A review on:
Major problems, control efforts and achievements in Leafy vegetables
Santosh Neupane ; Department of Horticulture, Institute of Agriculture and Animal Science, T.U.
Abstract
Various works of literature were reviewed to find out the major problems, control efforts and achievements in the leafy vegetables which is a wide group of horticultural plants cultivated for the edible part constituted of foliar structures, comprising lamina, petiole, midrib and veins, world-widely known for high nutrition and low-fat content. Leafy vegetables are subjected to rapid postharvest water loss because of their large surface area to volume ratio. The high amount of moisture loss presumes wilting that leads to freshness loss and water stress in the leaf that leads to yellowing acceleration. This effect of rapid water loss could be reduced using various anti-Ethylene plant growth regulators, using modified atmospheric packaging, lowering the storage temperature, providing light treatment during storage and providing heat-shock treatment before storage. Another major problem in the leafy vegetable is associated higher Nitrate level, which may lead factors to infant methemoglobinemia and the possible formation of carcinogenic nitrosamines. Vegetables supplied with organic fertilizers have low nitrate content, compared with minerally fertilized.
Introduction
Vegetables are commonly grouped according to the edible part as root, stem, leaf, immature flower bud, fruit, and sprout. Leafy vegetables are a wide group of horticultural plants that can be defined as vegetables cultivated for the edible part constituted of foliar structures, comprising lamina, petiole, midrib and veins (Alvino & Barbieri, 2015).
Green leafy vegetables are one of the widest categories of vegetables all over the world, with high nutritional quality and low-fat content (Koukounaras, 2009). The most important representatives are lettuce, spinach, cabbage, rocket, parsley, dill and coriander. Green leafy vegetables are most usually used in fresh salads.
Broadleaf mustard (Brassica juncea) is the most widely cultivated leafy vegetable in Nepal (Paudel, Pathak, Lamichhane, & Gauchan, 2018). It is a winter crop from Terai to the mid-hills whereas it is a summer crop for the high hills. It is rich in Vitamin A, B, C, E, iron, calcium and protein.
Table 1: Common leafy-vegetables of Nepal
| S.N. | Common name | Botanical name, family | Nepali name |
| 1 | Broadleaf mustard | Brassica juncea var. rugosa, Brassicaceae | Rayo |
| 2 | Cress | Lepidium sativum, Brassicaceae | Chamsur |
| 3 | Pak choi | Brassica rapa subsp. Chinensis, Brassicaceae | Paakchoi |
| 4 | Swiss chard | Beta vulgaris var. cicia, Chenopodiaceae | – |
| 5 | Spinach | Spinaceae oleraceae, Chenopodiaceae | Palungo |
| 6 | Lamb’s quarters | Chenopodium album, Chenopodiaceae | Bethe saag |
| 7 | Amaranthus | Amaranthus tricolor, Amaranthaceae | Lunde/ Latte |
| 8 | Lettuce | Lactuca sativa, Compositeae | Jiri |
| 9 | Fenugreek | Trigonella foenum-graecum, Leguminaceae | Methi |
| 10 | Pumpkin leaves | Cucurbita maxima, Cucurbitaceae | Farsi ko munta |
| 11 | Taro leaves | Colocasia esculenta, Araceae | Karkalo/ Gaava |
(Pathak 2012 and 2013)
Major problems in Leafy vegetables
Higher transpiration rate led to rapid freshness loss
After harvest, leafy vegetables are subjected to water loss because of their large surface area to volume ratio (Kays, 1991). The high amount of moisture loss presumes wilting that leads to freshness loss (Roura, Davidovich, and del Valle, 2000) and water stress in the leaf that leads to yellowing acceleration (Lazan, Ali, Al’ Ani and Nahar, 1987). The acceptable limit of weight loss for foliar vegetable marketability ranges between 3–7% (Burton, 1982).
