The Use Of Plant Growth Regulators On Citrus Fruits
Introduction
South Africa is an agricultural country with many hectares of land undergoing cultivation (Asami and Nakagawa, 2018). Climate, topography and soil differs from region to region and although multiple and intense cropping systems are used and proper irrigation tools are applied, but still less crop yield produced (Chanclud and Morel, 2016). About a decade ago scientists figured out that they could change the way some plants grow by applying hormones (William, 1948). Hormones are naturally produced chemicals synthesized in one part of the plant and then move to another part where they affects development and growth (Vanangamudi et al. , 2017). Plant hormones have been more studied for their roles in regulation of different aspects of plant development. However, in the past ten years crucial new insights have been made into their action during ripening and development, in both fleshy and dry fruits (Asami and Nakagawa, 2018).
The Plant Growth Regulators PGRs are grouped into five general compounds according to their effects on plants and chemical structures. The groups are Gibberellins, Auxins, Ethylene, Cytokinins and contains a group called inhibitors that includes Phenolics, Abscisic acids and Alkaloids (William, 1948). We are only going to focus on Auxins, Gibberellins, Ethylene, Cytokinins and Abscisic acid. Some new hormones do not fit well into these classifications but are described as having effects that have a similar to those known Plant Growth Regulators (Chanclud and Morel, 2016).
The use of plant growth regulators by the agricultural industry on Citrus fruits. Citrus is a genus of flowering shrubs and trees in the rue family Rutaceae, Scientific name Citrus and the Order is Sapindales (Asami and Nakagawa, 2018). Citrus are usually charecterized by a leathery and white pith that covers juicy parts, native to New Guinea, Australia, New Caledonian and possibly Southeast Asia (Vanangamudi, 2017). At the present time they are cultivated in tropical and subtropical climates all over around the world (Vanangamudi et al. , 2017). Large production hubs include Brazil, Spain, China, Mexico, the US and India, nearly a third of all citrus fruits are used to make juice (William, 1948). All kinds of citrus fruits are found all year round, peak season for grape fruits and oranges in the Northern Hemisphere is between April and mid-December (Asami and Nakagawa, 2018).
There is only three species of fruits which are pummelo, citron and mandarin orange (Chanclud and Morel. 2016). Most of the other citrus fruits especially those we see in famer’s markets and stores today are actually the products of crossing between the original species, including lemons, limes and common sweet oranges (William, 1948). Citrus fruits are an excellent source of vitamin C (Vanangamudi et al. , 2017). Now we are going to look at how plant hormones are used in Citrus fruits by the agricultural industry.
Body
Auxins are the plant hormones that are produced at the tip of the stem which promotes cell elongation and inhibits growth of lateral buds (Guardiola, 1988). Auxins are specific group of hormones for plant growth, they are not only critical to plant growth but they were discovered as a first class of plant hormones (Vanangamudi et al. , 2017). Frits Went, a Dutch graduates student initially described auxins in 1926 and named auxin from a Greek word ‘auxien’ which means ‘to grow’ (Guardiola, 1988).
Auxins move to the darker side of the plants which causes cells to grow bigger than corresponding cells on the lighter side of the plant (William, 1948). A curving of a plant tip towards light is plant movement known as phototropism, the stem growing towards light is positive tropism and stem growing away from light is negative tropism (William, 1948). Geotropism is growth of roots down into the ground in the direction of gravity and called positively geotropic, negatively geotropic when the stem shows (Asami and Nakagawa, 2018). There is also Thigmotropism which is growth response to touch and Hydrotropism which is growth response to plant (William, 1948). Auxins play a very crucial role in maintain apical dominance, many plants have lateral buds located at the nodes (Kumar, Khurana and Sharma, 2014). The embryonic meristems meristems maintained in a dormant state are called buds and auxins maintain this dormancy. Lateral buds remain dormant as long as sufficient auxin is produced by the apical meristem, the auxin is longer produced if the apex of the shoot is removed, which causes lateral buds to break their dormancy and then begin to grow (Vanangarumudi et al. , 2017).
Agricultural use of auxins (IBA, NAA and 2,4-D), citrus fruits like lemon for instance. Lemons vary from other citrus in that they may be subjected to lengthy packinghouse storage (Kumar, Khurana and Sharma, 2014). Post-harvest application of isopropyl ester of 2,4-D may be applied as an waxing or washing procedure just prior to the storage period, this treatment will slightly delay the loss of chlorophyll (Chanclud and Morel, 2016 ). NAA is thought to improve early fruit drop by increasing the competition between fruitlets and increasing production of abscission-inducing ethylene, which both encourages greater physiological drop usually in May (Amunulla, Sekar and Vincent, 2013). Gibberellins are plant hormones that control different development processes, including germination, stem elongation, flowering, dormancy, leaf and fruit senescence and flower development (Tan et al. , 2017).
