Attempts to demarcate the two processes of transportation: of inorganic, as well as organic nutrients in plants, dates back to the 17th century. A plant anatomist, Malpighi conducted a experiment in which he separated a ring of bark (phloem) from the wood (xylem) of young stems by detaching the two in the region of vascular cambium - ‘girdling’ or ‘ringing’ (Figure 13.1). Since the xylem remained intact, water and inorganic solutes kept on rising all the way upto the foliar region and the plant remained alive for a few days. However, girdled plants showed swelling of the bark in the area just above the girdle due to accumulation of photo-assimilates flowing downward. The downward stream also consisted of nitrogenous compounds and hormones, which caused cell enlargement above the girdle. Ultimately, the root system was subjected to starvation because of lack of nutrients and the girdled plants died away.

Evidences in Support of Phloem Transport

• 1. An analysis of phloem exudate obtained by making an incision into the phloem tissue provides evidence, supporting the fact that photoassimilates are translocated through the phloem.

• 2. Aphid technique: Aphids, constituting groups of small insects, feed on herbaceous plants by inserting a long mouth part (proboscis) deep into individual sieve-tube elements of the phloem. While aphids are feeding, they are anaesthetized with a gentle stream of CO2 and the proboscis is carefully removed with a sharp blade. Meanwhile, the uncontaminated phloem liquid continues to ooze out from the cut proboscis for a long period (Figure 13.2). This demonstrates that phloem sap is under pressure. The aphid technique has proved to be of great use in understanding the mechanism of phloem transport.

• 3. Radioactive tracers: Evidence is also garnered by employing radioactive elements, specifically 14C on the leaves of herbaceous and woody plants. The radioautographs that follow, reveal that radioactive photoassimilates being transported out from the leaves, are confined to only the phloem tissue.

Composition of the Photoassimilates Translocated in the Phloem

The major component of phloem sap in most of the plants is sucrose. But, a small number of plant families translocate oligosaccharides of the raffinose groups (raffinose, stachyose and verbascose) and sugar alcohols (mannitol and sorbitol).

Historical Background

The use of enzymes by mankind dates back to the Greek civilization, that first used enzymes in the process of fermentation to produce wine. After the discovery of the catalytic process in the early nineteenth century, a Swedish chemist, Brazelius (1836) suggested that the numerous chemical reactions in living organisms might depend upon the presence of catalysts within the tissues. In 1857, Louis Pasteur, a French scientist demonstrated the involvement of ‘living, intact’ yeast cells in the process of alcoholic fermentation and proposed the term ‘ferments’ for these biocatalysts. The term ‘enzyme’ was later coined by Kuhne (1878) for soluble ferment of yeast or bacteria. However, a significant breakthrough in the study of enzymes was made when the Buchner brothers (1897) in Germany accidentally discovered that the ‘squeezed out’ juice from non-living yeast extract when mixed with sugar could bring about fermentation. Yeast juice is now known to be a mixture of at least twelve different catalysts. The name ‘enzyme’ was coined for the postulated catalyst in juice.

The chemical nature of enzyme remained uncertain until Sumner (1926) purified and crystallized first, the enzyme ‘urease’ from Jackbean (Canavalia ensiformis) and further discovered that it was proteinaceous in nature. Thereafter, hundreds of enzymes have been separated in a pure or semi-pure state and all have proved to be proteins except for ribozymes.

Characteristics of Enzyme-catalyzed Reactions

• • High rates of reaction: Enzyme-catalyzed reactions have typically 106–1012 times higher rates as compared to uncatalyzed reactions. Most of enzymes have the capability to convert thousands of substrate into product molecules every second.

• • High specificity: Enzymes have the capablility to recognize extremely minute and specific differences in substrate as well as product molecules and can even distinguish between mirror images of any molecule (stereoisomers or enantiomers) e.g., D-Galactose and L-Galactose.

• • Mild reaction conditions: Enzymatic reactions usually take place at atmospheric pressure, relatively low temperatures and within a narrow range of pH (approximately 7.0) except for certain protein-degrading enzymes in vacuoles which function at pHs near 4.0, or enzymes present in thermophilic bacteria that can survive in hot sulphur springs (at 100 °C).

