eChapters

Introduction

After water, nitrogen is the most critical nutritive element for crop productivity and human efforts to produce food and energy have greatly changed the nitrogen cycle of the earth. Sustainable cropping systems throughout history and across the world have relied on the combination of a cereal with an N-fixing legume. In recent history, cereals have dominated global agriculture, while legume areas and productivity have stagnated or even declined. An atmosphere around us contains nearly 78% nitrogen, which is in free form. Hence it is not available to plants. At present, India produces around 206 million metric tones of foodgrains for its growing population, increased foodgrain cannot be produced unless we carefully make use of biological nitrogen. Biological nitrogen fixation is the key to sustain agricultural productivity through the application of biofertilizers in the field. However, there is an urgent need to transfer this technology on the field to farmers and industry by producing these fertilizers on large scale.

INTRODUCTION

One of the most dangerous ecological crises being faced by all living organism now-a-days is the pollution of the environment. It is easier to describe pollution rather than to define it. It is presumed that the environment in past was more or less pure. But due to various activities of man such as industrialization, urbanization, agricultural practices etc, composition of the environment gets changed which is called environmental pollution. Any substance that creates pollution is called ‘pollutant’. There are different types of pollution among which pollution caused by toxic level of heavy metal pollutants is called heavy metal pollution.

Heavy metal pollution is a problematic field of research and is gaining immense importance now-a-days. We have 104 elements in the periodic classification of which 80 are metals. According to Passow et al. (1961), heavy metals are those having densities more than five (5 g cc^-1) and atomic weight twenty three. The term ‘heavy metal’ is imprecise but is widely used although others such as ‘toxic metal’, ‘trace element’ are possible alternatives. Sources of these heavy metals are varying; some of these are encountered in water, air effluent, solid waste and sewage. They are emitted out from various sources e.g. waste water from electroplating industry (Cr, Ni), air emission from fluorescent lamps (Cd, Be), paint pigment waste (Pb, Cr), auto-exhaust emissions (Pb, Te) etc.

INTRODUCTION

The purpose of writing this chapter is to develop a comprehensive understanding of the role of functions of water in plant growth and development. Water is essential to life on earth as the living organisms contain 80-90 per cent water. A typical crop or grassland will transpire approximately 500 ml of water per 10 g of dry matter produced. Water has a tremendous effect on the environment near the earth, which on regional basis we call ‘climate’. Ecological importance of water is due to its physiological importance because water affects the plant growth through physiological processes. All metabolic activities in the plant cell or tissue depend on water status of the plant. For example, in a maturing seed, respiration rate is highest as it moves towards maturation stage, water content decreases and hence rate of respiration slow down. At the harvesting stage, when the water content is 10-20%, the respiration rates become negligible. Now in a stored seed if the water content will be high it will result into higher respiration, thus release energy, which will increase the temperature and will finally destroy the seed.

INTRODUCTION

Among plant nutrients, our soils are particularly low in nitrogen and therefore, require is high doses of nitrogenous fertilizers for getting optimum productivity. Added to this is the problem of poor efficiency of nitrogeneous fertilizers which under normal conditions is only 40 – 50 %, still less for rice ranging from 25 – 35 %, and the residual effect for the succeeding crops is never more than 10 %. The rest of the applied nitrogen is lost either through volatilization, leaching or denitrification (Kashyap, 1978). The high cost of nitrogenous fertilizers and their poor utilization efficiency very much add to the high cost of cultivation. Hence, the need of the hour is to decrease our dependence on chemical fertilizers and increase the efficiency of the fertilizer thus used. The increase in nitrogen efficiency in crop plants can be achieved by reducing denitrification with the use of nitrification inhibitors, reducing the soil compaction, improving drainage conditions, use of soil improving crops, and adopting carefully the irrigation procedures (Carlson, 1980). No doubt exploration of this possibility is a big task, nevertheless, the understanding of physiology and biochemistry of nitrogen nutrition at molecular level is obligatory and is likely to add insight to achieve the hard task goal.

INTRODUCTION

The metabolism of carbohydrates is vital for plant growth and development, and to the yield of major plant products such as starch and sucrose. Starch, sucrose and fructose are the three important carbohydrates which are utilized in most of the higher plants. The majority of sucrose that is synthesized in higher plants during photosynthesis is exported to non-photosynthetic cells (ap Rees, 1992). Starch serves as an important storage for carbohydrate residues. The degradation (breakdown) of starch ensures retaining of important life processes in plants when there is carbon shortage, for instance, during night period in leaves or in resting organs such as potato tubers.

