Date palm Phoenix dactylifera L. is known as tree of life plays an imperative role in the life of the people living in scorched regions of the world. It is an ancient and valuable fruit, as the date's name has been and is mentioned in holy books like the Quran, Torah and Buddha (Belarbi et al., 2000; Falade and Abbo, 2007). Prophet Muhammad (PBUH) is reported to have said that the best assets is the date palm, that it is used for curing many disorders, and He (PBUH) urged Muslims to consume dates (Zaid and de Wet, 1999). Muslims deem it as a virtue to eat dates at 'Iftar' in the month of 'Ramadan'. It has been cultivated in the Middle East since at least 6000 BC (Al-Qarawi et al., 2003). At least 2000 or more different cultivars of date palm exist all over the world (Ali-Mohamed and Khamis, 2004). The Date fruit provides a good source of carbohydrates, fibre, minerals, and vitamins, but it contains a minute amount of fat and protein (Baloch et al., 2006; Al-Farsi et al., 2005; Mohamed, 2000). Date fruit is also suitable for hypertensive persons because of its high amount of potassium and low content of sodium (Al-Hooti et al., 2002). New studies have reported that date fruit has antimutagenic and anticancer action (Ishurd and Kennedy, 2005; Vayalill, 2002).
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The ripening of the date fruit is considered by four different stages/phases based on color, softness, moisture, and sugar content. (1) Kimri stage: at this phase the fruit is quite hard, the color is green and it is not fit for edible purpose. At this phase the fruit attains a rapid increase in weight, volume and build up of reducing sugars that will weaken at the end of this stage. (2) Khalal stage: at this stage, date fruit gains its maximum weight and size. The date's total sugar and acidity will boost as its water content decreases. At the end of Khalal stage, the fruit is physiologically mature and hard. (3) Rutab stage: ripening of date fruit starts at this stage. Its colour changes and its texture become soft. It begins to lose astringency and starts acquiring a darker and less attractive colour than the previous stage. (4) Tamar stage: the date fruits are fully ripe at Tamar stage and texture of the flesh is soft. At this stage, date fruit contains its maximum total solids and it is in the best condition for storage. Some references have mentioned another a very early stage named called "Hababauk". This term is used for the female flower and also used for the period after pollination, in which the young fruit is still creamy before gradually turning green at the Kimri stage. Generally, date fruit is harvestable and marketable at three stages including Khalal, Rutab and Tamar that depend on cultivar characteristics, especially soluble tannins levels (Ismail et al., 2001; Hong et al., 2006; Awad, 2007).
Although there are many cultivars of dates, some have become pre eminent in the world market (Krueger, 2001; Nixon, 1950). 'Deglet Noor', meaning "date of the light" in Arabic, comes from the Algerian Sahara and is one of the leading cultivars grown in North Africa and California. 'Halawy', meaning sweet in Arabic, is a soft, high-quality date with rich flavour from Iraq. 'Khadrawy', meaning green in Arabic, is a short and moderately productive tree with soft fruit from Southern Iraq.
Climate is one of the major factors that affect all aspects of life and realistic crops cultivation depends on proper understanding of climatic condition. Proper understanding of climatic condition can help farmers in doing cultivation at an opportune time and supplying plant's needs during growth period. The ripening season of date palm starts with the rise in summer temperature with July and August, which is the peak production period. Unfortunately, the monsoon rains also falls within these months of the year, which is a real bottleneck for this crop. Hillawi and Khadrawi are the major varieties cultivated in Punjab. These varieties are semi dry and ripened early in July and face a huge problem of monsoon rains. The coincidence of date ripening period with the monsoon season means the crop receives heavy damage by rain and a few minutes of rain can destroy upto 80% of the date crop (ASF, 2010) The fruit during this period is at eatable (Khalal/Rutab) stage and prone to infestation by insects/birds and diseases that invade at a rapid rate under the favourable climate of relatively reduced temperature with high humidity. This adverse situation persists for several weeks. The extent of the losses contributes to accumulate so long as the fruits stay on the trees for want of Dong formation until the end of July (Saleem et al., 2005). Rain and high humidity may cause physical damage to the fruit in period preceding the ripening. When this happens, cracks appear on the fruit surface through fungi and bacteria may enter causing fermentation and souring of the fruit rapidly (Olin, 2002). It is worth mentioning that the amount of any particular rain is of less importance than the conditions under which it occurs (Nixon and Carpenter, 1978).