Under both natural and agricultural situations, plants are often subjected to environmental stresses. Stress plays an important role in determining how soil and climate restrict the distribution of plant species. Stress is usually defined as a disadvantageous impact on the physiology of a plant, induced upon a sudden shift from optimal environmental condition where homeostasis is maintained to suboptimal level which disturbs the initial homeostatic state. In most cases, stress is measured in terms of plant survival, crop productivity or the primary assimilation processes, which are all co-related to overall growth.

Plant stress can be divided into two categories: Abiotic and Biotic.

Abiotic Stress

Plants grow and reproduce in hostile environments containing large numbers of abiotic chemical and physical variables, which differ both with time and geographical location. The primary abiotic environmental parameters that affect plant growth are light, water, carbon dioxide, oxygen, soil nutrient content and availability, temperature, salts, and heavy metals. Fluctuations of these abiotic factors generally have negative biochemical and physiological impact on plants. Being fixed, plants are unable to avoid abiotic stress by simply moving to a more suitable environment. Instead, plants have evolved the ability to compensate for stressful conditions by switching over physiological and developmental processes to maintain growth and reproduction.

Responses to abiotic stress depend on the extremity and time duration of the stress, developmental stage, tissue type and interactions between multiple stresses (Figure 18.1). Experiencing stress typically promotes alterations in gene expression and metabolism, and reactions are frequently centred on altered patterns of secondary metabolites. Plants are complex biological systems comprising of thousands of different genes, proteins, regulatory molecules, signalling agents, and chemical compounds that form hundreds of interlinked pathways and networks. Under normal growing conditions, the different biochemical pathways and signalling networks must act in a coordinated manner to balance environmental inputs with the plant's genetic imperative to grow and reproduce. When exposed to unfavourable environmental conditions, this complex interactive system adjusts homeostatically to minimize the negative impacts of stress and maintain metabolic equilibrium.

Absorption of Water

Water is the essential medium of life. Thus, Plants that are land-based (as opposed to aquatic ones) are faced with potentially lethal desiccation through water loss to the atmosphere. This problem is aggravated by the large surface area of leaves, that are exposed to high levels of radiant energy (sunlight) and their need to have an open pathway for CO2 uptake. Thus, there is a conflict between the need for water conservation and the need for CO2 assimilation.

The need to resolve this vital conflict determines much of the structure of plants that grow on land, namely: (i) the development of an extensive root system to absorb water from the soil; (ii) a low-resistance pathway through the tracheary elements to bring water to the leaves; (iii) hydrophobic cuticle lining the surfaces of the plant to reduce evaporation; (iv) microscopic stomata on the leaf surface to allow gaseous exchange; (v) guard cells to regulate the diameter and diffusional resistance of the stomatal opening.

Algae and simple land plants like mosses and lichens may absorb water through their entire surface but in the vascular plants, the absorption of water takes place mostly through the roots. The source of water supply, with few exceptions, is the soil. The principal source of soil water is rain.

Different types of water

• • Run-away water (not available to the plant): After a heavy rainfall or irrigation, some of the water drains away along the slopes. This is called run-away water.

• • Gravitational Water (also not available to the plant): Some of the water percolates downwards through the larger pore spaces between the soil particles under the influence of gravitational pull until it reaches the water table.

• • Hygroscopic Water (again not available to the plant): Water gets adsorbed on the surface of soil colloids and held tightly by them.

• • Chemically combined Water (again not available to the plant): A small amount of water is bound to the molecules of some soil minerals by strong chemical bonds.

• • Capillary Water (the form that is available to the plant): The remainder of the water that fills the spaces between the non-colloidal smaller soil particles.

Capillary water plays a major role as it is readily adsorbed by the roots. The total amount of water held in the soil is called ‘holard’.

Our earlier book, Comprehensive Practical Plant Physiology, was first published in 2012 and was very warmly received by students and teachers alike. In a large measure, the book succeeded in generating a lot of interest amongst students and scholars alike, in the field of Botany and Agricultural Sciences. However, the readers have urged us to restructure the text and update the information so that the book becomes self-contained in itself, matching other leading titles in the field of plant physiology.