Phospholipids are the most abundant lipids of biological membranes. They serve primarily as structural components of membranes and are never stored in large amounts. As the name of phospholipid implies, this group of lipids contains phosphorus in the form of phosphoric acid groups. The major phospholipids found in membranes are the phosphoglycerides (phosphatides) (Fig.1). Which have two fatty acid molecules esterified

INTRODUCTION

Iron in the terrain’s crust is more oxidized now than it was when life evolved. It has been hypothesized that dissolved ferrous iron could have been a convenient oxygen acceptor and that deposition of oxidized iron in sediments must have taken place before oxygen could have entered the atmosphere in significant quantities. Obviously, during early evolution on this planet, the essential elements are present only in reduced form. Today a number of elements are present only in a highly oxidized form and considerable energy must be expanded to reduce them to a form similar to that on early earth which plants can utilize.

Iron represents essential elements which are usually available in oxidized form but must be reduced, often using metabolic energy, before they can be utilized. For the first time, Gris (1844) observed yellowing of grapevine leaves in absence of iron in the nutrient solution in which these plants were growing. Later, iron was reported to be essential for diverse group of plants. Due to a great variation in iron requirement by different plant species, it is difficult to decide whether iron is a macro or micronutrient.

Introduction

Salinity is a significant environmental stress for crop plants it account for about 70% of the losses in the crop yield. It adversely affects seed germination, seedling growth and different metabolic activities in plants (Begum et al. 1997). It reduces DNA, RNA and protein synthesis and differentially influences the activities of the hydrolytic enzymes (Kumar et al. 1996).

Plants are classified as glycophytes or halophytes according to their capacity to grow on high salt medium. Most crop plants are glycophytes and cannot tolerate salt stress. High salt concentration decreases the osmotic potential of soil solution creating a water stress in plants. Secondly they cause severe ion toxicity since Na+ is not readily sequestered into vacuoles as in halophytes.

The widespread nature of saline soils compounded by the geographical distribution of man’s population and agricultural practices are largely succeeded in increasing salinization in arid and semi-arid lands. Most of the crops are sensitive to even low concentration of NaCl.

Introduction

Chickpea, the world’s third most important food legume, is currently grown on about 10 m ha worldwide, with 95% cultivation in the developing countries. India is the largest producer of chickpea in the world. Chickpea is grown under rainfed conditions in winter in India subjecting it to abiotic stresses. Consequently it experiences water deficit stress at one or the other growth stage(s) which affects the crop productivity.  Among the abiotic stresses to which chickpea is subjected to drought stands to be the number one problem in major chickpea growing regions because the crop is grown on residual moisture and the crop is eventually exposed to terminal drought (Johansen et al., 1994). A distinctive variation exists between the plant in sensitivity to water stress. Chickpea is considered relatively more tolerant, possibly because of its deeper root system is and its smaller leaves and canopy. Several adaptive mechanisms are evoked by plants in response to water stress (Chaves et al., 2003). These traits enable chickpea to ensure its survival in different environmental conditions.

Introduction

It is an established fact that water in terms of direct deficit (drought) as well as water surfeits (flooding and water logging) and poor irrigation water quality (salinity)- are major constraints on the use of solar energy for food, feed and fiber production in almost all the agricultural regions of the world. The various biophysical, physiological, genetic, agronomic and ecological aspects of plant water relations have been thoroughly reviewed in recent years with the objective aimed at water availability as a constraint on crop production. This review will attempt a limited synthesis of these aspects, drawing mainly from research on drought resistance in grain crops, rice in particular, and emphasizing the progress- theoretical and practical- that could result from collaboration between physiologists, geneticists and plant breeders. A central theme will be research and development aimed at fitting crop plants to fluctuating drought prone environments. It is noteworthy that shortage of water restricts crop productivity all over the world-not just in those areas classified as arid or semiarid, but in any area in which the evaporative demand exceeds rainfall during the growing season. As such, the interaction between genotype and environment in situations has an important bearing affecting the physiology and also biochemistry at cellular level vis-à-vis the crop phenology. Cultivars with better ability to access soil water and improved water use efficiency could increase yields in an economic and environmentally sustainable way.

Introduction

The study of abiotic stresses in plants has advanced significantly in recent years. However, the majority of experiments testing the response of plants to changes in environmental conditions have focused on a single stress treatment applied to plants under controlled conditions. In contrast, in the field, a number of different stresses can occur simultaneously. These may include conditions such as high irradiance, low water availability, extreme temperature, or high salinity and may alter plant metabolism in a novel manner that may be different from that caused by each of the different stresses applied individually. The response of plants to abiotic stresses in the field may therefore be very different from that tested in the laboratory (Cushman and Bohnert, 2000).

