eChapters

Abstract

Microbes are endowed with a high and exploitable potential of being useful associates with their surrounding environment and host in case of symbiotic relationship. Most of the microbes live alone but many of them require association of other living system for its survival. Hence, the beneficial role of microbes is always correlated with growth promotion and survivability of other associated living system. Now it is established that microbes play an important and key role in plant growth and development, even its survivability in case of symbiotic mycorrhizal association with orchids. They are nothing but potential tool protecting plants from different adverse conditions like diseased state, environmental stress like cold temperature, drought, metal contamination, salt and acidic or alkaline stress.

Introduction

Plant growth in agricultural soils is inf luenced by a myriad of abiotic and biotic factors. As a routine practice growers normally use physical and chemical approaches to manage the soil environment to improve crop yields while the application of microbial products for this purpose is less common. An exception to this is the use of rhizobial inoculants for legumes to ensure efficient nitrogen fixation; a practice that has been occurring in North America for over 100 years (Smith, 1997). The region around the root, the rhizosphere, is relatively rich in nutrients, in expense of as much as 40% of plant photosynthates from the roots (Lynch and Whipps, 1991). Consequently, the rhizosphere supports large and active microbial populations capable of exerting beneficial, neutral, or detrimental effects on plant growth. The importance of rhizosphere microbial populations for maintenance of root health, nutrient uptake, and tolerance of environmental stress is now recognized (Bowen and Rovira, 1999; Cook, 2002). These beneficial microorganisms can be a significant component of management practices to achieve the attainable yield for in other words a crop yield limited only by the natural physical environment of the crop and its innate genetic potential (Cook, 2002).

The prospect of manipulating crop rhizosphere microbial populations by inoculation of beneficial bacteria to increase plant growth has shown considerable promise in laboratory and greenhouse studies however, responses have been variable in the field (Bowen and Rovira, 1999). The potential environmental benefits of this approach, leading to a reduction in the use of agricultural chemicals are fitting the sustainable management practices. Realizing the biological interactions occurring in the rhizosphere, recent progress are in need to fulfill the practical requirements of inoculant formulation and timely delivery which would increase the technology’s reliability in the field and facilitate its commercial development.

Plant Growth Promoting Rhizobacteria (PGPR) are root-colonizing bacteria that form symbiotic relationships with many plants. The name comes from the Greek rhiza, meaning root. Though parasitic varieties of rhizobacteria exist, the term usually refers to bacteria that form a relationship beneficial for both parties (mutualism). They are an important group of microorganisms used in biofertilizer. Biofertilization accounts for about 65% of the nitrogen supply to crops worldwide. The term PGPRs was first used by W. Kloepper in the late 1970s and has become commonly used in scientific literatures. PGPRs have different relationships with different species of host plants. The two major classes of relationships are rhizospheric and endophytic. Rhizospheric relationships consist of the PGPRs that colonize the surface of the root, or superficial intercellular spaces of the host plant, often forming root nodules. Endophytic relationships involve the PGPRs residing and growing within the host plant in the apoplastic space.Some common examples of PGPR genera exhibiting plant growth promoting activity are: Pseudomonas, Azospirillum, Azotobacter, Bacillus, Burkholdaria, Enterobacter, Rhizobium, Erwinia, Mycobacterium, Mesorhizobium, Flavobacterium, etc.An ideal PGPR should possess high rhizosphere competence, enhance plant growth capabilities, have a broad spectrum of action, be safe for the environment, be compatible with other rhizobacteria, and be tolerant to heat, UV radiation, and oxidizing agent.

Abstract

The greatest challenge facing us in the coming years is to produce adequate amounts of basic necessities of food, feed, fibre, fuel and raw materials for the rapidly shrinking per capita agricultural land. One alternative is to make a sustainable use of existing marginal lands. In many regions of the tropics and sub-tropics, the need to use marginal low-input soils with usually inadequate nutrient(s) supply permits only very low or unreliable yields to be achieved. It is frequently not possible  to supply required amounts of mineral fertilizers or chemical pesticides because of the low purchasing power of the marginal farmers. In particular, it is not possible to compensate for the deficient nutrient supply and this constitutes a yield limiting factor. The role of soil microorganisms in sustainable productivity has been well construed. In recent years, root inhabiting (rhizozosphere) bacteria (rhizobacteria) is gaining importance.

Introduction

Lithosphere, including the earth’s crust, is helpful in providing the base or medium for sustenance of most life processes and forms, interwoven in fine network interdependencies within themselves and the environment surrounding them i.e. ecosystems. Terrestrial ecosystem contains an important dynamic living matrix termed soil, which is critical for agricultural production and food security. Soil forms the medium for growth and maintenance of a variety of plant species while the plant roots in the soil represent a four dimensional region in space and time of profuse activity relative to the bulk soil, revolving around pH, nutrient, redox potential, and exudate gradients changing as distance from the root increases (Marschner, 1995). The soil region adhering the plant roots is being strongly influenced by the presence of these roots and is characterized by high rates of microbial population and activity and referred as rhizosphere (the soil adhering to plant roots).

