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For short day plants the duration of the dark period is the critical condition, rather than the duration of the light period. Phytochromes which absorb red and far red lights, is the pigment necessary for the response in photoperiodims.
In day neutral plants flowering is not affected by photoperiods. A few specific examples of vegetables fitting into the three categories of photo- periods are listed below: Exotic vegetables in the tropics are the ones mostly affected by day length as they generally originate mostly from temperate regions.
These regions experience day lengths longer than hours per day. Cultivar selection is therefore necessary in such crops so that the farmer does not produce poor quality produce due to unsuit- able day lengths.
This information is also useful for timing production of specific vegetables. Effect of Preharvest Factors 1 Other responses that can also be caused by different daylenghts beside flow- ering in vegetables are listed below: Long days for bulbing in onions; b. Short days for tuber initiation in Irish potato, yams and Jerusalem artichoce; c.
Short days for root enlargement in cassava and sweet potato. Winds Winds are caused by differences in air pressure. Temperature differences produce pressure gradients which give rise to air movements.
Winds affect the humidity in the atmosphere. They can remove the humid air adjacent to leaf surfaces, increasing transpiration rates and decreasing temperature. Winds can replenish the carbon dioxide supply to the leaves deep within the canopy thereby improving photo- synthesis and vegetable growth.
Winds can also injure or break the above ground portions of the plant thereby affecting quality of the product. All the above must be considered as they do not only affect growth and devel- opment of vegetable but the quality of the produce.
It is therefore necessary to check before introducing any vegetable into the tropics or any area, if its optimal growth temperature and photoperiodic requirements are compatible with local climatic conditions. Even for the quality of local varieties, it is important not to ignore the environmental conditions as they also affect quality of the produce.
Where local con- ditions are marginal, artificial means can be used to promote the growth of the crop for better quality. These can be windbreaks, shades and green houses to protect the crops from wind and high solar radiation respectively. Chilling injury Chilling injury is the permanent or irreversible physiological damage to mostly warm season plant tissues, cells or organs, which results from the exposure of chilling sensitive plants to temperatures below some critical threshold for that species or tissue.
A chilling temperature is any temperature above freezing below the critical threshold temperature that causes injury. All stages of growth and development of the entire plant except perhaps dry seed are susceptible to chilling injury in warm season crops. Factors that determine the extent of this injury include the temperature, duration of exposure to that low temperature, whether exposure is continuous or intermittent, physiological age or condition of the plant, part exposed and the relative responsiveness sensitivity of the crop to chilling.
Chilling sensitive species except asparagus and potatoes are mostly warm season crops of tropical and subtropical origin. Prolonged exposure to temperature below the chilling threshold in these species results in death. Low temperature limits the geographical areas and time of year for production of chilling - sensitive crops.
Causes of chilling injury The primary cause of chilling injury is thought to be damage to plant cell mem- branes. The presence of these polyunsaturated species and their breakdown products may induce membrane damage and dysfunction. The membrane damage sets off a cascade of secondary reactions, which may include ethylene production, increased respiration, reduced photosynthesis, interference with energy production, accumulation of toxic compounds such as ethanol and acetaldehyde, electrolyte leakage and altered cellular structure Skog, Some physiological parameters of chilling injury include electrolyte leakage, carbon dioxide and ethylene production in squash and cucumber McCollum and McDonald, These parameters increase with increasing severity of chilling injury Lee and Yang, In bell peppers chilling injury was also accompanied by high internal carbon dioxide and ethylene production Lin et al.
Ethylene - forming enzyme activity can also be used as an indice for thermotolerance of crops McCollum and McDonald, Chilling injury can be measured non-destructively in fruits using pulse ampli- tude modulated fluorometry before tissue damage is visible Lurie et al.
Qp was similar at both chilling and non-chilling storage temperature. Whether this method can be used in a field situation is not known. Symptoms of chilling injury Symptoms of chilling injury as summarized by Morris and Skog include the following: Surface lesions - pitting, large sunken areas, and discolouration.
These symptoms occur most frequently in products with firm thick peel such as citrus or cucumber. Water soaking of tissues - this disruption of cell structure and accompanying release of substrates favours the growth of microorganism.
Water soaking commonly occurs in vegetables with thin or soft peels such as peppers and asparagus. Internal discolouration browning of pulp, vascular strands, and seeds. Effect of Preharvest Factors 9 6. Failure of fruits to ripen in the expected pattern following removal to ripening conditions. Accelerated rate of senescence, but with an otherwise normal appearance. Increased susceptibility to decay due to leakage of plant metabolites that encourage growth of microorganisms.
A shortened storage shelf life due to one or more of the above responses. Composition changes - especially in relation to flavour and taste.
Loss of growth sprouting capacity - important with stored propagules. These symptoms are not unique to chilling injury and many can also be induced by water stress and anoxia.
The symptoms may not be evident while the com- modity is held at the chilling temperature and develop rapidly when removed from cool storage. The factors that influence the relative susceptibility of vegetables to chilling injury are, genetic diversity, stage of physiological development and the conditions under which the commodities are grown. Cultivars within a sensitive species can differ in symptom expression.
King and Ludford showed that differences in chilling sensitivity among 5 tomato cultivars as measured by electrolyte leakage could be discerned prior to the development of visible symptoms. Tomato fruit are quite susceptible to chilling in their mature green stage Autio and Bramlage, Chilling sensitivity decline, as ripening progress and then increase again during the late stages of ripening.
