फल फसलों में रंग विकास: कायिक एवं उद्यानिक परिप्रेक्ष्य
Fruit colour is one of the most reliable quality indicators for both eating and marking parameters. Fruit colour pigments are necessary for fruit attractiveness, and they tend to accumulate in the skin during the ripening process. Fruit pericarp colour is a critical quality trait that influences market value and consumer acceptance.
Fruit ripening is a complex, irreversible process that involves a succession of physiological, biochemical, and organoleptic changes that result in the production of a soft, edible ripe fruit with ideal quality characteristics. The shift in colour is caused by either the production of plant pigments or the unmasking of previously hidden colour.
Chlorophyll, a magnesium organic compound, is responsible for the colour change. The deterioration of chlorophyll structure causes the loss of green colour. Carotenoids and anthocyanins are two of the most important pigments found in fruit.
They are crucial for human health as a source of vitamin A and antioxidant chemicals, in addition to their involvement in pigmentation. Carotenoids, which are made up of carotenes derived from terpenoids, are produced in large quantities in fruit during the transition from chloroplast to chromoplast.
Anthocyanins are water-soluble flavonoids that are generated in the cytoplasm and found in vacuoles. They are formed eventually from phenylalanine. They come in a variety of colours, from orange/red to violet/blue. Except for avocado, kiwi fruit, and Granny Smith apple, most fruit crops change colour over time. Genetics and mutations, environmental conditions, crop load, plant nutrition, plant stressors, and plant growth regulators all play a role in fruit surface colour.
Degradation of chlorophyll results in the formation of yellow/orange/red/purple pigments. Only a few plant lineages have betalains, nitrogen-containing water-soluble chemicals generated from tyrosine that also impart yellow-to-red hues. All three groups of pigments serve as visual cues to attract pollinators and seed resettlement, such as insects, birds, and animals. They also protect plants from UV and visible light harm.
The accumulation of pigments is influenced by a variety of elements and signals, including light, temperature, hormones, and so on. In terms of horticultural viewpoints, various orchard management strategies were discovered to affect improved fruit colour.
Changes in pH, oxidation processes, and enzymes chlorophyllases are the primary causes of chlorophyll breakdown. Carotenoids are long-lasting pigments that last till senescence. They are either generated or concealed by the presence of chlorophyll during the developing phase.
This type of transformation can be observed in the instance of bananas. They are red-purple or blue water soluble phenolic glucosides discovered in beet root vacuoles and apple and grape epidermal cells. They produce a bright colour that frequently obscures carotenoids and chlorophyll. Anthocyanins have a red colour at acidic pH levels and a blue colour in alkaline pH levels.
This results in a phenomenon known as blueing in which the colour of a rose changes from red to blue as it ages. This is due to carbohydrate depletion and the release of free amino acids that leads in a more alkaline pH in the cell sap. With ripening, the texture of the fruit softens.
Hydrolases (poly galacturonase, pectin methyl esterase, and cellulases), which break down pectins, cellulose, and hemicellulose, are responsible for this. Propectin is a pectic molecule that binds to calcium and carbohydrates in the cell wall and is insoluble.
Propectin breaks down into lower molecular weight fractions that are more water soluble as it matures and ripens. The rate at which pectic compounds degrade is proportional to the rate at which the fruit softens. The development of sweetness in fruits during ripening is the fundamental change in flavour.
The sweetness comes from the breakdown of starch into simple sugars like glucose, fructose, and sucrose as the fruit ripens. Various enzymes such as amylase, invertase, phosphorylase, and others play a role in this transformation.
The formation of a complex mixture of volatile molecules such as ocimene and myrcene, as well as the breakdown of bitter principles, flavanoids, tannins, and related compounds, is ascribed to the rise in flavour and scent during fruit ripening.
Light, temperature, hormones, and other elements and signals all influence the accumulation of carotenoids and anthocyanin pigments. Apart from horticultural considerations, numerous orchard management procedures including as bagging, pruning, and fertilizing have been shown to have a significant impact on fruit colour pigmentation.
Fruit colour
The colour of fresh fruit is an essential indicator of its quality. It also serves to estimate the stage of fruit development, develops eating quality expectations, and promotes the fruit to consumers. Brightness, hue, and colorfulness are the three essential characteristics of colour. The achromatic aspect of colour is brightness, which refers to the amount of light absorbed by a surface.
As the amount of light reflected from an object's surface diminishes, the brightness of the object's surface falls. Red, green, and blue are examples of hues; the wavelengths of light reflected by various colour tones differ from one another. Monochromatic lights at 530 nm and 680 nm, for example, appear green and red, respectively.
Colorfulness refers to the color's purity. Above 600 nm, a bright red fruit is extremely reflecting, whilst wavelengths below 600 nm are minimally reflected.
