Even though the acidity of fruit increases as it ripens, the higher acidity level is not reflected in its flavor, which can lead to the misunderstanding that the riper the fruit the sweeter. This curious fact is attributed to the Brix-Acid Ratio.
Ripening agents speed up the ripening process.
They allow many fruits to be picked prior to full ripening, which is useful, since ripened fruits do not ship well. For example bananas are picked when green and artificially ripened after shipment by being gassed with ethylene. A similar method used in parts of Asia was to cover a bed of slightly green-harvested mango and a few small open containers of clumps of calcium carbide with a plastic covering. The moisture in the air reacted with the calcium carbide to release the gas acetylene, which has the same effect as ethylene. Ethylene is not emitted by the plant naturally, and cannot activate the ripening of nearby fruits, rather, it used as a hormone within the plant.
Calcium carbide is used for ripening the fruit artificially in some countries. Industrial-grade calcium carbide may contain traces of arsenic and phosphorus, and, thus, use of this chemical for this purpose is illegal in most countries. Calcium carbide, once dissolved in water, produces acetylene which acts as an artificial ripening agent. Acetylene is believed to affect the nervous system by reducing oxygen supply to brain; however, it has been shown that in practice, acetylene is not sufficiently reactive to affect consumers.
Covered fruit ripening bowls are commercially available to increase fruit ripening. The manufacturers claim the bowls increase ethylene and carbon dioxide gasses around the fruit which promote ripening.
Climacteric fruits are able to continue ripening after being picked, a process accelerated by ethylene gas. Non-climacteric fruits can ripen only on the plant and, thus, suffer from short shelf-lives.
Smart Fresh is a technology useful to maintain fresh-picked quality of whole fruits and vegetables. 1-Methylcyclopropene (1-MCP 0.14%) works with the ripening process to dramatically slow down ethylene production and prevent over-ripening and problems associated with aging.
Iodine (I) can be used to determine whether the fruit is ripening or rotting by showing whether starch in the fruit has turned into sugar. For example a drop of iodine on a slightly rotten part (not skin) of an apple will turn a dark-blue or black color, since starch is present. If it stays yellow, then most of the starch had converted to sugar.
Ethylene as a Plant Hormone
Ethylene serves as a hormone in plants. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers and the abscission of leaves.
History of Ethylene in Plant Biology
Ethylene has been used in practice since the ancient Egyptians, who would gash figs in order to stimulate ripening (wounding stimulates ethylene production by plant tissues). The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. In 1864, it was discovered that gas leaks from street lights led to stunting of growth, twisting of plants, and abnormal thickening of stems. In 1901, a Russian scientist named Dimitry Neljubow showed that the active component was ethylene. Doubt discovered that ethylene stimulated abscission in 1917. It wasn't until 1934 that Gane reported that plants synthesize ethylene.
Ethylene Biosynthesis in Plants
Ethylene is produced from essentially all parts of higher plants, including leaves, stems, roots, flowers, fruits, tubers, and seedlings.
“Ethylene production is regulated by a variety of developmental and environmental factors. During the life of the plant, ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. Ethylene production can also be induced by a variety of external aspects such as mechanical wounding, environmental stresses, and certain chemicals including auxin and other regulators”.
The biosynthesis of the hormone starts with conversion of the amino acid methionine to S-adenosyl-L-methionine (SAM, also called Adomet) by the enzyme Met Adenosyltransferase. SAM is then converted to 1-aminocyclopropane-1-carboxylic-acid (ACC) by the enzyme ACC synthase (ACS); the activity of ACS determines the rate of ethylene production, therefore regulation of this enzyme is key for the ethylene biosynthesis. The final step requires oxygen and involves the action of the enzyme ACC-oxidase (ACO), formerly known as the Ethylene Forming Enzyme (EFE). Ethylene biosynthesis can be induced by endogenous or exogenous ethylene. ACC synthesis increases with high levels of auxins, especially Indole acetic acid (IAA), and cytokinins. ACC synthase is inhibited by abscisic acid.