Fully mature honeydew melons were less susceptible to chilling injury than less mature melons Lipton, The conditions under which the commodities are grown may influence the extent and development of chilling injury Kader, Lyons and Morris, Some winter-grown tomatoes were more susceptible to chilling injury than summer grown fruit of the same cultivar Abdel-Madsoud et ah, Fully ripe, firm fruit can be held at Some of the important vegetables susceptible to chilling injury and the potential symptoms are listed in Table 3.
Alleviation of chilling injury Chilling injury can be avoided by limiting growing, handling and storage of sen- sitive crops above the critical threshold temperatures.
Growing cultivars that are less sensitive or tolerant of chilling injury is probably the only way of effectively reducing chilling injury in the field. Several other temperature treatments have been tried with various degrees of success to control mostly chilling injury in storage. These will be discussed briefly below.
High temperature conditioning treatments have been reported to enhance the chilling tolerance of a number of chilling sensitive tissues McCollum and McDonald, Vegetables susceptible to chilling injury. Beans, snap 3 days below 4. Melons Muskmelons Honey Dew Watermelons 15 days at Surface pitting, loss of flavour and fading of red colour. Potatoes 20 weeks at Pumpkins and winter squash Rots Sweet potatoes days at 7. Lutz and Hardenburg and Skog The beneficial effects of warming after a period of exposure to chilling may be related to either, the restora- tion of normal metabolism so that potentially toxic compounds accumulated during chilling can be removed or the availability of some essential factor that became deficient can be re-supplied Lyons and Breidenback, or to repair of damage incurred to membranes, organelles or metabolic pathways before degenerative changes occur.
Intermittent warming is warming the commodity to room temperature at intervals during storage before permanent injury has occurred and will allow the product to recover and prevent chilling injury symptom development. Intermittent warming plays a role in the control of chilling injury that occurs in the field prior to the harvest of fall grown tomatoes. Lyons and Breidenback showed that accumulation Ejfect of Preharvest Factors 11 of more than 13 hours of night temperature below This treatment may, however, cause undesirable softening and increase decay and may cause condensation to form on the product especially in storage Skog, Preconditioning can also be used.
It is the stepwise cooling of the commodity and can allow the vegetable to adapt to the cooler temperature and minimize chilling injury development in storage. Controlled atmospheres can also allow longer storage of chilling sensitive crops when stored above their critical temperature Skog, Modifying the atmosphere surrounding certain fruits subject to a number of low temperature disorders has been reported in some instances to alleviate or delay the symptoms usually associated with chilling injury Lyons and Breidenback, Reduced O 2 and elevated CO 2 can overcome the impact of low temperature injury on the ripening process Wade, ; Ilker and Morris, Controlled atmospheres may in some cases further stress crops and increase chilling injury susceptibility for crops such as cucumbers, tomatoes and asparagus Skog, Maintaining a high RH around the commodity during storage, as occurs with film wraps and modified atmosphere storage, reduces the severity of chilling injury symptoms by minimizing desiccation.
Proper pre-harvest nutrition can minimize chilling susceptibility. Calcium treat- ments may stabilize cellular membranes and reduce chilling injury in certain commodities. Ilker and Morris found that treatment with solutions of calcium and potassium salts could afford some protection against chilling injury in okra.
Reports examining possible approaches to alleviating chilling injury conclude that the most likely solution occurs through genetic modification Graham and Patterson, ; Couey, ; Bramlage A better understanding of the primary causes of chilling injury will provide information for the development of rapid methods for screening potential germ-plasm and possibly control of this disorder.
The debate as to whether there is a single primary response and whether the membrane plays a central role as the primary temperature sensor remains unresolved. Freezing injury Commodities may be frozen in the field before or during harvest, in transit and storage. Vegetables vary in their susceptibility to freezing injury. Some may be able to go through freezing and thawing several times with little or no apparent damage, while others are damaged by slight freezing.
The relative susceptibility of fresh vegetables to freezing injury is shown in Table 4. The inherent suscepti- bility of tissues to freezing stress may be partially responsible for this variation in sensitivity. Ice nucleating bacteria may also play an important role. Whether or not ice formation in the tissues is accompanied by permanent injury and death 12 R.
Relative susceptibility of fresh vegetables to freezing injury. Frozen leafy tissues and storage tissues such as turnips and rutabagas, lose their natural luster and appear glossy. Immediately upon thawing, they become water soaked. Water-soaked areas of leafy green tissues also appear dirty or a muddy green colour. In colourless or fleshy tissues like cauliflower heads, there is no initial discoloration, later, the more sensitive tissues such as the fibro- vascular bundles may turn yellowish - brown to black.
Fleshy roots, such as turnips, radishes, rutabagas, and horseradish, often show no discoloration except for the vascular tissues. Vegetables may arrive at the market bearing symptoms of frost damage incurred during some earlier stage of their growth. Glove artichoke Lynara scalymus L. Severe freezing kills the immature flower heads and causes them to turn black.
Slight freezing results in breaking, cracking, and blistering of the epidermis on exposed outer bracts. The damaged epidermis becomes whitish and buds may become brown.
This detracts from the market appearance of the buds Ramsey et ah, When freezing injury occurs prior to harvest in Irish potatoes Solanum tuberosum it is often referred to as field frost. This condition can usually be diagnosed by the presence of bluish-gray blotches beneath the skin. Tissues at the stem end of tubers are more sensitive than those at the bud end, and the differen- tiated vascular cells, such as tracheae, sieve tubes, and tracheids, are more susceptible than are the starch-filled parenchymatous cells.