During the course of reorganisation, we were greatly influenced by the feedback reviews we received from the many experts based within the country and in Southeast Asia, as well as the suggestions offered to us by the editorial staff at Cambridge University Press, India. We are much pleased to present to our readers an altogether new book, in which we have ensured a continuous flow of information so that the readers can assimilate the knowledge without too much effort. The material is presented in a concise and lucid manner, so that the readers can comprehend the conceptual complexities, come to know about the recent achievements in the field, and share the joy that we feel for this subject.

Salient features of this edition are as follows:

In addition to the generalized and well-informed textual content in each chapter, we have attempted to highlight the important information through models and flow charts; such as the information in the chapter on ‘growth and development’ regarding various topic like, mode of action, physiological role, biosynthesis and inactivation of auxins, gibberellins, ethylene, and abscisic acid. The role of the recently discovered hormones such as jasmonates, polyamines, salicylic acid and nitric oxide, etc., has also been emphasised upon appropriately. The mode of action of phytochrome-mediated responses based on their requirements such as LFRs, VLFRs and HIRs, also finds a special mention. Included also in the discussion is the work on Arabidopsis thaliana as an experimental tool and model system for research in genetics and molecular biology, enumerating its advantages over other plant materials. We have also included a discussion on Reactive Oxygen Species (ROS) and Asada-Halliwell or Ascorbate-Glutathione Pathway in the chapter ‘Stress Physiology and Secondary Metabolites’.

Sulphur Metabolism

Sulphur is an essential macronutrient for all living organisms, and it occurs in nature in different states of oxidation as a component of inorganic and organic compounds. The chief inorganic form of sulphur, sulphate, is obtained predominantly from the weathering of parent rock material. Natural organic sulphur compounds include gases such as hydrogen sulphide, dimethylsulphide and sulphur dioxide, which are discharged into the air both by geochemical processes and by activity of the biosphere. In the gaseous phase, sulphur dioxide (SO2) reacts with a hydroxyl radical and oxygen to form sulphur trioxide (SO3) which upon dissolving in water produces a strong sulphuric acid (H2SO4), the major source of acid rain. A high concentration of sulphate is also found in the oceans (approximately 26mM or 2.8 g L-1). Plants have a capacity to metabolize sulphur dioxide absorbed in the gaseous phase through their stomata. A prolonged exposure of nearly more than eight hours to high atmospheric concentrations of SO2 (i.e. more than 0.3 ppm) tends to damage plant tissues extensively due to formation of sulphuric acid.

Sulphate uptake and transport

Sulphate enters a plant from the soil solution, primarily through the roots by an active proton cotransport. Sulphate uptake is generally inhibited by the presence of anions such as selenate, molybdate, and chromate anions, which can compete with sulphate for binding to the transporters. Sulphate uptake systems can be classified as either sulphate permeases or facilitated transport systems. Plant sulphate permeases are similar to fungal and mammalian co-transporters. They consist of a single polypeptide chain with 12 transmembrane domains, characteristic of cation/solute transporters. A second mechanism, ATP-dependent transport, is exemplified by a system in cyanobacteria that includes a multiprotein complex of three cytoplasmic membrane components and a sulphate-binding protein in the periplasmic space.

Roots take up sulphate from the soil via electrogenic- symporters (SULTRs). Two high-affinity transporters are expressed in root epidermis and cortex. Although sulphate assimilation occurs in roots, most of the sulphate is transported to the shoots. In photosynthetic cells, sulphate can be translocated either into chloroplasts for assimilation, or into the vacuoles for storage. It has been found that leaves are usually more active in assimilation of sulphur as compared to roots, perhaps because photosynthesis supplies reduced ferredoxin, and photorespiration produces an amino acid serine, which might activate the synthesis of O-acetylserine.

Sucrose and Starch Metabolism

Sucrose and starch, the principal leaf storage products, are biosynthesized in two different compartments – sucrose in the cytoplasm of photosynthetic cells and starch in the chloroplast. Both the compounds accumulate in daylight and are mobilised to meet the metabolic demands during night time or at the time of minimum photosynthetic yield. The pathway for sucrose synthesis is highly regulated and synchronized with the pathway associated with starch synthesis.

The appropriation of carbon fixed by the Calvin cycle into either starch or sucrose synthesis is referred to as carbon allocation.