Ethylene Perception in Plants
Ethylene could be perceived by a transmembrane protein dimer complex. The gene encoding an ethylene receptor has been cloned in Arabidopsis thaliana and then in tomato. Ethylene receptors are encoded by multiple genes in the Arabidopsis and tomato genomes. The gene family comprises five receptors in Arabidopsis and at least six in tomato, most of which have been shown to bind ethylene. DNA sequences for ethylene receptors have also been identified in many other plant species and an ethylene binding protein has even been identified in Cyanobacteria.
The Triggers of Ethylene
Environmental cues can induce the biosynthesis of the plant hormone. Flooding, drought, chilling, wounding, and pathogen attack can induce ethylene formation in the plant. In flooding, root suffers from lack of oxygen, or anoxia, which leads to the synthesis of 1-Aminocyclopropane-1-carboxylic acid (ACC). ACC is transported upwards in the plant and then oxidized in leaves. The product, the ethylene causes epinasty of the leaves.
One speculation recently put forth for epinasty is the downward pointing leaves may act as pump handles in the wind. The ethylene may or may not additionally induce the growth of a valve in the xylem, but the idea would be that the plant would harness the power of the wind to pump out more water from the roots of the plants than would normally happen with transpiration.
Physiological responses of plants
Like the other plant hormones, ethylene is considered to have pleiotropic effects. This essentially means that it is thought that at least some of the effects of the hormone are unrelated. What is actually caused by the gas may depend on the tissue affected as well as environmental conditions. In the evolution of plants, ethylene would simply be a message that was coopted for unrelated uses by plants during different periods of the evolutionary development.
List of plant responses to ethylene
Seedling triple response, thickening and shortening of hypocotyl with pronounced apical hook. This is thought to be a seedling's reaction to an obstacle in the soil such a stone, allowing it to push past the obstruction.
In pollination, when the pollen reaches the stigma, the precursor of the ethylene, ACC, is secreted to the petal, the ACC releases ethylene with ACC oxidase.
Stimulates leaf and flower senescence
Stimulates senescence of mature xylem cells in preparation for plant use
Inhibits shoot growth except in some habitually flooded plants like rice
Induces leaf abscission
Induces seed germination
Induces root hair growth – increasing the efficiency of water and mineral absorption
Induces the growth of adventitious roots during flooding
Stimulates epinasty – leaf petiole grows out, leaf hangs down and curls into itself
Stimulates fruit ripening
Induces a climacteric rise in respiration in some fruit which causes a release of additional ethylene. This can be the one bad apple in a barrel spoiling the rest phenomenon.
Affects neighboring individuals
Inhibits stem growth outside of seedling stage
Stimulates stem and cell broadening and lateral branch growth also outside of seedling stage
Interference with auxin transport (with high auxin concentrations)
Inhibits stomata closing except in some water plants or habitually flooded ones such as some rice varieties, where the opposite occurs (conserving CO2 and O2)
Where ethylene induces stomata closing, it also induces stem elongation
Induces flowering in pineapples
Commercial Issues of Ethylene
Ethylene shortens the shelf life of many fruits by hastening fruit ripening and floral senescence. Tomatoes, bananas and apples will ripen faster in the presence of ethylene. Bananas placed next to other fruits will produce enough ethylene to cause accelerated fruit ripening. Ethylene will shorten the shelf life of cut flowers and potted plants by accelerating floral senescence and floral abscission. Flowers and plants which are subjected to stress during shipping, handling, or storage produce ethylene causing a significant reduction in floral display. Flowers affected by ethylene include carnation, geranium, petunia, rose, and many others.
Ethylene can cause significant economic losses for florists, markets, suppliers, and growers. Researchers have developed several ways to inhibit ethylene, including inhibiting ethylene synthesis and inhibiting ethylene perception. Aminoethoxyvinylglycine (AVG), Aminooxyacetic acid (AOA), and silver ions are ethylene inhibitors. Inhibiting ethylene synthesis is less effective for reducing post-harvest losses since ethylene from other sources can still have an effect. By inhibiting ethylene perception, fruits, plants and flowers don't respond to ethylene produced endogenously or from exogenous sources. Inhibitors of ethylene perception include compounds that have a similar shape to ethylene, but do not elicit the ethylene response. One example of an ethylene perception inhibitor is 1-methylcyclopropene (1-MCP).
