temperature regions. Mandarin orange (Citrus reticulata) is the most important commercial citrus in India followed by sweet oranges and acid limes. Citrus fruits are not only delicious and refreshing but also they provide vitamins, minerals and other substances. Different species of Citrus have different chemical composition. In sweet oranges, principal constituents of edible portions are sugars (glucose + sucrose) and acids (primarily citric acid and a little of malic acid), the fruits of acid groups contain primarily acids in fruit juice. Rind of citrus fruits is rich in pectin and contains essential oils and glucosides (hesperidin, naringin). Citrus fruits are a good source of vitamin C (ascorbic acid - 25 to 85 mg/100 ml juice), besides vitamin B, A and D. World-wide demand of orange is as fresh fruits, but orange marmalade is an important product and is one of the main uses of sour and sweet oranges. Grape fruit and mandarins are also taken as fresh fruits. Citrus fruit can be utilized in a number of other ways as salads, juices, squashes, cocktails, syrup, concentrates and pickles. Peel of thick-rinded fruits can also be made to delicious candy. Lemon is used in many culinary purposes. Grape fruit is utilized in preparation of wines and brandies and also for flavour and in perfumery.

Tomato, the ‘Apple of Love’ or ‘Golden Apple’ is one of the most popular, widely grown and highly praised vegetable in the world. In India, commonly referred as ‘Poor Man’s Orange’, has wider coverage in comparison to other vegetables. Tomato universally treated as ‘Protective Food’, is eaten raw or cooked or in processed form (soup, juice, ketchup, puree, paste and powder, green tomato - pickles and preserves) or it supplies vitamin C and adds variety of colours and flavours to food. It is a very good source of income to small and marginal farmers and contributes to nutrition of consumers. Tomato seed contains 24% oil used as salad oil and in manufacture of margarine. By virtue of its many attributes, it is considered as a favourite crop for research in physiology and cytogenetics, all over the world. Tomato is also rich in medicinal value. The pulp and juice are digestible, mild aperient, a promoter of gastric secretion and blood purifier. Tomato, a primary source of lycopene, showed significant association with low prostate cancer risk and cancers of digestive tract. It is one of the best vegetables which keeps our stomach and intestine in good condition. Tomato juice has become an exceedingly popular appetizer and beverage.

Onion is one of the most important commercial vegetable crops grown all over India. Onions are bulbous, biennial, herbaceous and are typically grown as annual, which give off a distinctive and pungent odour when the tissues are crushed. Characteristic pungency and flavour in onion are due to a volatile sulphur compound, allyl-propyl disulphide. This organo sulphur compound has potential for anticarcinogenic secondary metabolites. Pungency varies with cultivar, growing condition, stage of maturity and storage conditions.

It is such a unique vegetable, used throughout the year in the form of salad or condiment or for cooking with other vegetables. Onions are used for flavour, aroma and taste in preparation of vegetables and used in the form of dehydrated, freezing, canning and pickling. Onion salt is prepared by dehydration and used in flavouring.

Coconut is the most important palm of the wet tropics as a  major source of vegetable oil which also provides food, health drink, medicine, shelter, fuel, timber and fibre.  Because of its versatile nature, it is eulogised as ’Tree of Heaven’, ‘Tree of Life’, Kalpavriksha, Tree of Abundance, Natures supermarket and king of palms.  Oil is extracted from dried endosperm  which is called ‘copra’. The unopened flowering spathe, on tapping process, yields a delicious sap (Coconut Inflorescence Sap) rich in sugars and vitamin B, is called toddy from which sugar syrup and many products can be extracted.  An intoxicating drink called arrack can be prepared by distilling fermented toddy. The hard endocarps of fruits supply fuel and charcoal and mesocarp yields fibre for coir industry. Leaves are used for thatching houses.  Young unopened leaves are used to decorate festive places.

Black pepper is popularly known as ‘King of spices’, it is the dried berry of the pepper vine, so important in the International Trade Market. The crop has its origin in the tropical rain forests of Western Ghats of India.  Kerala is the leading State in the production of black pepper with the ubiquitous presence of this crop in coconut, arecanut and coffee plantations. Chief pepper producing countries are Indonesia, Brazil, Malaysia, Cambodia, Sri Lanka and Vietnam.

The term ‘Spices and Condiments’ applies to such natural plant or vegetable products or mixtures thereof, used in whole or ground form, mainly for imparting flavour, aroma and pungency to food. They are low volume high value crops which play a pivotal role in our national economy and provide a strong footing in the international market. According to the latest report of the International Organization for Standardization (ISO), there are about 109 spices grown in different parts of the world. India grows about 52 of them. The list of those spices with botanical and English names which come under the purview of Spices, Board, Ministry of Commerce, Government of India is given in Table 1.