Fruit pigments
Pigments are necessary for the attractiveness of fruits, and they collect most often in the skin throughout the ripening process, while pigments can also accumulate in the pulp tissue of certain climacteric fruits. Carotenoids and anthocyanins are the most important pigments in fruit. They are crucial for human health as a source of vitamin A and antioxidant chemicals, in addition to their involvement in pigmentation. Carotenoids include carotenes like lycopene and ß-carotene, as well as xanthophylls like lutein. They're made up of terpenoids and are produced in large quantities in fruit during the transition from chloroplast to chromoplast. Anthocyanins are a type of flavonoid that is generated from phenylalanin are soluble in water, produced in the cytoplasm, and found in vacuoles. They come in a variety of colours, from orange/red to violet/blue. They can be found all over the plant kingdom. In grapes, ethylene (or ethylene generator ethephon) promotes berry coloration, which has been linked to anthocyanin biosynthesis regulation. Betalains are nitrogen-containing water-soluble chemicals generated from tyrosine that are exclusively found in a few plant lineages and give them a yellow-to-red colour. Insects, birds, and animals are attracted to all three groups of colours, which aid in pollination and seed dissemination. They also protect plants from UV and visible light harm.
Fruit colour pigments have health advantages.
Anthocyanins: Treatment of visual issues and brain protection
Betalains: Antiviral and antibacterial characteristics and normal body metabolism
Carotenoids: Protection from photooxidative damage, immune system booster, suppression of cardiovascular disease, and cancer chemoprevention
Carotenoids
Carotenoids are either carotenes (hydrocarbons) or xanthophylls, and they give plant tissues a yellow-red colour (derived from carotenes with additional oxidation). Fruit carotenoids have a wide range of structures, including some that are unusual. Apocarotenoids (e.g., citraurin, citranaxanthin) are found in citrus fruits, for example.
Only a limited number of carotenoids contribute to the colour of many fruits (e.g., lycopene in tomato, cryptoxanthin, and zeaxanthin in mango and persimmon, capsanthin and capsorubin in capsicum). Fruit peels contain more carotenoids than fruit pulp, and the carotenoid content of some fruits (such as tomato and peach) increases after harvest.
Carotenoids are formed in chromoplasts, which are derived from chloroplasts after chlorophyll breakdown, during the ripening of tomatoes, oranges, mangoes, and other fruits.
However, carotenoid synthesis may occur even before the onset of chlorophyll disappearance in some fruits (e.g., grapefruit), whereas fruit maturity at high temperatures is not associated with a higher carotenoid concentration in some early orange or mandarin varieties because chlorophyll degradation is inhibited.
Environmental factors influence carotenoid formation in ripening fruits both before and after harvest. Temperature, light, and the oxygen concentration in the atmosphere are among them.
Anthocyanins
Anthocyanins, the most abundant category of water-soluble pigments in plant tissues, are phenolic compounds with a wide range of chemical structures that are found within the plant cell's vacuole and produce colours ranging from red to blue-purple to black.
They're mostly found in mature epidermal cells (apple, apricot, fig, nectarine, peach, pear, plum, and pomegranate), although they're also found in the flesh (e.g., apple, blackberry, blueberry, cherry, cranberry, red- and black-currant, fig, grape, peach, plum, pomegranate, olive, Sanguine, orange, raspberry, strawberry).
Anthocyanin occurrence and accumulation differ depending on the fruit species, cultivar, tissue structure, geographical location, fruit position on the tree, and cultivation conditions. Nasunin (delphinidin 3-p-coumarylrhamnoside) is the main anthocyanin found in the peel of purple eggplants but not white or green eggplants. It has been isolated in crystallised form and possesses potent antioxidant properties.
Fruit pigmentation biochemical aspects
Fruit colour pigmentation is connected with fruit maturity and ripening, where ripening is a genetically regulated stage of development that overlaps with senescence. Fruit is said to be ripe when it has reached its full flavour, aroma, and other qualities of the best fruit of that cultivar.
When describing these fruits that ripen on non-climacteric plants, the words "mature" and "ripe" are essentially equivalent. However, in the case of climacteric fruits, a mature fruit will need to ripen before reaching a good degree of edibility.
Degradation of chlorophyll and enzymatic change
The most visible change that occurs when fruit matures is the degradation of chlorophyll, which is accompanied by the production of other pigments, which are mainly anthocyanins or carotenoids. Key enzymes for the synthesis of anthocyanins are phenylalanine ammonia lyase (PAL) and flavone synthase.
Fruits can be categorized as follows based on the mechanism of colour change and pigment composition at ripeness:
- Fruits that have lost all of their chlorophylls, revealing previously created carotenes and xanthophylls and usually have a distinctive yellow colour. (e.g., banana, plantain, lemon).