Any freezing during storage and transit may manifest as or all the necrotic patterns known as ring necrosis, blotch, or net necrosis. Generally the symptoms of freezing progresses from the ring to net to blotch type as the freezing progress and as the freezing interval lengthens. Often the different types of symptoms will overlap.
The various internal symptoms of freezing injury may not occur unless potato tubers are bumped, jarred, or dropped during the freezing period. Varieties of potatoes differ in their reaction to low Effect of Preharvest Factors 13 temperatures. Some show serious internal discoloration after prolonged storage at several degrees above their actual freezing point, while others may not show injury at Freezing injury of celery can be readily recognized at harvest time by the flabby water- soaked condition of the leaves and leaf stalks.
Frozen leaves, if not attacked by bacteria, dry out and become papery. A second type of freezing symptom is the appearance of isolated sunken lesions on the leafstalks.
These two types of injury are most often apparent at harvest but are of little importance on the market.
Mineral nutrient disorders In this section the key nutrients causing physiological disorders and their role in vegetables will be discussed.
A mineral nutrient can function as a constituent of an organic structure, as an activator of enzyme reactions or as a charge carrier and osmo-regulator. Calcium Calcium is a relatively large, divalent cation. The Calcium content of plants varies between 0. Ca deficiency in plant tissues causes many physiological disorders which lead to significant losses in plant production.
Calcium shortage in plants is related to poor Ca uptake, its limited movement to above - ground plant parts and strong competition for Ca between leaves and generative plant parts fruits, seeds Wojcik, Calcium readily enters the apoplasm and is bound in an exchangeable form to cell walls and at the exterior surface of the plasma membrane. Its rate of uptake into the cytoplasm is severely restricted and seems to be only loosely coupled to metabolic processes.
The mobility of Ca from cell to cell and in the phloem is very low. It is the only mineral nutrient other than Bo which functions mainly outside the cytoplasm in the apoplasm. Most of its activity is related to its capacity for co-ordination by which it provides stable but reversible intermolecular linkages; predominantly in the cell walls and the plasma membrane.
Calcium is a non- toxic mineral nutrient, even in high concentrations, and is very effective in detoxifying high concentrations of other mineral elements in plants. It can be demonstrated most readily by the increased leakage of low molecular weight solutes e. Kwaramba and a loss of cell compartmentalization. Calcium stabilizes cell membranes by bridging phosphate and carboxylate groups of phospholipids Caldwell and Hang, and proteins, preferentially at membrane surfaces Legge et al.
In Calcium - deficient tissue polygalacturonase activity is increased Konno et al. The proportion of Ca in the cell walls is of importance for determining the sus- ceptibility of the tissue o fungal infections and for the ripening of fruits.
This shift is associated with a sharp increase in ethylene formation in the fruit tissue. Vitrescence, a physiological disorder in melons has been reported as being caused by Ca deficiency Jean Baptiste et al.
The symptoms are an intense colour, with a vitreous aspect and a delinquescent texture. Potassium Potassium is a univalent cation. It is characterised by high mobility in plants at all levels, within individual cells, within tissues, as well as in long-distance trans- port via the xylem and phloem.
Potassium is the most abundant cation in the cytoplasm, and potassium salts make a major contribution to the osmotic potential of cells and tissues of the glycolytic plant species. Cytoplasmic concentrations are maintained in the relatively narrow range of to mM.
The various functions of in cell extension and other turgor regulated processes are related to the concentration in vacuoles.
Potassium ions act as a charge carriers of high mobility that form weak complexes in which they are readily exchangeable Wyn Jones et al.
The high concentrations of in the cytoplasm and the chloroplasts are required to neutralize the soluble e. Potassium is required for enzyme activation and membrane transport processes. There are mechanisms pumps at the plasma membrane and probably also at the tonoplast for concentrating in the cytoplasm. Potassium deficiency causes chlorosis and necrosis in mature leaves and stems wilting and logging. The lower tolerance of deficient plants to drought is related to a the role of in stomatal regula- tion which is a major mechanism controlling the water regime of higher plants and b the role of as the predominant osmotic solute in the vacuole, main- taining a high tissue water level even under drought conditions.
Plants receiving inadequate are often more susceptible to frost damage, which at cellular level, is related in some respect to water deficiency. Frost damage is inversely related to the content of leaves Marschner, Table 5. The changes in enzyme activity and organic compounds that take place during deficiency are in part responsible for the higher susceptibility of plants to fungal attack. They also affect the nutritional and technological processing quality of harvested products.
This is most obvious in fleshy fruits and tubers with their high requirement for growth. There are more than 50 enzymes which either depend on or are stimulated by potassium Suelter, Potassium and other univalent cations activate enzymes by inducing conformational changes in enzyme protein. In general, potassium induced conformational changes of enzymes increase the rate of catalytic reac- tions, 3nd in some cases also the affinity for substrate, Evans and Wildes, In deficient plants some gross chemical changes occur, including an accumulation of soluble carbohydrates, a decrease in the levels of starch, and an accumulation of soluble N compounds.
Potassium activates the following enzymes pyruvate kinase, phospofructokinase Table 5. Relationship of potassium supply to potassium content in leaves, percentage of leaves damaged by frost and tuber yield in potatoes. Effect of increasing potassium concentrations in potato tubers, on the composition and quality of the tubers. Potassium is required for protein synthesis in higher plants. The role of in protein synthesis is reflected in the accumulation of soluble nitrogen compounds, e.
In higher plants affects photosynthesis at various levels. An increase in the leaf potassium content is accompanied by increased rates of photosynthesis, photo- respiration and RuBP carboxylase activity, but a decrease in dark respiration.