Introduction

The phytohormone abascisic acid (ABA) was discovered in connection to studies related to shedding of fruits and leaves, also called abscission, and the dormancy of buds. It was first identified and characterized by Frederick Addicott and his associates in 1963 while studying the compounds responsible for the abscission of cotton fruits. Inhibition of growth and maintenance of the dormancy of buds are the most important visible effects of ABA. The concentration of ABA does not remain constant as its concentration decreases in the buds significantly after sprouting and rises during seed and fruit production. Its concentration in dormant seeds is very high and considered as an efficient inhibitor of germination. The role ABA plays during the abscission of fruits and leaves is not clearly understood as its effect is very clear in case of fruit abscission where as it is not very clear on the abscission of leaves. However, the regulatory effect of ABA on the water balance is established in plants. It also induces stomatal closure to inhibit further loss of water. Being a growth inhibitor, ABA also reverses the effect of growth-stimulating hormones like auxin, gibberellins and cytokinin in different tissues of a plant. One of the main goals of phytohormonal ecology is to study the interactions between biotic and abiotic stress at hierarchical levels of biological organization.

Introduction

There are a number of chemicals in the environment, some natural other man-made, some toxic other non-toxic. These are toxic chemicals, whether natural or man-made, which has created problems for the environment and biota on the earth. Of these toxic chemicals, heavy metals are extremely hazardous. Metals are present naturally in the earth’s crust at various levels. However, increasingly widespread heavy metal pollution resulting from anthropogenic activities has caused serious environmental problems and is posing threat to both wild life and human population (Nriagu and Pacyna, 1988). Natural sources include weathering of mineral and metal ions from rocks, displacement of certain contaminants from groundwater or subsurface layers of soil, atmospheric deposition from volcanic activity, and transport of dust. The anthropogenic sources include deposition of industrial effluents, application of sewage sludge, deposition of air-borne industrial wastes, military operations, mining, landfill operations, use of agricultural chemicals, gas exhausts and energy and fuel production (McIntyre, 2003).

Introduction

The term ‘lectin’ was coined by William C. Boyd in 1954 for their property of selectivity (Latin ‘legere’, to pick up or choose). In 1888 H. Stillmark of Russia gave the first description of what we now know as lectin. While investigating the toxic effects on blood of extracts of the castor bean (Ricinus communis), Stillmark observed that the red blood cells were being agglutinated by a protein and gave it the name ‘ricin’. Shortly afterward ‘arbin’ was extracted from Arbus precatorius. Because they were first isolated from plants, lectins came to be known as phytohaemaglutinin. Now it has become clear that lectins are present not only in plants but also in some bacteria, vertebrates and invertebrates. However, lectins are most widely distributed in plants, reported in almost 1000 plant species.

Plant lectins are a very heterogeneous group of proteins; as far as their occurrence within the plant kingdom and distribution over tissues are concerned. They show high degree of heterogeneity with respect to biochemical and physiological roles, because of their widely different carbohydrate binding specificities (Etzler 1986; Rüdiger 1988; Goldstein and Hayes, 1978). Besides the fact that they all exhibit carbohydrate binding activity, most of the plant lectins behave as typical storage proteins (Peumans and Van Damme, 1995 b).

Introduction

Phenolic compounds have been shown to be of importance in regulation of plant growth, development and metabolism, therefore now they are not considered as passive by products of any catabolic or anabolic processes. Salicylic acid is one of them and it is a naturally occurring phenolic which has received much attention due to its association with economically important plant responses to disease and other stresses (Raskin, 1992; Borsani et al., 2001; Clarke et al., 2004). Centuries before medical scientists had identified numerous therapeutic effects of salicylates. Leaves and barks of willow tree were used by women as a pain reliever during child birth in 4th century B.C. It also cured aches and fevers. Due to a number of curative properties of willow bark, in 1828 Jahann Buchner, a scientist from Munich had isolated a small amount of silicin - a salicylic alcohol glucoside, the major salicylate in willow bark. In 1838, Rafaela Piria had given the name salicylic acid (SA), from the Latin salix, a willow tree. Further during 19th century SA and other salicylates, mainly methyl esters and glucosides easily convert to SA, isolated from a number of plants. Salicylic acid has an aromatic ring which bears a hydroxyl group or its functional derivative (Fig. 1).