Sucrose biosynthesis

Sucrose is the most common type of carbohydrate present in the translocation stream (Figure 8.1). Plants such as wheat, oats and barley temporarily accumulate large quantities of sucrose in the vacuole. Sucrose (non-reducing sugar) is a disaccharide and each molecule is composed of two monosaccharides, glucose and fructose (reducing sugars). Instead of free glucose and fructose, phosphorylated forms of these sugars serve as precursors of sucrose.

Uridine diphosphate-glucose (UDP-G) is the most preferred precursor for sucrose biosynthesis in the cytoplasm of cells of the leaf.

Reactions involved in the pathway of sucrose formation are as follows:

Overall equation for sucrose synthesis is

Only one ATP molecule is essential for the biosynthesis of sucrose. Since three ATP molecules are consumed in the Calvin cycle for each carbon in each hexose of sucrose (a total of 36 ATPs), one additional ATP needed to form a glycosidic bond in sucrose is a small additional requirement.

Sucrose exported from the leaf cell to non-photosynthetic tissues may be oxidized quickly or stored in the vacuoles for a short period or gets transformed to starch for long-term storage in the chloroplast.

Regulation of sucrose-6-phosphate synthase: Sucrose phosphate synthase (SPS) is a key regulatory enzyme in sucrose biosynthesis and catalyzes the transfer of a glucose molecule from UDP-glucose to fructose-6-phosphate to yield sucrose-6-phosphate and UDP.

SPS is regulated via reversible protein phosphorylation and dephosphorylation. For example, SPS kinase causes phosphorylation of the enzyme sucrose phosphate synthase, thus converting this to an ‘inactive’ form while SPS phosphatase enzyme dephosphorylates the enzyme, thus, activating it.

Plant physiology as we know, deals with different life processes operating within the cell, and interactions between the cell and the environment based upon physical, chemical and biological concepts. All these make it highly dynamic and exacting in nature. The science of plant physiology is never static but always changing as new facts are discovered and fresh concepts are developed. With new instrumentation and advances in the knowledge of working of the cell and the discovery of structure of DNA by Watson and Crick (1953), plant physiology has become first increasingly biochemical and then molecular.

A knowledge of plant physiology is essential for different fields of applied botany, whether agronomy, floriculture, forestry, horticulture, landscape gardening, plant breeding, plant pathology, or pharmacognosy. All these applied courses depend upon plant physiology for information regarding how plants grow and develop.

Until the forties, laboratories in England and Germany dominated the scientific scene and many Indian scientists began their career in these countries. However, after the Second World War, during which much of Europe was completely devastated, the focus of major research activity shifted to USA and Canada where new schools were established and many of our scientists went over there to enrich their expertise.

Compared to other parts of the world in India, the discipline of plant physiology has not received the attention it ought to have deserved but still some of our scientists earned distinction at the international level. The undisputed Indian pioneer in experimental research on plants was J. C. Bose (1858–1937), who was knighted by the British in recognition of his contributions to scientific endeavour; entitling him to the use of ‘Sir’ before his name. Basically a physicist, he was widely acclaimed for his discoveries on radio waves and wireless. But in his later years, and particularly after retirement (which to most people spells the end of one's career), Bose was also greatly interested in plant life and became hugely involved in research on plants. In fact, in 1917, he founded an autonomous institute, the Bose Institute (now among the pioneering research centres in India).

Plants contain 80–90 per cent of water by weight and the remaining 10–20 per cent is dry matter. Nearly 95 per cent of the dry matter is comprised of carbon, hydrogen and oxygen. Carbon and oxygen are obtained from the atmosphere in the form of carbon dioxide and water vapour, and hydrogen being derived from water absorbed by the roots from the soil. All other minerals are obtained from the soil. Dry matter when burnt at high temperature (400–500°C) in a muffle furnace, leaves behind non-volatile residue, commonly called plant ash, i.e., metal oxides. The ash content of different plants and tissues varies from 1 to 4 per cent of fresh weight – being the least in aquatic plants and the highest in plants growing in saline or dry soil. Succulent tissues and fleshy fruits are poor in mineral proportion, whereas the leaves contain a relatively high proportion. A careful chemical analysis of the ash reveals that it is made up of as many as sixty different mineral elements found in the plants.