Black pepper, christened as “King of spices” and “Black gold” is the most important and the most widely used spice in the world occupying a position that is supreme and unique. It was the lure of Indian pepper that brought the western world to Indian sub continent.

The black pepper of commerce is the dried mature fruit of the tropical perennial climbing plant Piper nigrum. Pepper originated in the tropical evergreen forests of the Western Ghats. The Malabar coast of India was the centre of pepper trade from time immemorial. Black pepper was essentially a minor forest produce in the past, people collected it from forests where it abundened. Domestication of pepper appears to be a much later event. The recorded history shows that Arabs, followed by the Portuguese, the Dutch and the British traders took pepper to all over the world and spread its use and cultivation. Initially black pepper was taken from the Malabar coast to the Indonesian islands, then it spread to various Pacific islands, South East Asian countries and later to tropical Africa and America. Currently, pepper is grown in about 26 countries, major being India, Indonesia, Brazil, Malaysia and Srilanka. In India, pepper is grown mainly in Kerala, Karnataka, Tamil Nadu and to a little extent in Goa, Orissa, Assam and Andaman group of islands. Kerala is the original home of pepper. This state accounts for 95% of country’s area and production.

Myristica fragrans Houtt., the nutmeg tree is unique among spice plants as the donor of two distinct spices; nutmeg and mace.Native of Indonesia (Moluccas islands), nutmeg tree grows there abundantly and is now naturalised in West Indies, Srilanka, India, Phillipines, Tropical America and Pacific Islands. (Varghese, 2000). In India, the plant is grown in certain pockets of Kerala, Tamil Nadu, Karnataka, Goa, Maharashtra, North East India and Andamans (Krishnamoorthy, 2000). It is also grown in a small scale in Srilanka, Trinidad, China, Tobago, Zanzibar and Mauritius.

Coriander is used by man since long as coriander seeds were found in Egyptian tombs of 960 BC. Further, Persia grew coriander 3,000 years ago and it added fragrance to Hanging Gardens of Babylon. In 3^rd century BC, Romans also found coriander seed as an excellent seasoning for popular foods.

Besides seeds, fresh leaves are being used for garnishing of food in many Latin American, Spanish, Russian, Chinese and Indian dishes since long. Fresh coriander leaves are also known as Chinese Parsely. In India, coriander seed is used almost daily in scores of curries for flavouring and leaves for garnishing. Coriander is a native of Mediterranean region.

Marjoram is native of Southern Europe. It grows in West Asia, South and North America, France, Germany, Hungary, Spain, Portugal, England and North Africa. It is an important kitchen herb and its dried leaves with or without flowering tops in small proportion constitute the spice of commerce. In India, the crop finds its cultivation only in small scale in Karnataka, Andhra Pradesh and Tamil Nadu. France and Egypt are the main exporters of marjoram oil in the world.

Botany and chemical composition

Marjorana hortensis is an aromatic branched perennial, 30-60 cm high. Though a perennial, it is treated as an annual under cultivation. Main stem is woody with many softer branches. Leaves are round, soft, mat green and with a marvelous sweet smell. Flowers small, whitish or purplish in terminal clusters, seeds minute, oval and dark brown in colour.

Garlic is recognized all over the world as a valuable condiment for food and medicine. It is a native of Western Asia and Mediterranean area where it is in cultivation for centuries. China, Korea, India, USA, Spain, Argentina and Egypt are the major garlic growing countries, China being the largest producer (Pandey, 2004). Europeans, especially the Italians and the Spanish used this spice regularly for 2000 years and more. Garlic is one of the important spicy bulbs grown commercially in India. The plant is cultivated in temperate and tropical climates. Important states producing garlic on commercial scale are Gujarat, Maharashtra, Tamil Nadu, Karnataka, Andhra Pradesh, Madhya Pradesh, Orissa, Uttar Pradesh and Haryana. Demand of garlic is increasing day by day in domestic and export markets. Garlic and garlic products, especially dehydrated garlic and garlic powder are gaining considerable economic importance recent times.

Weeds are the plants which grow where they are not wanted or simply plants growing out of place. A plant may be desirable in one situation and a weed in another situation. Those plants that interfere with human activity in crop and non-crop areas are considered weeds. Weeds compete with crops for soil nutrients, water and light; they host insects and plant pathogens harmful to crop plants, and their root exudates and/or leaf leachates may be toxic to crop plants. Weeds also hinder crop harvest and increase the costs of such operations. In addition, at harvest, weed seeds often contaminate the crop produce. Thus, the presence of weeds in crop areas reduces the efficiency of inputs such as fertilizer and irrigation water, enhances the density of other pest organisms, and finally severely reduces crop yield and quality (Labrada and Parker 1999).  Weeds are serious menace in crop production.