Consumers choose leafy vegetables based on appearance (Abbott, 1999), and raises serious problems when it declines; due to yellowing, wilting (Saltveit, 1999). An important parameter of the display is colour which is affected by chemical, biochemical, microbiological and physical changes that occur during development, maturation and post-harvest handling (Pankaj et al., 2011). Chlorophyll degradation is the major symptom of senescence (Heaton and Marangoni, 1996), and is considered as a quality reduction for leafy vegetables, which results in shortening of the shelf-life.
Among all vegetable species, the leafy vegetable groups show the highest rate of respiration, which has the function of releasing energy from stored chemical compounds in the plant (Alvino & Barbieri, 2015). The depletion of energy reserves leads to the reduction of shelf life of the product. During the shelf life, the fresh product loses water continuously (transpiration), which causes a moisture reduction of tissues and thus an irreversible loss of turgor.
Water loss is the major cause of postharvest losses in leafy vegetables (Alvino & Barbieri, 2015). Leafy vegetables are perishable because senescence starts very soon in detached vegetables. For this reason, the quality of leafy vegetables is strongly dependent on freshness, such as the time elapsed from cutting (in the field or a controlled environment) and the cold storage chain (Alvino & Barbieri, 2015).
Efforts and achievements in the management of rapid freshness loss
Post-harvest senescence can be induced by several stresses, such as darkness, water and nutrient availability stress, wounding, high-temperature storage, ethylene concentration in storage atmosphere and pathogen attack (Ella, Zion, Nehemia & Ammon, 2003). Gomez et al., (2008) have proposed that ascorbic acid may be involved in the initiation of spinach senescence.
Using PGRs
Gibberellins and Cytokinins have a positive effect on delaying chlorophyll degradation, while Abscisic acid and Ethylene induce senescence (Nooden, 1988). 1-Methylcyclopropene (1-MCP) is an Ethylene action inhibitor that can minimise the negative effect of Ethylene on leafy vegetables to extend their shelf-life (Blankenship and Dole, 2003). Jiang, Sheng, Zhou, Zhang and Liu (2002) found that 1-MCP at a concentration of 100 nL/L delayed postharvest senescence of coriander during storage in the presence of ethylene (10μL/L). Pak Choy sprayed with 50 ppm kinetin 1 hour before harvest had a shelf life of 23 days (Jiang and Pearce 2005).
Table 2: Ethylene concentration to negatively affect the shelf life
| Vegetable | Ethylene concentration (ml/L) | Reference |
| Lettuce | 0.005-1 | Kim and Wills, (1995) |
| spinach | 10 | Hodges and Forney, (2000) |
| Parsley | 10 | Lers et al., (1998) |
Using modified atmosphere packaging
Modified atmosphere packaging (MAP) is a packaging technology that modifies or alters the gas composition around the products in food packages from normal air to provide an atmosphere for increasing shelf life and maintaining the quality of food (Zhuang et al., 2014). An atmosphere of 2% oxygen in conjunction with 5% carbon dioxide was considered most appropriate for commercial use (Jiang and Pearce 2005). The successful use of MAP will be based on the specific O2 and CO2 permeation properties of polymer films used and also on the respiration activity of packed food (Jayanty et al., 2005; Kader, 1986 in Zhuang et al., 2014)
To minimize weight loss of fruits and vegetables, the use of packaging film is necessary. Several studies indicate the beneficial effect of high CO2 or low O2 in reducing chlorophyll degradation and delaying senescence in many green leafy vegetables (Lers, Jiang, Lomaniec & Aharoni, 1998; Yamauchi & Watada, 1993; Aharoni, Reuveni & Dvir, 1989). The recommended concentrations of CO2 and O2 for the majority of foliar vegetables are 5–10% and 1–5%, respectively (Lipton, 1987).