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Traditional methods of date fruit ripening/curing are popular in many areas of the country, in which fruit at the Dong stage are spread on the mats or plastic sheets and exposed to sun in an open air. The quality of sun dried product under dusty condition becomes very poor and non-uniform with a low yield. Due to persistent rain and stormy conditions a large amount of the harvested dates become mouldy, fermented, and dusty, damaged by the birds and insects.
The present study will be therefore conducted on the ripening aspects and fruit quality of date palm with respect to monsoon rains, by evaluating the potential of preharvest ethephon application on ripening enhancement and fruit quality of date palm at Kimri and Khalal stages, studying the role of different chemicals on the ripening behaviour and fruit quality and ripening and quality assessment of date fruit will also investigated by the influence of hot water treatment by harvesting the fruit at the physiological maturity (Doka/Khalal) stage.
V) REVIEW OF LITERATURE:
Fruits should be harvested at the correct physiological maturity and state of ripeness (Harman and Patterson, 1984). They are self sufficient with their own catalytic machinery to maintain an independent life, even when detached from the parent plant. Based on their respiratory pattern and ethylene biosynthesis during ripening, harvested fruits have been classified as climacteric and non-climacteric based on the respiration pattern and ethylene production during ripening. Climacteric fruits, harvested at physiological maturity, can be ripened off the parent plant. The respiration rate and ethylene formation though minimal at maturity, raise dramatically to a climacteric peak, at the onset of ripening, after which it declines (Gamage and Rehman, 1999). In climacteric fruit, onset of ripening is accompanied by a sharp increase in respiration and ethylene production. The climacteric ethylene is thought to regulate fruit ripening by inducing the expression of many ripening-related genes responsible for autocatalytic ethylene production, cellwall metabolism, chlorophyll degradation, synthesis of carotenoids and volatiles, and conversion of starch to sugar (Gray et al., 1992; Theologis, 1993; Alexander and Grierson, 2002). Non-climacteric fruits are not capable of continuing their ripening process, once they are detached from the parent plant. Also, these fruits produce a very small quantity of endogenous ethylene, and do not respond to external ethylene treatment. Such fruits show comparatively low profile and a gradual decline in their respiration pattern and ethylene production, throughout the ripening process (Gamage and Rehman, 1999). In non-climacteric fruit, there is no dramatic change in the rate of respiration, and ethylene production remains at a very low level. However, in some plant species, some aspects of ripening, such as chlorophyll degradation and fruit softening, are controlled or at least partially controlled by ethylene (Goldschmidt et al., 1993; Wills and Kim, 1995). Ethylene is biosynthesized from methionine via a welldefined pathway in which 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) and ACC oxidase (ACO) function as key enzymes.
Ripening is defined as changes that "occur from the latter stages of growth and development through the early stages of senescence and result in characteristic aesthetic and/or food quality" (Watada et al., 1984). It is a highly co-ordinated, genetically programmed, and an irreversible phenomenon involving a series of physiological, biochemical, and organoleptic changes that lead to the development of a soft and edible ripe fruit with desirable quality attributes. A wide spectrum of biochemical changes such as increased respiration, chlorophyll degradation, biosynthesis of carotenoids, anthocyanins, essential oils, and flavor and aroma components, increased activity of cell wall-degrading enzymes, and a transient increase in ethylene production are some of the major changes involved during fruit ripening (Brady, 1987).
The color change during fruit ripening is due to the unmasking of previously present pigments by degradation of chlorophyll and dismantling of the photosynthetic apparatus and synthesis of different types of anthocyanins and their accumulation in vacuoles, and accumulation of carotenoids such as Î²-carotene, xanthophyll esters, xanthophylls, and lycopene (Tucker and Grierson, 1987; Lizada, 1993). The increase in flavor and aroma during fruit ripening is attributed to the production of a complex mixture of volatile compounds such as ocimene and myrcene (Lizada, 1993), and degradation of bitter principles, flavanoids, tannins, and related compounds (Tucker and Grierson, 1987).