Unlike root-free soil, the rhizosphere around growing plants is a very dynamic environment that hosts a much higher number of microbes, in particular bacteria. Microbes interact with each other and with plants in symbiotic, associative, neutral or antagonistic ways. A plant colonized or penetrated by a microorganism can either result in asymptomatic or disease or form an association or symbiosis, depending on the cells’ perception of each other and how the environment affects that interaction. Microbes that penetrate and colonize plants have developed elaborate strategies to subvert plant defenses.

INTRODUCTION

Biological control involves the suppression of pests/organisms with other organisms.  It is an eco-friendly method for management of plant pathogens which can be done exploiting the fungi (Ampelomyces, Trichoderma, Coniothyrium, Candida etc.) or bacteria (Agrobacterium, Bacillus, Pseudomonas and Streptomyces) from rhizosphere,  as well as phylloplane and endophytes (Jayaraman and Verma, 2002). The rhizosphere microfloras are predominantly composed of gram negative pseudomonads.  The fluorescent pseudomonads constitute a large proportion of the rhizosphere microflora presumably due to their nutritional variability being able to use a large number of organic substances (Stolp and Gadkari, 1981). However earlier workers used fluorescent pseudomonads extensively they now prefer Bacillus sp. due to their spore forming ability and heat tolerance which make its formulation easier.

The rhizosphere is the narrow zone of soil surrounding the root that is under the immediate influence of the root system. This zone is rich in nutrients when compared with the bulk soil, due to the accumulation of a variety of organic compounds released from roots by exudation, secretion and deposition. Organic compounds released by plant roots include amino acids, fatty acids, nucleotides, organic acids, phenolics, plant growth regulators, putrescine, sterols, sugars and vitamins. Because these organic compounds can be used as carbon and energy source by microorganisms, microbial growth and activity is particularly intense in the rhizosphere.

Abstract

Rhizobia are soil bacteria that fix atmospheric dinitrogen after establishing symbiotic relationship inside the root nodules of legumes (Fabaceae). Plant Growth Promoting Rhizobacteria are the group of bacteria which thrive in the rhizosphere region and enhance the plant growth through direct & indirect mechanisms. Pulses are being inoculated with specific rhizobial strains to have better nodulation in India. The native rhizobial strains from the root nodules of legumes from different agroclimatic zones of Odisha was isolated, characterised and screened for PGP activities. Out of all eleven isolates showed similar characteristics with the family Rhizobiacae. Among them six isolates contained nif-gene and PGPR activity (B2, B4, B5, B6, B7 and B8) were applied to seeds, that helped in faster germination and better elongation of root and shoot. The observations were statistically significant.

Any substance or mixture of substances intended through physiological action, for accelerating or retarding the rate of growth or maturation, or otherwise altering the behaviour of plants or the produce thereof are known as plant growth regulators. It does not include plant nutrients, trace elements, nutritional chemicals, plant inoculants and soil amendments.

Plant hormones are organic compounds other than nutrients, synthesize by plants and regulate plant growth and development. They are active in very low concentrations, produced in certain parts of the plants and are usually transported to other parts where they elicit specific biochemical, physiological and morphological responses. They are also active in tissues where they are produced. The term plant growth regulator is usually used to denote a synthetic plant hormone, but most of the synthetic compounds with structures similar to those of the natural hormones have also been called hormones. For instance, the synthetic cytokinin, kinetin is considered a hormone.

Introduction

Plant growth regulators are defined as small, simple chemicals produced naturally by plants to regulate their growth and development and are essential for regulating their own growth. They act by controlling or modifying plant growth processes, such as formation of leaves and flowers and elongation of stems.

In modern forest nursery raising, the benefits of extending the use of plant hormones to regulate the growth of economically important plants have been established. There are two possible goals in inducing trees to form floral buds, in particular, female buds. The first is to achieve precocity. Precocious flowering in normally immature trees can provide advantages in breeding and seed production because it overcomes a substantial seed production delay intrinsic to late-maturing conifers. The second reason is the phase change from juvenile growth to mature growth. The induction of either precocity or increased male flower and female cone formation are generally the result of applied treatments. A few studies have focused on the intrinsic physiological differences between high and low producing clones.

Plant growth regulators are organic substances other than nutrient, which is required in minute quantity to modify the plant growth processes. PGR has very important role in fruit breeding which is used in every facet of crop improvement e.g. multiplication of hybrids/selection through asexual means or by micro propagation, breaking of seed dormancy, growth control, regulation of flowering, fruiting and induction of sterility in reproductive organs etc (Dhatt, 1985). Details of the various uses of plant growth regulators are given as under.

Growth

Growth means quantitative increase in the planwt body e.g., increases in the length of stem, roots, number of leaves, fresh and dry weight of fruits etc.

Development

It refers to the qualitative changes from the initiation of growth to the death of a plant or plant parts like seed germination, formation of flowers, fruits and seeds, emergence of buds, leaves and falling of leaves and fruits.

