Erasmus Darwin loved seeing, touching, smelling, using, studying and celebrating plants and wanted others to share in his enjoyment, from members of his family to his patients, who utilised their virtues or powers as drugs to combat their illnesses and improve their lives. In Phytologia, his treatise upon agriculture and gardening, he exclaimed that ‘the beautiful colours of the petals of flowers with their polished surfaces’ were ‘scarcely rivalled’ by those of shells, feathers or ‘precious stones’. Many of these ‘transient beauties’ that gave such ‘brilliancy to our gardens’ delighted ‘the sense of smell with their odours’ and were employed extensively ‘as articles either of diet, medicine, or the arts’. Knowledge of plants was always important for Darwin, given that vegetable products were the basis of so much of the materia medica (substances prescribed as treatments), and botany was taught in the medical schools in order that practitioners could distinguish useful from useless or even dangerous plants.

During the 1770s and 1780s, however, botany and gardening assumed greater significance in Darwin’s life, encouraged by a community of plant enthusiasts and what Sylvia Bowerbank has described as literary ‘defenders’ of ‘environmental stewardship’ who were centred upon Lichfield and invested it with ‘deep feeling and value’ as a multilayered ‘storied place’. Among them were the poet Anna Seward, Rev. John Saville (1736–1803), Darwin’s future wife Elizabeth Pole and the members of the botanical society that Darwin formed, as noted above, with Brooke Boothby and William Jackson (1743/5–1798), who were engaged upon a project to translate some of the works of Carl Linnaeus (1707–1778) from Latin into English. Encouraged by his medical and scientific botanical concerns, his growing love of gardening and his experience of the countryside around Lichfield and the local coterie of plant lovers, Darwin developed a botanic garden in a small well-watered valley near the city, which inspired the two parts of his epic poem The Botanic Garden (1791). The formation of the Lichfield botanical garden was a crucial moment in both Darwin and Seward’s poetical careers. Through the success of the Botanic Garden, which, as the poet Samuel Taylor Coleridge (1772–1834) remarked, was ‘for some years greatly extolled, not only by the reading public in general’ but by poets of his generation, Darwin’s Lichfield creation became one of the most influential British botanical gardens.

Erasmus Darwin’s most important contribution to the science of farming was the publication Phytologia, a major treatise on agriculture and gardening that attracted some contemporary praise. More recently, Robert Schofield was puzzled that an ‘elderly, established doctor’ with ‘little public indication of an interest in agriculture’ should have published a treatise on the subject. And while Maureen McNeil argued that Darwin’s inspiration lay primarily in Scottish writings concerning agricultural improvement, the Agricultural Revolution and the ‘scarcity crisis of the 1790s’, along with some aspects of Linnaean botany and the creation of the Lichfield botanic garden, she still found it hard to explain how these provided ‘sufficient stimulus for a six-hundred page treatise on agriculture’. While some interest in the technologies of agricultural improvement and plant physiology were already evident in Darwin’s commonplace book from the 1770s, for McNeil ‘explanations of Darwin’s agrarian interest’ founded only upon his ‘personal situation’ are ‘inadequate’ and she saw ‘no necessary transition’ from prose and poetical ‘nature studies’ to agriculture. His medical interests did not provide a full explanation either, given the small part of agriculture that was devoted to generating ‘tools of medicine’. However, this underestimates the extent to which he mixed with farmers and landowners throughout his career, the extent of his experiences combating animal and plant diseases and the impact of his medical ideas and experiences upon his analyses of agriculture and horticulture.

Darwin was a strong supporter of the Georgian ideology of improvement, which, as Raymond Williams argued, like the notion of ‘cultivation’, contained meanings that were ‘historically linked but in practice so often contradictory’: ‘working agriculture’ existed alongside the costly ‘improvement of houses, parks, artificial landscapes’, interweaving the desire for increased wealth and productivity with landscape aesthetics reinforced by moral judgement. This chapter argues that Darwin’s belief that the sciences and medicine could be used to better harness nature and help realise a more productive countryside is evident in Phytologia, his medical treatise Zoonomia and the references to agriculture and gardening in his poetry and correspondence. McNeil has argued that, as such, his emphasis was on the ‘intellectual, rather than on the manual aspect’ of farming and that he was ‘eager to celebrate’ both agricultural as much as industrial ‘capitalists’ as ‘social heroes’ as well as those who helped encourage farmers to better organise their ‘space, capital and time’.

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.