The taste development is due to a general increase in sweetness, which is the result of increased gluconeogenesis, hydrolysis of polysaccharides, especially starch, decreased acid-ity, and accumulation of sugars and organic acids resulting in an excellent sugar/acid blend (Lizada, 1993; Grierson, Tucker, and Robertson, 1981; Selvaraj, Kumar, and Pal, 1989). The metabolic changes during fruit ripening include increase in biosynthesis and evolution of the ripening hormone, ethylene (Yang and Hoffman, 1984), increase in respiration mediated by mitochondrial enzymes, especially oxidases and de novo synthesis of enzymes catalyzing ripening specific changes (Tucker and Grierson, 1987). Alteration of cell structure involves changes in cell wall thickness, permeability of plasma membrane, hydration of cell wall, decrease in the structural integrity, and increase in intracellular spaces (Redgwell, MacRae, Hallet, Fischer, Perry, and Harker, 1997). Fruit softening is associated with cell wall disassembly (Seymour and Gross, 1996) and modifications to the pectin fraction are some of the most apparent changes that take place in the cell wall during ripening (Marin-Rodriguez, Orchard, and Seymour, 2002). The general observation is that softening is accompanied by solubilization of pectin, involving the action of enzymes pectinesterase (PE), polygalacturonase (PG) and pectate lyases (PL) (White, 2002) and hydrolysis of starch and other storage polysaccharides (Selvaraj et al., 1989; Fuchs, Pesis and Zauberman, 1980). This notion was supported by reports of changes in cell wall pectic material in ripening mango (Roe and Bruemmer, 1981), tomato (Besford and Hobson, 1972) and pear (Ahmed and Labavitch, 1980).
Fruit ripening and ethylene:
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The gaseous hormone ethylene regulates a number of plant growth and developmental processes, including fruit ripening. Ethylene plays a major role in fruit ripening in a wide range of plant species (Abeles et al., 1992; Lelievre et al., 1997; Giovannoni, 2004). It is a fruit ripening phytohormone, in minute amounts can trigger many events of cell metabolism including initiation of ripening and senescence, particularly in a climacteric fruit. A number of reviews have been published on the role of ethylene in fruit ripening, particularly in mangoes as well as its biogenesis (Adams and Yang, 1979; Kende, 1993). The pathway for ethylene biosynthesis has been elucidated in apple, and other fruits such as avocado, banana, and tomato (Kende, 1993; Yang and Hoffman, 1984). The first step is the conversion of S-adenosylmethionine (SAM) to 1- aminocyclopropane carboxylic acid (ACC) by the enzyme ACC synthase. At the onset of fruit ripening, expression of multiple ACC synthase genes are activated, resulting in increased production of ACC. In most cases, it is the ACC synthase activity, which determines the rate of ethylene biosynthesis. ACC is then oxidized to ethylene by ACC oxidase.
Potential of ethephon in fruit ripening:
Fruit ripening has been described as an oxidative phenomenon that requires a turnover of active oxygen species, such as H2O2 and superoxide anion (Jimenez et al, 2002). Ethephon (2-chloroethylphosphonic acid), releases ethylene in plant tissues, has aroused interest because of its physiological effects in many fruit species (Cooke and Randall, 1968; Warner and Leopold, 1967). The responses to ethephon appear to be mainly caused by the action of ethylene (Anderson, 1968). Now ethephon is widely used in agriculture for promotion of flowering, fruit ripening, defoliation and so on. Orange, grapefruit, tangerine and green lemon fruits dipped in ethephon solutions for a few seconds to several minutes developed satisfactory marketable color in seven to ten days after treatment. (Fuch and Cohen, 1969; Yong et al., 1970). In pomegranates, however pre-harvest application of ethephon decreased the soluble solids, pH and vitamin C content of the fruit juice (Shaybany and Sharifi, 1973).
Rouhani and Bassiri (1977) reported that when date fruits were treated with 0, 125, 250, 500, 1000 or 2000 ppm ethephon and stored in sealed bags or bags with 10 holes. The percentage dry weights of pulp and seed, titratable acidity, soluble solids and respiration rates increased, whereas pH, firmness and astringency decreased with greater maturity. The application of ethephon increased respiration and titratable acidity significantly. Although ethephon concentration and bag type affected ripening, their effects were relatively small; endogenous factors controlling ripening were more important. Ethephon application at 1500 ppm shortly after full bloom on 'Zaghloul' and 'Samani' date palms grown in Egypt advanced fruit ripening by about one month (Kamal, 1995). Musa (2001) reported that effectiveness of ethrel in enhancing the fruit ripening of 'Mishrigi Wad Khatib' and 'Mishrigi Wad Lagi' dates grown in Khartoum, Sudan, was 2-3 fold higher by injecting 2ml of ethrel (480 g/l a.v.) into a pit made in the peduncle compared with 1000 ppm of ethrel spraying over the fruit.