Commercial growers of bromeliads including pineapple plants, use ethylene to induce flowering. Plants can be induced to flower either by treatment with the gas in a chamber, or by placing a banana peel next to the plant in an enclosed area.
Process Of Over Ripening or Rotting
It begins with leaching by water; the most easily lost and soluble carbon compounds are liberated in this process. Another early process is physical breakup or fragmentation of the plant material into smaller bits which have greater surface area for microbial colonization and attack. In smaller dead plants, this process is largely carried out by the soil invertebrate fauna, whereas in the larger plants, primarily parasitic life-forms such as insects and fungi play a major breakdown role and are not assisted by numerous detritivore species. Following this, the plant detritus undergoes chemical alteration by microbes. Different types of compounds decompose at different rates. This is dependent on their chemical structure. For instance, lignin is a component of wood, which is relatively resistant to decomposition and can in fact only be decomposed by certain fungi, such as the black-rot fungi. Said fungi are thought to be seeking the nitrogen content of lignin rather than its carbon content. Lignin is one such remaining product of decomposing plants with a very complex chemical structure causing the rate of microbial breakdown to slow. Warmth determines the speed of plant decay, with the rate of decay increasing as heat increases, e.g. a plant in a warm environment will decay over a shorter period of time.
In most grassland ecosystems, natural damage from fire, insects that feed on decaying matter, termites, grazing mammals, and the physical movement of animals through the grass are the primary agents of breakdown and nutrient cycling while bacteria and fungi play the main roles in further decomposition. The chemical aspects of plant decomposition always involve the release of carbon dioxide.
Fruit Crop Types
Simple Fruit Epigynous berries are simple fleshy fruit. From top right: cranberries, loganberries, blueberries red huckleberries
Simple fruits can be either dry or fleshy, and result from the ripening of a simple or compound ovary in a flower with only one pistil. Dry fruits may be either dehiscent, or indehiscent (not opening to discharge seeds). Types of dry, simple fruits, with examples of each, are:
Achene - Most commonly seen in aggregate fruits (e.g. strawberry)
Capsule – Brazil nut
Caryopsis – (wheat)
Cypsela - An achene-like fruit derived from the individual florets in a Capitulum (e.g. dandelion).
Fibrous drupe – (coconut, walnut)
Follicle – is formed from a single carpel, and opens by one suture.
Legume – (pea, bean, peanut)
Loment - a type of indehiscent legume
Nut – (hazelnut, beech, oak acorn)
Samara – (elm, ash, maple key)
Schizocarp – (carrot seed)
Silicle – (shepherd's purse)
Fruits in which part or all of the pericarp is fleshy at maturity are simple fleshy fruits. Types of fleshy, simple fruits are:
Berry – (redcurrant, gooseberry, tomato, cranberry)
Stone fruit or drupe (plum, cherry, peach, apricot, olive
An aggregate fruit or etaerio, develops from a single flower with numerous simple pistils.
Magnolia and Peony, collection of follicles developing from one flower.
Tulip Tree, collection of samaras.
Sweet gum, collection of capsules.
Sycamore, collection of achenes.
Teasel, collection of cypsellas
The pome fruits of the family Rosaceae, (including apples, pears, rosehips, and Saskatoon berry) are a syncarpous fleshy fruit, a simple fruit, developing from a half-inferior ovary.
Schizocarp fruits form from a syncarpous ovary and do not really dehisce, but split into segments with one or more seeds; they include a number of different forms from a wide range of families, carrot being an answer.
Aggregate fruits form from single flowers that have multiple carpels which are not joined together, i.e. each pistil contains one carpel. Each pistil forms a fruitlet, and collectively the fruitlets are called an etaerio. Four types of aggregate fruits include etaerios of achenes, follicles, drupelets, and berries. Ranunculaceae species, including Clematis and Ranunculus have an etaerio of achenes, Calotropis has an etaerio of follicles, and Rubus species like raspberry, have an etaerio of drupelets. Annona have Etaerio of berries.