In most plant species also has the major responsibility for turgor changes in the guard cells during stomatal movement. An increase in the concentration in the guard cells results in the uptake of water from the adjacent cells and thus stomatal opening Humble and Raschke, Boron Boron occurs mainly as boric acid, H3BO3 a very weak acid in aqueous solution. This is also the preferred form taken up by the roots.
Boron is strongly complexed to cell wall constituents even in the roots Thelier et ah, Long distance transport of boron from the roots to the shoot is confined to the xylem, and uptake and translocation are closely related not only to the mass flow of water to the root surface but also to the xylem water flow.
There is no indication that boron is an enzyme component and there is only little evidence Birnbaum et ak, that the activity of any enzyme is enhanced or inhibited by boron. Interactions between boron and other mineral nutrients during uptake and in the plant itself seem to be of minor importance Marschner, Bo deficiency is a widespread nutritional disorder in vegetables.
Boron deficiency usually occurs under: Plant species differ characteristically in their capacity for Bo uptake when grown in the same soil. This is shown in Table 7 below.
Boron content of the leaf tissue of plant species from the same location. The critical deficiency range, expressed as milligrams boron per kg dry weight is about in monocots, in red clover, in carrots and in sugar beet Bergmann, Differences in boron requirements are most likely related to differences in cell wall composition. High light intensities seems to increase sensitivity to boron deficiency, by raising the requirements for boron in the tissue. Symptoms of B deficiency in the shoots are noticeable at the terminal buds or youngest leaves, which become discoloured and may die.
Internodes are shorter, giving the plants a bushy or rosette appearance witches broom. Interveinal chlorosis on mature leaves may occur and misshaped leaf blades are also symptoms of B deficiency in leaves. Buds, flowers and developing fruits drop. In heads of vegetable crops e. In storage roots of celery or sugar beet, necrosis of the growing areas lead to heart rot. With severe deficiency the young leaves also turn brown and die, subsequent rotting and microbial infections of the damaged tissue being common.
Nodule number was lower in B - deprived plants of Vigna subterranean Reduction or even failure of seed and fruit set are also common B deficiency symptoms. There was also reduced levels of crude fat and protein in soybean seed grown in B deficient soils Eguchi, The application of boron either to the soil or as a foliar spray, different sodium borates, including borax or sodium tetraborate can be used. Boric acid can also be used as foliar sprays.
The amount of boron applied varies from 0. Boron toxicity may occur when large amounts of municipal compost are applied Purves and Mackenzie, , and it is of much concern in semi-arid and arid 18 R. Kwaramba areas where irrigation water contains high levels of boron. The critical toxicity levels expressed as mg boron per kg dry weight of leaves is for cucumber and for squash El-Sheik et ah, Typical boron toxicity symptoms on mature leaves are marginal or tip chlorosis or both.
They reflect the distribution of boron in shoots, following the transpiration stream. Phosphorus The phosphorus requirement for optimal growth of vegetables is in the range of 0. Plants suffering from P deficiency exhibit retarded growth, and often a reddish coloura- tion occurs because of enhanced anthocyanin formation. P-deficient plants also often have a darker green colour than do normal plants. Deficiency leads to a general reduction of most metabolic processes including cell division and expansion, respiration and photosynthesis because of the functions of P in the growth and metabolism of plants Terry and Ulrich, The regulatory function of inorganic phosphate P; in photosynthesis and carbohydrate metabolism of leaves can be considered to be one of the major factors limiting growth particularly during the reproductive stage.
After uptake at physiological phosphate either remains as inorganic phosphate Pj or is esterified to a simple phosphate esteri or attached to another phosphate by the energy - rich pryophosphate bond e. Phosphate forms a bridging group connecting units to more complex or macromolecular structures using another type of phosphate bond C-P-C.
Phosphorus is responsible for the strongly acidic nature of nucleic acids and thus for exceptionally high cation concentration in DNA and RNA structures. The bridging form of P diester is also abundant in the phospholipids of biomembranes where it forms a bridge between a diglyceride and another molecule amino acid, amine or alcohol. In biomembranes the amine choline is often the dominant partner, forming phosphatidyl choline lecithin.
Most phosphate esters are intermediates in metabolic pathways of biosynthesis and degradation. Their function and formation is directly related to the energy metabolism of the cells and to energy rich phosphates. The energy required, for example, for biosynthesis or for ion uptake is supplied by an energy- rich intermediate or coenzyme, principally ATP. Energy liberated during glycolysis, respiration, or photosynthesis is utilized for the synthesis of the energy-rich pyrophosphate bond.
ATP is the principal energy-rich phosphate required for starch synthesis. Inorganic phosphate is also either a substrate or an end-product e. Inorganic phosphate controls some key enzyme reactions. In fruit tissue of tomato, P; released from the vacuoles into the cytoplasm can stimulate phos- phofructokinase activity Woodrow and Rowan, Phosphofructokinase is the key enzyme in the regulation of substrate flux into the glycolytic pathway.
An Effect of Preharvest Factors 19 increase in the release of Pj from vacuoles can therefore initiate the respiratory burst correlated with fruit ripening Woodrow and Rowan, Delayed fruit ripening in phosphorus deficient tomato plants Pandita and Andrew, may be related to this function of P;. ADP-glucose pyrophosphorylase is allosterically inhibited by P; and stimulated by triose phosphates.