For the actual recognition and the dependence of plants on mineral elements present in the soil, credit must be given to de Saussure. In his book Recherches chimiques sur la végétation, published in 1804, de Saussure clearly demonstrated that the inorganic minerals present in the ash of plants are obtained from the soil through the root system. He further stated that mineral elements, including nitrogen, derived from the soil were important for the growth and development of plants. But the essentiality of the inorganic constituents of plant ash to the general welfare of the plant was not recognized until it was reported by Liebig in 1840.

The earliest kinds of experiments to study mineral nutrition and soil improvement were done in the field. In 1843, the oldest existing Rothamsted Experimental Station, Harpenden, in Hertfordshire, UK was established by Sir John Lawes and John Gilbert. Together they established the first protocols for the extensive experimentation that followed on mineral nutrition of plants and were able to successfully convert insoluble rock phosphate to soluble phosphate(superphosphate).

Plants synthesize a vast array of organic compounds that seemingly have no known role in either assimilation or during growth and development of the organism. These compounds, which are natural products, are called secondary metabolites (Figure 19.1). Secondary metabolites are different from primary metabolites (e.g., proteins, lipids, carbohydrates and nucleic acids) because of their restricted distribution in various groups of plants (Figure 19.2). Scientists have long thought that these compounds provide protection to plants from predators and pathogens owing to their toxic effect and repellent nature to herbivores and microbes when studied in vitro. Recent studies have shown that the expression of secondary metabolites can be modified by advanced molecular techniques.

Plant secondary metabolites can be classified into four distinct groups, such as terpenes, phenolics, alkaloids and glycosides.

Terpenes (Terpenoids)

Terpenoids take their name from terpenes, the volatile constituents of turpentine (a solvent produced from the distillation of pine tree resin). These include both primary and secondary metabolites. The terpenes represent the largest group of secondary metabolites. The diverse substances belonging to this group are normally insoluble in water and biosynthesized either from acetyl CoA or intermediates of glycolysis. Terpenoids and their derivatives may be considered polymers of isoprene units which consist of the branched five-carbon isoprene skeleton (Figure 19.3). Consequently, terpenoids are often referred to as isoprenoid compounds. The repetitive 5-C structural motif from which terpenoids are built is called the prenyl group. Terpenes are grouped by the number of 5-C units they have in their skeleton:

Diterpenoids and triterpenoids include both primary and secondary metabolites. An important chemical group derived from di- and triterpenoid precursors is steroid which is widely distributed in plants, fungi and animals. They contribute much to the stability of membranes and hormone signalling.

The science of Plant Physiology deals with various life processes occurring both within cells, and between cells and the environment. The former includes metabolic pathways like photosynthesis, respiration, nitrogen and fat metabolism, translocation of food materials and growth regulators and similar processes; while the latter encompasses diffusion of gases, absorption of water and minerals, ascent of sap and transpiration and such. Understanding the physiological requirements of different crops is the key to successful agriculture. A deep understanding of water, nutritional and other edaphic and climatic parameters controlling the crop physiological functions is essential for optimizing productivity for the sustainable development of our exploding population. To emphasise this, we will examine the pre-green revolution era when we had tall varieties of wheat and rice with a tendency towards lodging when heavily fertilised. Using the Norin 10 dwarfing genes in wheat, and the Dee-geo-woo-gen genes in rice, agricultural scientists were able to change the morphological architecture (the Ideotype concept) by developing high-yielding dwarf and semi-dwarf varieties. But their success depended upon the crop's water, nitrogenous fertilizer and other edapho-climatic requirements which were optimised through the concerted efforts of plant physiologists, agronomists and others. This is the success story of the Green Revolution in the early 1960s.

Plant Physiology is a rapidly advancing field of botany and serves as the foundation for the numerous advances in agriculture (including horticulture), environmental sciences, floriculture, agronomy, plant pathology, forestry, and pharmacology. Adequate knowledge and understanding of crop physiology is the key to success in optimizing farm productivity in terms of yield and quality. In the case of crops, their genetic make-up and environmental factors play a crucial role in manipulating their growth. Once the breeders understand the physiology of crops they are trying to handle, it becomes easy for them to deal with the problems for selecting varieties for higher yield and to cope up with environmental stresses as well as insect invasions, etc.