Lowering the storing temperature
In terms of colour criterion the lettuce can be stored in marketable condition at 0 ° C for 13 days, at 5 ° C for 10 days, at 10 ° C for 6 days and at 20 ° C for 3 days, with 90 % Relative Humidity (Manolopoulou and Varzakas, 2016).
Table 3: Temperature affect Lettuce Chlorophyll degradation stored at 90% RH for 13 days
| Storage temperature | 0 OC | 5 OC | 10 OC | 20 OC |
| Lettuce Chlorophyll lost | 5 % | 30 % | 46 % | 64 % |
| Storability (days) | 13 | 10 | 6 | 3 |
(Manolopoulou and Varzakas, 2016)
Heat shock treatment
Heat treatment between 37oC for 3.5 min successfully delayed senescence of spinach leaves for 10 days of storage at 23 degree Celsius (Gomez et al., 2008).
Light treatment
Kasim and Kasim, (2017) concluded that white, red and green LED lighting treatments are useful for enhancing the storage life of leaf lettuce, whereas the blue LED light treatment showed negative effects.
Nitrates associated with leafy vegetables
There is a great deal of concern over the nitrate and nitrite content of both human and animal food and water supplies. This is due to several reports relating these factors to infant methemoglobinemia and the possible formation of carcinogenic nitrosamines (Mirvish, 1977 in Ezeagu & Fafunso, 1995).
The European Food Safety Authority, in their scientific studies, gave recommendations about the acceptable daily intake (ADI) of nitrate in the human body and for an adult, it amounts to 3.7 mg/kg body weight/day (EFSA, 2008).
Table 4: Nitrate content in Leafy vegetables in the Republic of Croatia
| Vegetables | Spring mg/Kg | Autumn mg/Kg |
| Lettuce | 703.7 | 1264.8 |
| Spinach | 706.2 | 2013.1 |
| Chard | 972.2 | 1024.7 |
| Kale | 603.0 | 1181.4 |
(Brkic et al., 2017)
Absorption of nitrate occurs most often from natural sources, but vegetables accumulate a significant portion of nitrate from nitrogen-based fertilizers, which are used for fertilizing plants for faster and bigger growth (Anjana & Iqbal, 2007)
Generally, nitrate-accumulating vegetables belong to the families Brassicaceae (rocket, radish and mustard), Chenopodiaceae (beetroot, Swiss chard and spinach), Amaranthaceae (Amaranthus), Asteraceae (lettuce) and Apiaceae (celery and parsley) (Santamaria, 2006). Vegetables are the major source of the daily intake of nitrate by human beings, supplying about 72 to 94% of the total intake (Dich et al., 1996). Vegetables supplied with organic fertilizers have low nitrate content, compared with minerally fertilized (Raupp, 1996 in Anjana & Iqbal, 2007)
Disease and insect pests
Table 5: Diseases of common leafy-vegetables of Nepal
Use of Pest Exclusion Net (PEN)
Neupane, Shrestha, Regmi, Fooyontphanich, & Devkota, (2018) reported that the use of PEN protects BLM against aphid and flea beetle, increases yield and can be considered as a viable technology for BLM production by smallholder growers of Nepal.
Use of Trichoderma spp.
Paudel et al., (2018) reported the possible use of Trichoderma in biological control and plant growth promotion as an alternative disease control strategy.
Conclusion
Leafy vegetables are subjected to rapid postharvest water loss because of their large surface area to volume ratio. The high amount of moisture loss presumes wilting that leads to freshness loss and water stress in the leaf that leads to yellowing acceleration. This effect of rapid water loss could be reduced using various anti-Ethylene plant growth regulators, using modified atmospheric packaging, lowering the storage temperature, providing light treatment during storage and providing heat-shock treatment before storage. Another major problem in the leafy vegetable is associated higher Nitrate level, which may lead factors to infant methemoglobinemia and the possible formation of carcinogenic nitrosamines. Vegetables supplied with organic fertilizers have low nitrate content, compared with minerally fertilized.
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