The ratio of P; to triose phos- phates therefore determines the rate of starch synthesis in chloroplasts Herdt et ah, Nitrogen Elevated nitrate levels are uneconomical in relation to nitrogen utilization and are also undesirable nutritionally. Sometimes nitrite is formed from nitrate during either the storage or processing of vegetables. Infants fed on nitrite-containing foods run the serious risk of developing methemoglobemia. The vegetable examples presented include tomato and squash which are two very different fruits, cabbage, a leaf vegetable and sweet potato a root vegetable crop.
Tomato Tomato is a warm season vegetable belonging to the Solanaceae family. The tomato is adapted to a wide range of climatic and soil conditions. It is produced throughout the whole world from near the Arctic Circle under protected environments to the equator.
Tomatoes are a major source of vitamins and minerals to human nutrition. Production of tomatoes can be either in the field or in the greenhouse. The tomato crop has several disorders that are caused by environmental factors that ultimately reduce yield and quality. These are discussed below. Chilling injury This disorder shows surface pitting, poor colour development upon ripening, pre- mature loss of firmness and increased susceptibility to decay.
This is a result of very low above freezing temperatures especially night temperatures. This could be achieved by reducing ventilation in the green- house Kang and Park, Recent studies have indicated that heat treatment administered prior to chilling reduces the incidence and severity of chilling injury 20 R.
Kwaramba in tomato fruits and other organs. There was a correlation between maintenance of heat - shock protein mRNA during storage and the inhibition of chilling injury in heated tomatoes. Under certain conditions, however, heat treatment of tomato fruits may not reduce chilling injury as effec- tively as partial ripening Whitaker, Sun scald Tomatoes at the mature green stage or beginning to show the pink colouration breaker stage are most susceptible to sun scald. The fruits develop a white necrotic tissue surrounded by a yellow halo.
The patch often becomes sunken and wrinkled. The damage to the fruit occurs on the side or top half exposed to sunlight. The area may remain white or yellow or become dry and papery and may later become infected by fungal infection when the fruit ripens.
It is caused by a sudden exposure of the fruit to direct sunlight during hot dry weather and can be worsened by prac- tices such as pruning or harvesting or other operations involving working moving through the field and exposing the fruits. Sudden defoliation leads to exposure of the fruits to sunlight. When supports collapse they expose fruit to direct sun. Heat and light cause direct irradiation with sunlight.
The fruits fail to turn red and in severe cases turn white and blistered. The exposed side of the fruit turns pale at first and becomes sunken and wrinkled as the fruit ripens. Catface This is a typical malformation with deep cavities of the fruit which occurs at the blossom end of tomato fruit.
These can range from mild to extreme deformations and scarring of the fruit. The fruit will have many bulges and be larger than the unaffected fruit produced by the same cultivar. Although hereditary, occurrence in larger fruited cultivars is mainly worsened by exposure of the plant to cool tem- peratures at the time of flower initiation.
Cultivars may vary in their susceptibility to this disorder. The large fruited cultivar like Moneymaker is very susceptible. High nitrogen has also been recorded as a cause for catfacing. The cracks usually form on unripe fruit often cutting deep into the flesh.
The shallow ripe cracks are known as bursting. This disorder is hereditary although it is facilitated by continuous rain and heavy dew during maturity of the fruit. It may extend even into harvesting period when there is frequent flood irrigation with small amounts of water. Sudden changes in soil moisture and atmospheric humidity cause this disorder due to alterations in growth rate. It is Effect of Preharvest Factors 21 however, more severe when foliage is sparse as exposed fruits crack more due to exposure to greater temperature fluctuations from exposure to direct sun rays.
Tomatoes grown under cold conditions are more prone to cracking. High nitrogen and low potassium result in succulent plants that produce tomatoes that are very susceptible to cracking. Blossom end rot Black bottoms Blossom end rot arises from a localized deficiency in calcium at or near the blossom end, which could be facilitated by several environmental conditions.
It is also associated with fluctuations in the soil moisture due to incorrect or poorly sched- uled irrigation and poor root development. A brown-black, dry, leathery depression at or near the blossom end occurs during development of the fruit commonly noticeable when the fruits are one-third to one-half full size. Occasionally it appears on the side of the fruits and sometimes produces internal black lesions not visible from the exterior of the fruit.
Affected fruits ripen more rapidly than normal fruits. At first a small water soaked light tan spot appears, enlarges and darkens as the fruit size increases. The spot may enlarge to cover as much as a third to half of the entire fruit surface. Large lesions soon dry out and become flattened, black and leathery in appearance and texture.
This is due to environmental conditions that cause water stress. Environmental conditions required for high yield, such as light, carbon dioxide concentration and temperature stimulate fruit expansion, but may not increase and may even reduce the transport of to the distal tissue of the fruit.
Incidence of blossom end rot is determined by the fruit growth response to the environmental conditions during the rapid phase of fruit enlargement. In an experiment carried out to determine the influence of N source, N application rate and soil moisture on the incidence of blossom end rot, a close relationship was observed between the incidence of blossom end rot and the ammonia concentration in the soil solution Morikuni and Shimada, When only nitrate N was applied the blossom end rot was decreased.
Longer NH4-N supply increased the amount of fruit with blossom end rot in the winter but had no effect in spring, however, higher NH4-N concentration in solution in spring greatly increased the number of fruits with blossom end rot Sandoval- Villa et ah, Sandy soils are also vulnerable to lack of water and nutrients due to high leaching. Some literature has suggested close spacing in the field and dry winds as encour- aging blossom end rot.