The raspberry, whose pistils are termed drupelets because each is like a small drupe attached to the receptacle. In some bramble fruits the receptacle is elongated and part of the ripe fruit, making the blackberry an aggregate-accessory fruit. The strawberry is also an aggregate-accessory fruit, only one in which the seeds are contained in achenes. In all these examples, the fruit develops from a single flower with numerous pistils.
A multiple fruit is one formed from a cluster of flowers. Each flower produces a fruit, but these mature into a single mass. Examples are the pineapple, fig, mulberry, osage-orange, and breadfruit.
In the photograph on the right, stages of flowering and fruit development in the noni or Indian mulberry (Morinda citrifolia) can be observed on a single branch. First an inflorescence of white flowers called a head is produced. After fertilization, each flower develops into a drupe, and as the drupes expand, they become connate (merge) into a multiple fleshy fruit called a syncarpet.
To summarize common types of fleshy fruit (examples follow in the table below):
Berry – simple fruit and seeds created from a single ovary
Pepo – Berries where the skin is hardened, like cucurbits
Hesperidium – Berries with a rind and a juicy interior, like most citrus fruit
Compound fruit, which includes:
Aggregate fruit – with seeds from different ovaries of a single flower
Multiple fruit – fruits of separate flowers, merged or packed closely together
Accessory fruit – where some or all of the edible part is not generated by the ovary
Seedlessness is an important feature of some fruits of commerce. Commercial cultivars of bananas and pineapples are examples of seedless fruits. Some cultivars of citrus fruits, satsumas, mandarin oranges, table grapes, grapefruit, and watermelons are valued for their seedlessness. In some species, seedlessness is the result of parthenocarpy, where fruits set without fertilization. Parthenocarpic fruit set may or may not require pollination but most seedless citrus fruits require stimulus from pollination to produce fruit.
Seedless bananas and grapes are triploids, and seedlessness results from the abortion of the embryonic plant that is produced by fertilization, a phenomenon known as stenospermocarpy which requires normal pollination and fertilization.
Types of Vegetables
1. Root vegetables grow and are harvested below the ground. These include potatoes, carrots, onions, garlic, turnips, rutabagas and parsnips.
Brassica or Crucifer Vegetables
2. The roots, stems, leaves and flowers of these vegetables are eaten. These include broccoli, Brussels sprouts, cauliflower, cabbage, collards, mustards, cress, kale, kohlrabi and Bok choi.
3. This family of plants have alternating leaves, flowers that normally have five petals and many internal seeds. These include eggplant, tomato, capsicum peppers and belladonna.
4. Legumes are pods that have their seeds attached to half of the inside of the pod. Legumes are string beans, peas and shell or dry beans.
5. Some leafy green vegetables are eaten raw, such as lettuce, spinach, escarole, endive and radicchio. Others are cooked first, such as Swiss chard, kale, collards, mustard greens, beet greens and turnip greens.
Keeping food quality
Food quality is enforced by the Food Safety Act 1990. Members of the public complain to trading standards professionals, who submit complaint samples and also samples used to routinely monitor the food marketplace to Public Analysts. Public Analysts carry out scientific analysis on the samples to determine whether the quality is of sufficient standard.
Food quality is an important food manufacturing requirement, because food consumers are susceptible to any form of contamination that may occur during the manufacturing process. Many consumers also rely on manufacturing and processing standards, particularly to know what ingredients are present, due to dietary, nutritional requirements, or medical conditions. Besides ingredient quality, there are also sanitation requirements. It is important to ensure that the food processing environment is as clean as possible in order to produce the safest possible food for the consumer.
Food quality also deals with product traceability, e.g. of ingredient and packaging suppliers, should a recall of the product be required. It also deals with labeling issues to ensure there is correct ingredient and nutritional information.
Crop Yields; The Green Revolution
With the experience of agricultural development begun in Mexico by Norman Borlaug in 1943 judged as a success, the Rockefeller Foundation sought to spread it to other nations. The Office of Special Studies in Mexico became an informal international research institution in 1959, and in 1963 it formally became CIMMYT, The International Maize and Wheat Improvement Center.