1. Classification of weeds based on life cycle (Ontogony)

(i) Annuals: Weeds that  complete their life cycle within a year or season are called annual weeds. They are associated with annual crops and propagated mainly through seeds. Annual weeds are subdivided according to the season of prevalence as follows:

(a) Kharif season  annuals or Kharif  weeds: The weeds that grow during the summer and complete their life cycle during Kharif or rainy season are called Kharif season annuals or Kharif weeds. The kharif weeds complete their life cycle during the warm wet season (i.e. from the month of June to September) and attain maturity with the onset of winter and finally dies.

Ecology is the inter-relationship between organisms and their environment. Weed ecology is study of inter-relationship between weed and their environmneent. Simply, Ecology of weeds means the inter-relationship of weeds with climate, soil, crops and other biotic factors. Terrestrial weed species persist in the soil by virtue of dormant structures, typically seeds or vegetative perennating organs such as rhizomes, tubers and taproots. In dense infestations, the banks of seeds or underground meristems from which new plants may be recruited into adult populations can be exceptionally large. Rao (1968) estimated that in Cyperus rotundus L. tuber populations of 10 million per hectare were possible, whilst Soerjani (1970) calculated that Imperata cylindrica may annually produce six tones of rhizomes per hectare. Typically seed banks of annual weeds in arable soils contain from 1000-10,000 seeds per m2whilst in grassland the upper limit to this range may extend at least to 1 million per m² (Mortimer 1994).

Weeds have some characteristics as follows:

    1.    Weeds are hardy and can grow under adverse edaphic (Soil), climatic and biotic condition.
      Example: Lantana camera,Eupatorium adoratum etc.

    2.    Weeds in general prolific with abundant seed production potentialities.
        Example: Amaranthus viridis, Chenopodium album etc.

No plant in the earth is completely worthless. Weeds have also some beneficial effects although they do much harm in different ways. The beneficial effects of weeds are as follows:

    1.    Most of weeds absorbs nutrient from the deeper layer of soil. When such weeds are incorporated in the soil, they add organic matter content of the soils. As a result, the fertility of plough layer from where plants absorb most of their nutrients is enriched.

    2.    Weeds help in controlling erosion by the soil binding effects of their roots and by forming a living mulch, which resist beating action of rain drops and sweeping action of wind.

Dispersal means the spreading of weeds from one place to another place. Dispersable of weeds generally takes place through seeds and vegetative parts. Dispersal of mature seeds and live vegetative parts of weeds is nature’s way of providing non-competitive sites to new individuals.

    (a)    Dispersal of weeds through seeds: Seeds are protector and propagator of their kind. Weeds seeds remain viable for a long time and their dispersable mainly takes place through natural and human agencies.

There are some weeds which prefer to in crop field and some of them have a distinct association with some crops.

Weeds interfere with man’s utilization of land and water in various ways. The extent of losses depends on the intensity, infestation, time of occurance and type of weeds.

1. Competition between weeds and crops for growth factors

Weeds compete with crop plants for  other factors in the micro-enviroment. Weeds are capable of absorbing more amount of nutrient from soil than the crop plants. In dry lands, weeds exhaust the soil moisture and put the crop under water stress earlier and thus reduce the yield. Weed can compete with productive crop or pasture land or convert the productive land into unusable scrub. Weeds are also often poisonous, produce burrs, thorn or other damaging body parts or otherwise interfere with the use and managemet of desirable plants by contaminating  harvest or excluding livestock.

Plant competition is a powerful force for the elimination of some weaker plants. Plants require nutrients, water, light, Oxygen and Carbondioxide for their growth and development. Weeds compete with the crop for growth factors and the competition is maximum at the early growth stages of the crop. But the competiting ability of the weeds becomes practically nil as soon as the crops attain maturity. Weeds need more nutrients and water than the crop plants. The weeds in general grow faster and shade the crop plant if they are not controlled at proper time and afterwards interfere in the growth of crop plant. Weeds have allelopathic effect on crop growth. Some weeds excrete certain chemicals into the soil which inhibit the germination and growth of other plants in their vicinity and the phenomenon is called allelopathy. Allelopathy is defined as the direct or indirect harmful effects of one plant on another plant through the production of chemical compounds that escape in the environment.

Weed control methods may be classified into two groups as follows:

    1.    Preventive method
    2.    Curative or remedial measures. Curative measures are sub-divided into eradication and control.

(a) Preventive Methods

Prevention is the measures taken to prevent the introduction or spread  of the weeds to new areas and to check the infestation prior to the germination of weed seeds. To achieve this, a wise step to be followed the principle- “Prevention is better than cure”.