This disorder is most prevalent when rapidly growing plants bearing fruits are suddenly exposed to a drought period and when roots fail to obtain nutrients and water due to any form of damage or otherwise. Factors that encourage rapid growth are high nitrogenous fertilizer application or high temperatures Tabatabaei et ah, and light intensity. These factors favour development of the disorder.
Thirty five percent of fruits in the unfiltered compartment had blossom end rot compared to none in the filtered compartment. To control blossom end rot, rapid growth caused by high temperature should be avoided Tabatabaei et ah, Hollowness Puffiness The fruits are light and soft. Fruits appear slab-sided or angular from the exterior. Various climatic conditions, varietal and nutritional factors cause the malforma- tion.
The cavities of a normal fruit are filled with a jelly-like substance which carries the seed between the walls and the core.
When hollowness occurs the cavities are not completely filled with the jelly and consequently the fruit becomes puffy or hollow, light in weight, soft and of reduced quality. Cultivars with two or three seed cavities are usually more subject to hollowness.
High nitrogen applications in the early stages of growth and incorrect irrigation scheduling facilitate devel- opment of hollowness. It is encouraged in plants that set fruits in cool weather although bad or inadequate pollination, fertilization and seed development may contribute to the disorder.
These can be due to improper nutrition. Low potassium nutrition, high nitrogen, high phosphorous coupled with low light and low dry matter content of the fruit all encourage development of puffiness.
The use of auxin growth regulators for fruit set can also lead to puffiness. Uneven colouring blotchy ripening This disorder is caused by different factors which include insufficient light, virus infection and potash deficiency. Too high or too low temperatures during fruit ripening will cause green or yellow shoulders on ripe tomatoes. Generally high Effect of Preharvest Factors 23 temperatures and luxuriant growth as a result of nitrogenous fertilizer coupled with cool overcast weather and potassium deficiency are all associated with uneven ripening which may resemble the mottling caused by Tomato mosaic virus TMV.
Tomato Mosaic Virus causes a green-yellow red mottling of the fruits while Tomato Spotted Wilt Virus causes circular spots with concentric red and yellow bands.
Grey wall vascular browning; internal browning A greyish-brown discolouration of the fruit wall shows through the healthy tissue of the skin. When the tomato is cut in cross-section, internal browning is evident on the tissue around the vascular bundles.
Apart from tomato spotted wilt virus the same conditions that cause uneven ripening will cause this particular disorder. These include TMV, luxuriant growth, heavy applications of nitrogenous fertil- izers, low light intensity, moist weather, high soil moisture and low temperature, but also high temperatures and water stress. Blackheart From the outside the tomato fruit looks healthy, but when cut, the core and other tissues inside are found to be black.
The same environmental conditions that cause blossom end rot are thought to cause this disorder. Creased stem This occurs when too much nitrogen is applied when tomato plants are still very young.
Once this occurs fruits become malnourished and become small and unhealthy. They are light in weight and fail to attain the red colour expected by the consumer. Internal white tissue The white tissues are usually in the outer wall of the fruit which shows when the tomato fruit is cut in half. The problem occurs in the placental area, proximal to the locules.
The tissues may extend from the core into the fruit with an increase in the amount of white tissues. There are cultivar differences to susceptibility to this disorder. Poor potassium nutrition and high temperatures or any other factors that impose some form of stress causes internal white tissues.
Sweet potato white fly has been involved and tends to increase the incidence of this disorder. Rain check Numerous tiny concentric cracks which may coalesce appear on the shoulders of the fruit. The cracks are rough to the touch. Affected areas may become leathery and remain green when the fruit ripens or may become blackish.
If the fruit becomes red the cracks are still visible. The cause is not well known but could be temper- ature alteration by rain or water uptake which disrupts the shoulder epidermis. The problem tends to be severe when a dry period is followed by heavy rains.
Blossom drop Yellowing of pedicels and calyx leads to the abortion of the flower. This may take place before or after anthesis. The flower withers and turns brown but does not abscise. Stress conditions increase the incidence of the disorder. Any factor that inhibits, pollination and fertilization for example low or high temperature, high relative humidity, excess wind lead to blossom drop.
Improper nutrition from deficiencies in fertilizer or excess nitrogen tends to also increase the problem. Foliar diseases and insect damage are some documented causes. Zippering Very thin longitudinal brown necrotic scars start at the stem scar and extend part or all the way to the blossom end. These long scars have small transverse scars along them resembling a zipper. Mostly one scar occurs per fruit but there can be several, at times a hole open at the locule forms in addition.
Under cool weather the anthers may be attached to the ovary wall of newly forming fruits causing zippering. Other weather conditions may also be involved however, susceptibility differs with cultivars.
Chilling injury Chilling injury is caused if squashes and pumpkins remain in the field at Symptoms of chilling injury are sunken pits on the surface and high levels of decay Cantwell and Suslow, Severe pitting and slight decay were observed after 12 days of exposure to low temperature Wang, In Zucchini squash Cucurbita pepo post harvest treatments that reduce chilling injury temperature conditioning and low O 2 storage were found to increase endo- genous levels of polyamines Wang, Exogenous treatment with poly amines by pressure infiltration was shown to increase the tolerance of squash to chilling injury.
Freezing injury Can occur in vegetables at temperatures below Oedema Oedema is a physiological disorder of cucumbers and is most frequently found with pumpkins and winter squash that are subjected to moisture stress.
This is most often associated with uneven availability of moisture when immature fruit are enlarging. On winter squash rinds the severity may be enough to make the fruit unmarketable. The crusty appearance is similar to the appearance of scab on hard shelled fruit, except that the oedema lesions never appear crater-like or shrunken Bodnar and Fitts, On buttercup squash the corking lesions may be circular, spindle or apostrophe shaped.