In 1961 India was on the brink of mass famine. Borlaug was invited to India by the adviser to the Indian minister of agriculture M. S. Swaminathan. Despite bureaucratic hurdles imposed by India's grain monopolies, the Ford Foundation and Indian government collaborated to import wheat seed from CIMMYT. Punjab was selected by the Indian government to be the first site to try the new crops because of its reliable water supply and a history of agricultural success. India began its own Green Revolution program of plant breeding, irrigation development, and financing of agrochemicals.
India soon adopted IR8 - a semi-dwarf rice variety developed by the International Rice Research Institute (IRRI) that could produce more grains of rice per plant when grown with certain fertilizers and irrigation. In 1968, Indian agronomist S.K. De Datta published his findings that IR8 rice yielded about 5 tons per hectare with no fertilizer, and almost 10 tons per hectare under optimal conditions. This was 10 times the yield of traditional rice. IR8 was a success throughout Asia, and dubbed the "Miracle Rice". IR8 was also developed into Semi-dwarf IR36.
In the 1960s, rice yields in India were about two tons per hectare; by the mid-1990s, they had risen to six tons per hectare. In the 1970s rice cost about $550 a ton; in 2001, it cost under $200 a ton. India became one of the world's most successful rice producers, and is now a major rice exporter, shipping nearly 4.5 million tons in 2006.
The projects within the Green Revolution spread technologies that had already existed, but had not been widely used outside industrialized nations. These technologies included pesticides, irrigation projects, synthetic nitrogen fertilizer and improved crop varieties developed through the conventional, science-based methods available at the time.
The novel technological development of the Green Revolution was the production of novel wheat cultivars. Agronomists bred cultivars of maize, wheat, and rice that are generally referred to as HYV’s or “high-yielding varieties”. HYV’s have higher nitrogen-absorbing potential than other varieties. Since cereals that absorbed extra nitrogen would typically lodge, or fall over before harvest, semi-dwarfing genes were bred into their genomes. A Japanese dwarf wheat cultivar, which was sent to Washington, D.C. by Cecil Salmon, was instrumental in developing Green Revolution wheat cultivars. IR8, the first widely implemented HYV rice to be developed by IRRI, was created through a cross between an Indonesian variety named “Peta” and a Chinese variety named “Dee-geo-woo-gen.”
With advances in molecular genetics, the mutant genes responsible for Arabidopsis genes, wheat reduced-height genes (Rht) and a rice semi dwarf gene were cloned. These were identified as gibberellin biosynthesis genes or cellular signaling component genes. Stem growth in the mutant background is significantly reduced leading to the dwarf phenotype. Photosynthetic investment in the stem is reduced dramatically as the shorter plants are inherently more stable mechanically. Assimilates become redirected to grain production, amplifying in particular the effect of chemical fertilizers on commercial yield.
HYVs significantly outperform traditional varieties in the presence of adequate irrigation, pesticides, and fertilizers. In the absence of these inputs, traditional varieties may outperform HYVs. Therefore, several authors have challenged the apparent superiority of HYVs not only compared to the traditional varieties alone, but by contrasting the monoculture system associated with HYVs with the polycultural system associated with traditional ones.
Cereal production more than doubled in developing nations between the years 1961–1985. Yields of rice, maize, and wheat increased steadily during that period. The production increases can be attributed roughly equally to irrigation, fertilizer, and seed development, at least in the case of Asian rice.
While agricultural output increased as a result of the Green Revolution, the energy input to produce a crop has increased faster, so that the ratio of crops produced to energy input has decreased over time. Green Revolution techniques also heavily rely on chemical fertilizers, pesticides and herbicides, some of which must be developed from fossil fuels, making agriculture increasingly reliant on petroleum products. Proponents of the Peak Oil theory fear that a future decline in oil and gas production would lead to a decline in food production or even a Malthusian catastrophe.