They are one of the longest classes of plant hormones produced in the plant cell’s plastids, and transferred to the endoplasmic reticulum of the cell where they are being modified and getting ready for use (Duan et al. , 2013). Gibberellins affect the plant growth and development of seeds and fruits and distributed evenly throughout the plant tissue (Tan et al. , 2017). They are used to increase fruit size, stimulate flowering and have an important effects on the stem growth (Hamooh, 2014). Once a plant produces gibberellins, its cell start a process of elongation. As plants a made of single cells stacked on top of another, elongation of thousand single cells results in the overall growth of the plant (Mumtazkhan et al. , 2008). Gibberellins moves both directions freely in plant as they are not only produced in shoot apex but also in the root structure (Hamooh, 2014). Gibberellins are used in agriculture to induce flowering, stimulating seed germination and seedling growth, stimulates growth by increasing cell numbers and size (Mumtazkhan et al. , 2008).
Timely spray of GA3, when the target crop is 0. 5 to 0. 75 full size and still green, this will delay fruit maturity of citrus like lemon (Duan et al. , 2013). GA3 may decrease bloom and fruit set the spring following treatment, an increased fruit set during the next summer also may happen. Therefore application of GA3 may result in a delay in harvest of fruit which allows larger fruit size (Hamooh, 2014). Gibberellic acid are used commercially to increase fruit size and also to regulate citrus flowering and rind maturation (Tan et al. , 2017). Cytokinins is a type of plant hormone that promotes cytokinesis in shoots and plant roots (Wang et al. , 2013). They are also found in fungi, mosses and bacteria. In the meristem of the roots is where most of the cytokinins are being produced (Mishra et al. , 1996). A region of tissue in plant that promotes cell division, when the cytokinin has been produced in the roots it moves through the xylem, to plant parts where continued growth occurs (Wang et al. , 2013). Cytokinins raise up the division of cells by stimulating the production of proteins required for mitosis and they delay senescence (Parthasarathy et al. , 2002). During spring as roots start to grow actively, large amounts of cytokinins are transported to the shoots, where they make dormant buds to expand and become active (Mishra et al. , 1996).
When cytokinins are combined with auxins they stimulate cell division and differentiation. In agriculture, cytokinins are used reduce fruit abscission in citrus therefore promoting cell division and enlargement. Application of callous tissue and it will begin to make shoots (Parthasarathy et al. , 2002). Farmers also use cytokinins to develop resistance to pathogens and high temperatures, and keep the citrus fruit healthy and fresh for many days (Wang et al. , 2013). Abscisic acid is hormone that occurs in several developmental plant processes, like environmental stress response and dormancy (Aung, Houck and Norman, 1991). It is produced in the terminal buds at the peak of the plant as well as in roots, slows growth and inhibits cell division (Suzuki et al. , 1975).
During winter primordial leaves develop into scales and protect the apical bud and it keeps the seeds dormant (Rodgers, 1980). It is involved in many plant functions. Stomata is an opening at the bottom side of plant leaves and its function is to regulate water content and control carbon dioxide (Suzuki et al. , 1975). Abscisic acid assist in the closing of the stomata when the plant does not need too much carbon dioxide and when the plant cannot lose too much water during the dry season (Rodgers, 1980). Abscisic acid has one of the important functions which is inhibits seed germination once the seed is put in the soil immediately stop the seed germinating (Aung, Houck and Norman, 1991).
Abscisic acid causes the seeds to wait until the conditions are favourable before it grows, this allows good growth and successful reproduction (Suzuki et al. , 1975). Abscisic acid content varies in fruit of different citrus species. In agriculture abscisic acid is used to introduce flowering during short day plants kept below un-favourable photoperiod. In citrus fruits abscisic acid encourages rooting and prolonging of dormancy of buds a seeds (Aung, Houck and Norman, 1991). Ethylene is a naturally occurring small hydrocarbon gas and promotes the ripening of the fruit and involved in leaf abscission (Fuchs, 1970). Ethylene is colourless flammable gas may occur as a result of combustion, oppose some auxin effects inhibits growth and development of roots, flowers and leaves (Yang, 1967).
In ripening of citrus fruits no full agreement has been made concerning the role played by ethylene and at some fruit age ethylene is produced by citrus fruit. This means that there might be an ethylene-producing system in a citrus fruit that is inhibited below normal ripening conditions (Aharoni, 1968). Citrus fruits are said to be non-climacteric but being exposed to exogenous ethylene like ethylene de-greening they tend to stimulates many ripening-related processes, it has been yet found whether exogenous ethylene has impacts internal ripening process in citrus flesh (Aharoni, 1968). In citrus fruits, ethylene enhanced peel colour break, also respiration and production of ethylene (Fuchs, 1970).
Conclusion
In conclusion, using plant growth hormones in agriculture for citrus is highly recommended since they can be used to regulate and control growth of citrus. In general, hormones may be used get rid of dormancy of a seed which allows it to germinate all year round. Plant Growth Hormones plays a huge role in agriculture and food production, without plant hormones lower yields and poor quality can be produced.