While on butternut squash oedema appears as linear growth cracks usually on the neck portion of the fruit. Stigma death The development of female flowers may be affected by temperature. The ovaries turn yellow and then shrivel and the stigma of the unopened flower exhibits black streaks into the ovary. This reduces the yield of squash. Cabbages Cabbage is a cool season crop belonging to the Cruciferae family. Cabbage is a very important crop in the temperate regions of the world but it is also gaining importance in the high altitude areas of the tropics especially for small scale farmers as it has a fairly long shelf life.
Sauerkraut is the main processed product from cabbages Yamaguchi, In most of the tropics, cabbage can also be dried blanching is necessary before drying. Kwaramba short periods of time. Most cultivars are short day plants but others appear to be day length neutral. For a high acceptable quality of cabbage removal of outer wrapper leaves should reveal the typical shape and colour of the cultivar green, or purple or pale yellow-green , firm, heavy for the size head, and the head must be free of insect, decay, seed stalk and other defects.
Leaves should be crisp and turgid Cantwell and Suslow, Some disorders caused by environmental factors that impact on quality of cabbages are discussed below. Bolting This refers to production of a seed stalk instead of a head. In the tropics this rarely happens but it can occur at very low temperatures especially night temperature accompanied by extreme variations with day temperatures.
This is undesirable as cabbage is grown mainly for the heads. High-veld areas are generally colder there- fore cabbages can be grown in summer, autumn and spring to avoid bolting as winter temperature cause bolting even in the tropics.
Deterioration in storage can also be associated with bolting Cantwell and Suslow, Frost injury Heavy losses of cabbage occur annually as a result of freezing, both in the field and during storage. Cabbage tissue has one of the highest freezing points There is no way of determining by examination of a frozen head whether it will exhibit injury when thawed. Immediately after thawing, frozen areas appear water soaked due to suffusions due to water liberated by the thawing of ice in intracellular spaces.
If cells have not been killed, some of this water will be reabsorbed and the tissue will only appear slightly wilted or shriveled due to excessive water loss. The margins of lower leaves become flaccid and turn brown and may even die. There is often rupturing of the epidermis on the underside of leaves and cracking of main veins.
The veins then become spongy, pithy, and tough, losing their characteristic flavour. Some heads can withstand freezing several times before the effects become pronounced; others are injured the first time.
If cells have been killed, water does not re-enter them and is either lost due to evapora- tion, with attendant drying out of the tissues, or it remains and the tissues become a leaking, disorganized mass which soon succumbs to saprophytic fungi and bacteria attacks.
The outer leaves of cabbage appear to be more resistant to frost damage than the inner leaves and stem. They generally thaw without injury. Following prolonged exposure to freezing temperatures, the inner tissues, especially the stem pith, are killed and subsequently become affected with bacterial soft rot. A good quality cabbage should be crunchy and fresh. Head cracking Cabbage heads crack when approaching maturity during hot months. Uneven pro- vision of soil moisture coupled with high temperature cause cracking.
Very dry Effect of Preharvest Factors 27 conditions followed by sudden high moisture or heavy irrigation after the dry period also leads to cracking. Too rapid growth at high nitrogen levels will lead to coarse, loose heads, poor processing and storage quality and cracking.
Cracking may also occur as a result of boron deficiency in the soil. Blindness Blindness may result from planting blind seedlings from the beginning. The apical meristem is damaged or is missing from the beginning. Physical damage to the growing tip will result in cabbages that will not head due to the absence of the shoot tip.
Freezing injury during the initial stage of head formation may lead to blindness Yamaguchi, Foul smell Very dry conditions cause the cabbage leaves to be tough and rough and the cabbage produces a typical strong cabbage smell when cooked.
The leaves are supposed to be tender and slightly succulent as cabbage is generally consumed fresh and raw in salads. As a heavy feeder cabbage requires higher nitrogen content than other vegetables. Tip burn Leaf margins become brown and papery later turning dark brown to black and finally necrotic. Injured tissue becomes predisposed to attack by soft rot bacteria.
This disorder is believed to be very similar to blossom end rot in tomatoes in not only its causes but also the environmental factors affecting. Conditions favouring include fluctuating growth rates especially rapid growth caused by high temperatures, light intensities and excess nitrogen http: Late harvesting and wide spacing of crop in the field can also increase the incidence of this disorder.
Oedema These are wart-like swellings that appear mainly on the underside of leaves of cabbage. Over-watering or prolonged rainy weather cause such swellings. Button This refers to unusually small heads due to premature generative stage. This happens under poor environmental conditions which arrest vegetative growth. Cabbage heads also become small due to boron deficiency and will also be yellow Yamaguchi, Leaf abscission Cabbages are very sensitive to ethylene Cantwell and Suslow, Very low levels of the ethylene will lead to leaf fall and yellowing.
In the field, if cabbages are exposed to other plants or fruits nearby that produce ethylene the disorder will appear. In storage poor ventilation and mixing with ethylene producing plants and fruits will result in leaf abscission. Black speck, pepper spot, petiole spot, gomasho Very small to moderate size discolourations in the form of lesions appear on the midrib and leaf veins. In the field black speck is as a result of low temperatures coupled with harvesting over-mature cabbage heads whereas in storage and in transit this is due to exposure to low temperatures suddenly followed by warmer temper- atures Cantwell and Suslow, Sweet potatoes The sweet potato Ipomoea batatas is a member of the Convolvulaceae family and is a warm season tender perennial root crop grown as an annual for its storage roots.