A single active ingredient often is sold in several different kinds of formulations as follows:

(a) Dry Formulations
1. Granule (G)

Granule is, a ready-to-use, dry mixture of a small amount of active ingredient with inert carriers. Most are made by applying a liquid formulation of the active ingredient to coarse particles (granules) of some porous material such as clay, sand or plant material. Granule particles are much larger than dust particles; will pass through a 4-mesh sieve but not through an 80-mesh sieve (the number of wires per inch). The herbicide is absorbed into the granule, or coats the outside of it, or both. Inert ingredients may be added to make the formulation handle well. The amount of active ingredient usually ranges from 1 to 15 percent.

Aquatic weeds are those unwanted plants which grow in water and complete at least a part of their life cycle in water. Aquatic weed problem is a global one but more pronounced in tropical and subtropical zones, where warm weather supports profuse growth of aquatic plants. In India, large irrigation projects have been reported to be adversely affected by profusely growing aquatic weeds, that block canals reducing water flow by as much as four-fifths. (Dr. K.T. Chandy,  2010).

1. Harmful Effects of Aquatic Weeds

The important harmful effects of aquatic weeds are as follows
 
    (i)    The aquatic weeds mainly Submerged, immersed, and marginal weeds in and along irrigation canals, ditches, and drainage channels impede water flow, increase evaporation, cause damage to canals and structures and clogs gates, siphons, valves, sprinkler heads, bridge piers, pumps, etc.

(1) Striga
 
Striga, commonly known as witchweed or witches weed, is a genus of parasitic plants that occur naturally in parts of Africa, Asia, and Australia. Witchweeds are characterized by bright-green stems and leaves and small, brightly colored and attractive flowers, Striga has been given the common name of “witchweed” because of attaching to the roots and robbing the host of water and nutrients. The genus Striga (Family: Scrophulariaceae) includes a large number of tropical parasitic or hemi-parasitic plants as follows

The mode of action is the overall manner in which a herbicide affects a plant at the tissue or cellular level. The mode of action is the way in which the herbicide controls susceptible plants. It usually describes the biological process or enzyme in the plant that the herbicide interrupts, affecting normal plant growth and development. Herbicide may act on the life process of plant and dislodge the normal functioning of plants resulting in the death of plants. Herbicides  do damage of plants by inhibiting the biological process of plant as follows:

Herbicides degrade over time. The persists of herbicides is dependent on several factors including light, temperature, and substrate moisture. Several processes are responsible for herbicide degradation and include the following:

    1.    Photodegradation: The decomposition of herbicides caused by duration or intensity of sunlight is called as Photodegradation. Photodegradation occurs when ultraviolet (UV) light breaks chemical bonds of the herbicide active ingredient. Secondary molecules resulting from the cleavage of the parent molecule are generally less effective in providing weed control. Herbicides such as Monuron, Trifluralin, PCP, Chloramben etc are prone to photodegradation. So these herbicides should be applied in evening or incorporated in to the soil just after application in order of avoiding photodegradation.

Herbicides are applied to the soil in hopes of obtaining season-long weed control. It is desirable for the chemicals to control weeds during the season of application, but they should not remain long enough to affect subsequent crop growth.

    1.    Selection of herbicides: The type of herbicide and the method of application can influence the extent of the residual carryover. Herbicides differ in their physical and chemical properties such as water solubility, volatility and susceptibility to breakdown by sunlight, and microbes. When applied at recommended rates, most herbicides breakdown within a few days or weeks after application and impose no restrictions on cropping options in the next year. Some herbicides however do not degrade quickly and can persist in the soil for weeks, months or years following application. The use of residual herbicides can be beneficial as the residues prevent growth of sensitive weed species throughout the season. These residues however can restrict the crops that can be grown in rotation. Some herbicides can remain active in the soil for weeks, months or years. This can be an advantage as it ensures good long term weed control. However, if the herbicide stays in the soil longer than intended, it may damage sensitive crop or pasture species sown in subsequent years. For example, chlorsulfuron is used in wheat (Triticum sp) and barley (Hordeum vulgore), but can remain active in the soil for several years and damage legumes and oilseeds.

1. Herbicide –herbicide interaction

Herbicides cause some effects as follow:

(i) Synergistic: Synergism is defined as the cooperative action of different chemicals in a mixture such that the total effect is greater than the sum of the independent effects (Green et al., 1997). In relation to herbicides, the total  induced by an herbicide mixture is greater than the sum of the responses by each herbicide alone (Anderson, 1996). Synergistic  is often used to describe the effect of drugs working together — where one drug increases the other’s effectiveness. Synergy is the interaction of multiple elements in a system to produce an effect different from or greater than the sum of their individual effects.