Effects on Food Security
The effects of the Green Revolution on global food security are difficult to understand because of the complexities involved in food systems. The world population has grown by about four billion since the beginning of the Green Revolution and many believe that, without the Revolution, there would have been greater famine and malnutrition. India saw annual wheat production rise from 10 million tons in the 1960s to 73 million in 2006. The average person in the developing world consumes roughly 25% more calories per day now than before the Green Revolution. Between 1950 and 1984, as the Green Revolution transformed agriculture around the globe, world grain production increased by over 250%.
The production increases fostered by the Green Revolution are often credited with having helped to avoid widespread famine, and for feeding billions of people.
There are many claims that the Green Revolution has decreased food security for a large number of people. One claim involves the shift of subsistence-oriented cropland to cropland oriented towards production of grain for export or animal feed. For example, the Green Revolution replaced much of the land used for pulses that fed Indian peasants for wheat, which did not make up a large portion of the peasant diet.
Green Revolution agriculture relies on extensive use of pesticides, which are necessary to limit the high levels of pest damage that inevitably occur in monocropping - the practice of producing or growing one single crop over a wide area.
Industrialized agriculture with its high yield varieties are extremely water intensive. In the US, agriculture consumes 70% of all fresh water resources. For example, the Southwest uses 36% of the nation’s water while at the same time only receiving 6% of the country's rainfall. Only 60% of the water used for irrigation comes from surface water supplies. The other 40% comes from underground aquifers that are being used up in a way similar to topsoil that makes the aquifers, as Pfeiffer says, “for all intents and purposes non renewable resources.” The Ogallala Aquifer is essential to a huge portion of central and southwest plain states, but has been at annual overdrafts of 130-160% in excess of replacement. This irrigation source for America's bread basket will become entirely unproductive in another 30 years or so.
Likewise, rivers are drying up at an alarming rate. In 1997, the lower parts of China’s Yellow River were dry for a record 226 days. Over the past ten years, it has gone dry an average of 70 days a year. Famous lifelines such as the Nile and Ganges along with countless other rivers are sharing in the same fate. The Aral Sea has lost half its area and two-thirds its volume due to river diversion for cotton production.
Also the water quality is being compromised. In the Aral Sea, water salinization has wiped out all native fish, leaving an economy even more dependent on the agricultural model that originated the problem.
Fish are disappearing through another form of agricultural run off as well. When nitrogen-intensive fertilizers wash into waterways it results in an explosion of algae and other microorganisms that lead to oxygen depletion resulting in “dead zones”, killing off fish and other creatures. Over 90% of runoff incidents are linked to small-scale family farming, typically organic-based operation.
The spread of Green Revolution agriculture affected both agricultural biodiversity and wild biodiversity. There is little disagreement that the Green Revolution acted to reduce agricultural biodiversity, as it relied on just a few high-yield varieties of each crop.
This has led to concerns about the susceptibility of a food supply to pathogens that cannot be controlled by agrochemicals, as well as the permanent loss of many valuable genetic traits bred into traditional varieties over thousands of years. To address these concerns, massive seed banks such as Consultative Group on International Agricultural Research’s International Plant Genetic Resources Institute have been established.
There are varying opinions about the effect of the Green Revolution on wild biodiversity. One hypothesis speculates that by increasing production per unit of land area, agriculture will not need to expand into new, uncultivated areas to feed a growing human population. However, land degradation and soil nutrients depletion have forced farmers to clear up formerly forested areas in order to keep up with production. A counter-hypothesis speculates that biodiversity was sacrificed because traditional systems of agriculture that were displaced sometimes incorporated practices to preserve wild biodiversity, and because the Green Revolution expanded agricultural development into new areas where it was once unprofitable or too arid. For example, the development of wheat varieties tolerant to acid soil conditions with high aluminum content permitted the introduction of agriculture in the Amazonian Cerrado ecosystem in Brazil.
Nevertheless, the world community has clearly acknowledged the negative aspects of agricultural expansion as the 1992 Rio Treaty, signed by 189 nations, has generated numerous national Biodiversity Action Plans which assign significant biodiversity loss to agriculture's expansion into new domains.