Other common names include Louisiana yam and Spanish potato. The crop is grown throughout most of the tropics. Sweet potato is the major staple crop in Papua New Guinea Hartemink et al. It is also an important secondary food crop for many Eastern and Southern African countries whose staple diets are based on cereals, particularly maize Gakonyo, Sweet potato is an impor- tant food security crop when maize is in short supply or in years of drought Mutuura et al.
Sweet potato is grown in tropical, subtropical and warmer temperate regions during the frost-free periods Yamaguchi, It is the most widely adapted of the agriculturally important root crops native to the humid tropics. It also requires approximately 2 cm of soil moisture per week, uniformly distributed during the growing season.
Sweet potatoes are adapted to a wide range of soil textural classes but sandy loams at a pH of 5. Heavy clay soils often give low yields of poor quality Kay, and irregular tuber shape Martin, Leonard and Stamp, The quality of the sweet potato root is affected by several environmental conditions during the growing Effect of Preharvest Factors 29 season.
The main environmental factors affecting root quality are soil moisture, nutrition, temperature and pests. These will be discussed separately below. Freezing injury Sweet potatoes are generally considered to be very susceptible to low temperature injury, however, their average freezing temperature of While this is uncommon in the tropics, such temperatures can be encountered during harvest. Sweet potatoes that have been only slightly frozen are characterized by yellowish-brown discoloration of the vascular ring and internal vascular elements and by a yellowish-green water-soaked appearance of surrounding tissues.
When exposure has been long enough for extensive ice formation to occur within the tissues, they collapse immediately upon thawing and the root becomes soft and flaccid as water is liberated. Such roots may dry and become brown, dis- colored mummies, although they usually decay due to infection by blue mold fungus Clark and Moyer, Sweet potatoes must be harvested before freezing weather occurs. Chilling injury Potatoes exposed to moderate chilling may be dug, cured and sold but they should not be placed in long-term storage Pierce, Lower temperatures signifi- cantly reduce root quality and storage life of sweet potatoes.
Low temperatures may cause internal breakdown in storage roots, internal discolouration, an increase in the frequency of roots with decay and sometimes, hard core. Hardcore is a disorder in which roots remain hard after cooking. The cold is thought to modify pectic substances in the middle lamella so that the tissue remains hard during cooking. Hardcore develops in roots exposed to 1.
Chilled roots do not exude latex when cut Clark and Moyer, High temperature and sunscald The sweet potato plants are relatively resistant to the combined effects of heat and sunlight. In contrast to the growing plants, storage roots left exposed to the sun after they are dug are commonly damaged by sunscald.
Many harvesting systems involve removing storage roots from the soil and leaving them on the soil surface. This facilitates drying and removal of soil from the roots, which are picked in a second step of the harvest operation. When roots are left in bright sun for as little as 30 minutes, serious sunscald can result. They become soft near the surface, and within a few days their exposed surfaces may turn purplish brown.
The incidence of storage rots, especially Rhizopus soft rot, surface rot, Fusarium root rot and charcoal rot. Kwaramba is greatly increased when sun scalded roots are stored Clark, L. Sweet potatoes left in containers to dry for brief periods in the field can be covered with vines to prevent sunscald. Soil structure ejfects Sweet potatoes require well prepared soil worked to a depth of 15 cm, supported by a heavy clay loam subsoil with good drainage.
In loose or too deeply ploughed soil the roots have a tendency to become long and slender Nonnecke, A firm subsoil beginning at 15 cm provides the right conditions for tuber development. Sweet potatoes produce better quality tuberous roots when following soybeans or other legumes in the rotation. Scurf is a disease characterized by roughened skin. Long rotations should be used to decrease the incidence of scurf and infection from Fusarium wilt Kemble et al. Heavy clay soils result in rough, irregular roots Peet, Soil moisture effects and related disorders Sweet potato is considered drought tolerant, and an even moisture supply throughout the growing season will enhance the yield and market appearance.
Water logging and fluctuating soil moisture in sweet potato production signifi- cantly affect the quality of the roots. Poor aeration caused by poor drainage decrease yields of sensitive cultivars and can cause either souring tissue breakdown in the storage roots with severely impeded drainage or water blisters enlargement of lenticels on the periderm if the drainage problems is less severe Peet, Wet soil conditions at harvest lead to an increase in tuber rots and adversely affect yield, storage life, nutritional and baking quality Ton and Hernandez, ; Akparanta, Skaggsa and Saunders, Souring Souring is caused by roots being in water - saturated soils for prolonged periods prior to harvest.
Souring can result in complete crop loss. Sweet potato roots sustain a high rate of metabolic activity. When soils are saturated with water, the exchange of oxygen and carbon dioxide is inhibited and the roots become asphyxiated. Ethanol and carbon dioxide accumulate in such roots. The result is a high percentage of roots that decay during curing, and surviving roots undergo a greater amount of shrinkage.
Excessive soil moisture may also reduce quality factors such as carotenoid pigments, dry matter content and baking quality. Tolerance to flood damage may be due to either less ethanol production Ahn et al.
Effect of Preharvest Factors 31 4. Water blisters Storage roots may develop small raised bumps at the lenticels called water blisters following extended periods of flooded soil.
The bumps are white at first and turn brown to black after the roots are harvested. They often develop prior to souring. Cultivars differ in their tendency to develop water blisters. Berry Punch Gummies mg - Envy. Black Cherry Fusion mg - CannaPunch.
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