Introduction

The respiratory electron transport pathway of plant mitochondria comprises the cytochrome (Cyt) pathway and alternative pathway (McDonald et al., 2002). Both the cyt and the alternative respiratory pathways start at protein complex I when NADH is being oxidized. One H+ (proton) is transported by the complex I to inner membrane space, whereas two electrons are transported within the inner membrane by the ubiquinone, which at its reduced state (Qr) transfers these electrons either to complex III or to the Alternative oxidase (AOX). Ubiquinone is the point in which reactions can proceed in different ways, and it is called the branch point. Cyt respiratory pathway is present in all living organisms. It proceeds while complex III pulls out a proton from the mitochondrial matrix to the inter membrane space. The electrons are received by cytochrome c which spreads up to the outer side of the inner membrane towards protein complex IV, which then pulls out another proton similar to complexes I and III, and transports the electrons back to inner domain of the mitochondria. As a result, oxygen is consumed with a proton and the two electrons to produce water (Fig 1).

Introduction

Seed, the basic input of crop production technology, is exposed to various stress factors right from its formation to the completion of its germination. Even after germination, the stressed seeds may fail to emerge into a healthy plant and thus, the subsequent seed formation is also affected both quantitatively and qualitatively.

The stress factors may be abiotic, biotic or mesobiotic. Environmental factors such as climatic parameters, nutritional deficiencies, soil factors and conditions of storage are the examples of abiotic stress factors. Insects, fungi, bacteria, nematodes and other plant parasites constitute biotic stress factors whereas viruses, viroids and virusoids and similar agencies are the examples of mesobiotics factors that bring about stress over the seed. Apart from these conspicuous causes of stress, there are certain other factors also, which may affect the influence of stress over the total performance of the seed. For example, the crop improvement efforts bring about changes in seeds quality that can have considerable effects on the vulnerability of the seed to invasion by various biotic factors.

Introduction

The life cycle of a plant passes through biotic and abiotic stresses and plants are being attacked at different stages of growth by a number of disease causing organisms. These organism including bacteria, fungi, viruses, and nematodes attacks plants. These pathogens caused large crop losses and probably since the beginning of agriculture have contributed to human hunger and malnutrition. The control of plant diseases is thus of fundamental importance and is a major objective of plant breeding and pathology programme and agriculture chemical industry. Plants resists pathogens attack both with preformed defenses such as antimicrobial secondary compounds and by inducing defense responses (Hammond and Jones, 2000, Heath, 2000). Inducible defenses can be activated upon recognition of general elicitor such as bacterial flagellin and even host cell fragments released by pathogens damage (Gomez and Boller, 2000, Hammond and Jones, 2000). However, plants also involved sophisticated recognition systems to detect proteins produced during infection by specific races of pathogens (Martin et al., 2003).

Introduction

Plants are responsive to the applied Non-scheduled caste, which constitute most of the vital macromolecules and metabolites related to its vegetative and reproductive cycle. The applied fertilizer N enhances crop productivity per unit area, as agricultural soil deficient in N worldwide. The fertilizer application is thus considered essential to meet the requirement of the burgeoning population, particularly in the developing countries (Abrol et al., 1999; Chanda and Sati, 2005). The farmers practicize heavy dressing of N-fertilizers in crop fields for high productivity, which has resulted into world including Indian sub-continent and China have resulted into the excessive use of N-fertilizers intending to maximize crop yield. Decrease in the organic fertilizer input and depletion of micronutrients in intensive cultivation zones have caused a decline or stagnation in the crop productivity, which  has been attempted with a further loading of soluble chemical fertilizers (Maleshwar, 2003; Kumar and Yadav, 2003; Anonymous, 2004; Chanda and Sati, 2005; Abdin et al., 2006).

The mites belong to the Class - ‘Arachnida’, Order - ‘Acarina’. Most of the phytopathogenic mites are classified under the two Families - Tetranychidae and Eriophyidae.

The mites belonging to the Family - Tetranychidae are generally known as Tetranychids. They are very minute, and less than 100 microns in size. The body is non-segmented and the head, thorax and abdomen are  not distinct, but rather fused together. The life cycle of a tetranychid mite consists of an egg stage, three nymphal stages viz., protonymph, deutonymph and tritonymph and an adult stage. In the protonymphal stage, which is also called the larval stage, the mites have only 3 pairs of legs. In all the other nymphal stages and the adult stage they have 4 pairs of legs. The mites are roundish or oval in shape, with biting and chewing, as well as piercing and sucking type of mouthparts (Fig.190).