Plant hormone is an organic compound synthesized in one part of the plant body and translocated to another part where in very low concentration it causes a physiological response. However, this definition could not demarcate the vitamins and other growth influencing substances from hormones. For this reason, the term Phytohormone was introduced with included vitamins, growth hormones and flowering hormones. Phytohormone can be defined as an organic substance produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of synthesis and active in minute amounts, whereas, an organic substance, which promotes growth, i.e., irreversible increase in height or volume, when applied in low concentration to the shoots of plant is known as growth hormone.

The plant hormones are naturally occurring as they are synthesized by the plant. On the other hand, there are several manufactured chemicals, which often resemble the hormones in physiological action and molecular structures, are called growth regulators. Generally, there are five classes of phytohormones.

The phytohormones universally present in all plants are classified as under:

The quantitative increase in plant body such as increase in the length of stem and root, the number of leaves etc., is referred to as plant growth whereas, the qualitative changes such as germination of seed, formation of leaves, flowers and fruits, falling of leaves and fruits is referred as development. Utilization of these substances for proper development of the plant is regulated by certain chemical messengers called plant growth substances or plant growth regulators, which in minute amounts increase or decrease or modifies the physiological process in plants. Plant growth substances are the chemical other than nutrient which required in very small amount but without which plant can not complete their physiology.

The two sets of internal factors, viz., nutrition and hormone control the growth and development of the plant. The raw material required for growth is supplied by nutritional factors which include the minerals, organic substances the protein, carbohydrates, etc.

Phytohormones: These are the hormones produced by plants which in low concentrations regulate plant physiological process. These usually move within the plants from a site of production to a site of action.

Plant growth regulators: These are organic compounds other than nutrients, which in small amounts promote, inhibit or otherwise modify any physiological process in plant.

It may be defined as any organic compounds which are active at low concentrations (1-10 ml) in promoting, inhibiting or modifying growth and development in plants. The naturally occurring (endogenous) growth substances are commonly known as plant hormones, while the synthetic ones are called growth regulators.

Plant hormones: It is an organic compound synthesized in one part of plant and translocated to other parts, wherein very low concentration causes a physiological response.

The plant hormones are identified as promoters (auxins, gibberellin, and cytokinins), inhibitors (abscisic acid and ethylene) and other hypothetical growth substances (Florigen, death hormone, etc.).

Plant hormones (phytohormones) are chemicals that regulate plant growth and are termed as ‘plant growth substances’. Plant hormones are signal molecules produced within the plant, and are present in extremely low concentrations. Hormones regulate cellular processes in targeted cells. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. Every cell is capable of producing hormones in plants. They shape the plant, affecting seed growth, time of flowering, the sex of flowers, senescence of leaves, and fruits. Plant hormones affect gene expression and transcription levels, cellular division, and growth.They direct which tissues grow upward and which grow downwards. Hormones are vital to plant growth. Hence, they are also known as growth factors or growth regulators.

Plant growth

Definition: The quantitative increase in plant body such as increase in the length of stem and root, the number of leaves etc., is referred to as plant growth.

Plant development

Definition: The qualitative changes such as germination of seed, formation of leaves, flowers and fruits, falling of leaves and fruits is referred as developement.

Phytohormones

Definition: These are the organic substance or hormones produced by plants which in low concentrations regulate plant physiological process. It is synthesized in one part of plant and translocated to other parts. These are endogenous substance means naturally occurring in the plant.

Plant growth regulators are defined as chemicals other than nutrients and vitamins which regulate (or) modify the physiological process ofthe plant. A plant hormones or phytohormones are chemical substances which are naturally produced by plant in specific organs and are translocated from site of synthesis to the site of action. They bring about specific physiological response, even in very low concentration. The quantity of synthesis of these hormones differ from plant to plant, plant internal and external factors. They are readily absorbed and these chemicals move rapidly through the tissues when applied to different parts of the plant. A plant growth hormone may be termed as a plant growth regulator, but not vice - versa, since other than natural plant hormones, plant growth regulators are synthetically manufactured chemicals.

Plant growth regulators have a diverse chemical composition. Based on the general response the chemical mediates in the plant system, they are classified as growthpromoters and growth inhibitors/retardants. However, a particular growth regulator may be growth promoter at a lower concentration and growth inhibitor at higher concentration.

The quantitative increase in plant body such as increase in the length of stem and root, the number of leaves etc., is referred to as plant growth whereas, the qualitative changes such as germination of seed, formation of leaves, flowers and fruits, falling of leaves and fruits is referred as development. Utilization of these substances for proper development of the plant is regulated by certain chemical messengers called plant growth substances or plant growth regulators, which in minute amounts increase or decrease or modifies the physiological process in plants. Plant growth substances are the chemical other than nutrient which required in very small amount but without which plant can not complete their physiology.

The two sets of internal factors, viz., nutrition and hormone control the growth and development of the plant. The raw material required for growth is supplied by nutritional factors which include the minerals, organic substances the protein, carbohydrates, etc.

Plant growth regulators or phytohormones are organic substances produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts. Thimmann in1948 proposed the term Phytohormone as these hormones are synthesized in plants (Thimmann, 1963).