- Fruits with marked de novo biosynthesis of carotenoids, referred to as carotenogenic fruits (e.g., orange, persimmon, red pepper).
- Fruits with marked de novo biosynthesis of anthocyanins (e.g., grape, apple, olive, pomegranate, red cherry, raspberry, cranberry).
- Fruits that retain chlorophyll during ripening (e.g., some cultivars of kiwi and avocado).
As a result, the most noticeable alteration that happens in many fruits is colour. The most noticeable change is the loss of green colour. The breakdown of the chlorophyll structure is responsible for the loss of green colour. pH shifts (owing to organic acid leaking from the vacuole), oxidative processes, and chlorophyllases are the primary agents responsible for this destruction.
The loss of chlorophyll is linked to the synthesis or revelation of pigments ranging from yellow to crimson. Aside from that, many of the chemical and physical effects of pigmentation are linked to enzyme action.
Fruit softening during ripening is linked to an increase in pectic esterase and poly galactouronase activity. Furthermore, the activity of oxidative enzymes, glycolytic enzymes, hydrolytic enzymes, invertase, transaminase, and chlorophyllase increased with the ripening of most fruits.
Guadarrama and Andrade reported enzyme alterations in Sweetsop (Annona squamosa L.) and Golden Apple (Spondiascitherea Sonner). The major enzymes in fruit softening were pectin methyl esterase and polygalacturonase.
The results showed that the trends of pectin methyl esterase and polygalcturonase enzyme activities follow the same pattern in both fruit species at all stages of ripening, while sweetsop fruits always had higher activities.
The strength of the fruit decreases as it ripens due to structural and compositional changes in the cell walls caused by enzymatic hydrolysis of cellulose components, pectic acid, and polygalacturonic.
Organic acids and flavoring agents
Non-volatile organic acids are among the most important cellular elements that alter during fruit colour. Because of the conversion of fruit to sugar, the acidity decreases significantly during ripening. Acids can be thought of as a reserve source of energy for the fruit, and as such, they are expected to fall during the increased metabolic activity that occurs during ripening. Banana and pine apple are outliers, as they have the highest degree of acid when fully ripe.
The decrease in organic acid level during fruit ripening could be due to an increase in membrane permeability, which permits acid to be stored in respiring cells. Aroma is vital in the development of excellent fruit eating quality. Esters of aliphatic alcohols and short chain fatty acids are the most common chemical compounds discovered.
When fermentation occurs in ripening fruit, alcohol and esters are produced. Reductive deamination of amino acids can result in the formation of branched chain alcohols. Oxidation is thought to have produced aldehydes and ketones from alcohols.
Factors affecting fruit color
There are some primary factors affecting red color development in fruit crop skin
Genetics
Some cultivars or strains of some cultivars lack the ability to synthesize large quantities of anthocyanin. In litchi Green cultivar and Golden Delicious and Granny Smith are examples of cultivars that develop little red color.
Light and temperature
If light is the factor limiting red color development, then exposing fruit to additional light during the final two or three weeks before harvest will often enhance color development of apple and peach and this can be done by summer pruning to eliminate upright non-fruiting shoots that shade the canopy interior or by placing reflective mulch under the trees.
The other primary environmental factor that can enhance late-season color development is exposure to low temperatures, but the optimum temperature and time of exposure depends on the cultivar and critical values have not been identified for most cultivars. Typically several cool nights followed by warm sunny days will enhance color development.
Tree nutrition
Some growers think they can enhance red color development by improving the nutritional status of the tree, but most inorganic elements have little effect on color, especially if temperature and light are not suitable for color development. High late-season nitrogen levels cause phenylalanine to be converted to proteins rather than anthocyanin, so high late season nitrogen levels should be avoided.
Adequate levels of potassium are needed for good color development. So if trees are deficient is potassium, applying potassium may enhance color development. If nutritional levels are adequate for good tree growth and fruiting, then the addition of any element will likely not enhance color development.
Crop load
Trees with heavy crops often have poorly colored fruit. The reason for this is likely that sugar levels in the fruit are low.
Stress
Most types of stress, such as water stress or damage from leaf-feeding insects, can reduce photosynthesis resulting in poor red color development. These stresses become more problematic when trees are carrying heavy crops because increased demand for photosynthetic assimilates accompanies the reduced capability of the tree to produce those assimilates.
Authors:
Ankit Kumar Pandey1 and Sakshi Shastri2
1Department of Horticulture (Fruit & Fruit Technology), BAU, Sabour, Bhagalpur (Bihar)
2Department of Agril. Extension, Indira Gandhi KrishiVishwavidyalay, Raipur (Chhattisgarh)
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