Preharvest ethrel application significantly increased the Rutab fruit yield per bunch (7 kg) as compared to the control (4.5 kg) and postharvest dipping of fruit at khalal stage in ethrel at 4.2ml/l and abscisic acid at 1.0 mM significantly enhanced the ripening, compared to the control (Awad, 2007). Ethephon accelerates ripening and improves the peel color of the mangoes (Lakshminarayana et al., 1975). Mixture of ethephon, sodium hydro oxide and water, kept in the vicinity of mango fruit, facilitate the ripening in natural way (Sudhakar, 2006). Nair and Singh (2003) reported that fruit quality in terms of TSS, TSS/acid ratio, sugars and eating quality of mango cv. Bocado was found to improve with ethrel at 2000 ppm. Increase in salt concentration progressively increased the Dong/Rutab formation of khadrawi and shamran date cultivars, and addition of acetic acid enhanced the effect, but acetic acid alone was ineffective (Kalra et al., 1977).
Ethephon treatment stimulated the decrement of titratable acidity, anthocyanin accumulation and fruit softening four days after treatment in rabbiteye blueberry. The ripening promotion effect of ethephon on total soluble solids content was observed only eight days after treatment. Ethephon treatment did not affect the fruit enlargement during the investigation period. They concluded that ethephon application for rabbiteye blueberry promote the fruit ripening, but the stimulatory effects of ethephon on fruit ripening were different in degree on each ripening characters (Ban et al., 2007). The stimulatory effect of ethephon on blueberry fruit ripening has been reported by some researchers (Eck, 1970; Forsyth et al., 1977; Lewis and Ju, 1993; Warren et al., 1973). The skin color enhancement effect of ethephon has been noted for apple and cranberry (Eck, 1972; Murphey and Dilley, 1988). In 'Jonagold' apple, ethephon application stimulated the anthocyanin accumulation in the skin, but did not affect the total soluble solids content, acidity and fruit firmness (Awad and Jager, 2002). From these results, it is concluded that ethephon application for fruit promotes ripening, but the stimulatory effect of ethephon on fruit ripening differs in degree for each fruit ripening character. Ethylene released by the breakdown of EthrelÂ® is the cause of softening of fruit and hastens the onset of ripening of several fruits, including mango, as reported by researchers (Rupinder, Poorinima, Pathak, Singh and Dwivedi, 2007).
Role of hot water, sodium chloride and acetic acid in fruit ripening:
Recently, wide international interest in heat treatment for quality maintenance and disease control has reflected in a range of literatures. With exposure of fresh agricultural commodities to high temperature, heat shock proteins transcripts and protein levels in such commodities have been shown to increase (Lurie, 1998). Further more, a wide range of fruit ripening processes are affected by heat, such as color (Cheng et al, 1988; Tian et al., 1996), ethylene synthesis (Ketsa et al., 1999), respiration (Inaba and Chachin, 1988), fruit softening and cell wall metabolism (Lurie and Nussinovich, 1996), volatile production. Postharvest heat treatment also can reduce chilling injury in many kinds of fruits during subsequent low temperature storage as well as reduce pathogen level and disease development. Agricultural commodities are large and respond differently by applied heat treatment. Inappropriate heat treatment can also lead to ripening acceleration or heat damage (McDonald et al., 1999; Lurie, 1997).
The influence of hot water treatment on the ripening/curing of Dhakki dates with 70 oC performed better than 35 and 93 oC furnishing with 55% product yield of acceptable quality. The yield of improved quality product is further increased to 70% on the optimization of treatment time to 3 minutes. They concluded that Dhakki dates does not require to stay on tree beyond fully mature doka stage for want of dong formation and hence saves at least two weeks hang-on period Saleem et al. (2004). When fruits of Khadrawi and Shamran treated with 2% NaCl alone achieved 72 and 75% ripening by weight, similar studies were also conducted on Khadrawi, Shamran, Zaidi and Thoory date varieties at the doka stage. Sodium chloride (0.5-3.0%), actic acid (0.5-2.0%), or sodium chloride at 1.0% + acetic acid. With Khadrawi and Shamran increased concentration of NaCl resulted in a progressive increase in the ripening percentage of fruits (Kalra and Jawand, 1974; Kalra et al., 1977). Shamshiri and Rahemi (1999) reported that sodium chloride and acetic acid either separately or combined, significantly increased the TSS and reduced fruit firmness and moisture content. Acetic acid at 2% had a better effect on fruit ripening and sodium chloride, but the fruits with sodium chloride were better in appearance. Mirza and Meraj-ud-Din (1988) treated the fruits of Dhakki and Basra cultivars in doka stage with 3% brine solution, 0.25% acetic acid solution and 0.25% citric acid solution for five minutes and sulfuring them for five hours. Different chemical treatments significantly enhanced the ripening percentage of fruits, brine solution was found to be superior ripening agent.