The climatic factors that influence crop maturity, ripening, and senescence include rainfall, light, temperature, relative humidity, air, and wind. They are abiotic components of the environment, including topography and soil, that influence plant growth and development, maturation, ripening, and senescence, and consequently the postharvest quality of crop production.
There are mainly three stages in the life span of fruits and vegetables: maturation, ripening, and senescence. Maturation is indicative of the fruit being ready for harvest.
At this point, the edible part of the fruit or vegetable is fully developed in size, although it may not be ready for immediate consumption. Ripening follows or overlaps maturation, rendering the produce edible, as indicated by texture, taste, colour, and flavour.
Senescence is the last stage, characterized by natural degradation of the fruit or vegetable, as in loss of texture, flavour, etc. (senescence ends with the death of the tissue of the fruit).
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Role of Rainfall in Crop Maturity and Postharvest Quality
Rainfall is the most common form of precipitation. The amount and regularity of rainfall vary with location and climate types and affect the dominance of certain types of vegetation as well as crop growth and yield.
Through precipitation, water is made available to plants as surface water, soil water or moisture, or groundwater. It comprises about 70–90% of the body or even more on a fresh weight basis, although only a small fraction of the water absorbed is utilised.
Most of the water absorbed is lost through transpiration and only about 1 per cent or less is used in the various biochemical processes.
Water is needed by plants from seed germination to plant development. It is a biochemical reactant in the different hydrolytic processes occurring in plants as well as in photosynthesis.
It dissolves nutrients to be absorbed and transported and serves as a coolant of the plant through the process of transpiration, the exit of water from plants in the form of vapour.
A deficiency or excess of water may influence the postharvest quality of some crops. Extreme water stress reduces yield and quality; mild water stress reduces crop yield but may improve some quality attributes in the fruit, and no water stress increases yield but may reduce post-harvest quality.
In strawberries, reduction of water stress by natural rainfall or irrigation during maturation and ripening decreases firmness and sugar content and provides more favourable conditions for mechanical fruit injury and rot.
If strawberry plants are over-irrigated, especially at harvest, the fruit is softer and more susceptible to bruising and decay.
Other examples include: heavy rainfall leads to waterlogging and affects adversely the flowers and fruits; at blossoming, rain washes away stigmatic fluid and pollen from stigma; rain before harvesting causes softening of fruits in banana and date palm and induces infection of fruit fly in guava and peach; water deficits have been shown to delay development of floral primordia in both grain sorghum and barley.
Excess rain or irrigation leads to brittle and easy damage in leafy vegetables and to reduced tendency to decay; lack of rain or irrigation leads to low juice content and thick skin in citrus fruit; dry condition followed by rain or irrigation leads to growth cracks in tomato or secondary growth in potatoes.
Effect of Temperature on Growth, Ripening, and Crop Quality
Temperature influences all plant growth processes such as photosynthesis, respiration, transpiration, breaking of seed dormancy, seed germination, protein synthesis, and translocation. At high temperatures, the translocation of photosynthesis is faster so that plants tend to mature earlier.
In general, plants survive within a temperature range of 0 to 50°C. Enzyme activity and the rate of most chemical reactions generally increase with the temperature rise. Up to a certain point, there is a doubling of enzymatic reaction with every 10°C temperature increase. But at excessively high temperatures, denaturation of enzymes and other proteins occur.
Excessively low temperatures can also cause limiting effects on plant growth and development. For example, water absorption is inhibited when the soil temperature is low because water is more viscous at low temperatures and less mobile, and the protoplasm is less permeable.
At temperatures below the freezing point of water, there is a change in the form of water from liquid to solid. The expansion of water as it solidifies in living cells causes the rupture of the cell walls.
The favourable or optimal day and night temperature range for plant growth and maximum yields varies among crop species. The growth of most crops ceases below a critically low temperature and very high temperatures (usually above 30–35°C) have adverse effects.
Crops are divided into five adaptability groups based on their photosynthetic carbon assimilation pathways (C3, C4 or CAM) and according to the effects of radiation and temperature on photosynthesis.
Between the minimum temperature for growth and the optimum temperature for photosynthesis, the rate of growth increases more or less linearly with temperature; the growth rate then reaches a plateau within the optimum temperature range before falling off at higher temperatures.
Temperature interacts with radiation; the highest growth potential is achieved with both radiation and temperatures in the optimal range. In many temperate climates and at high altitudes in tropical countries, the temperature for growth is below optimum during part of the growing season.
Low temperatures occurring four to five weeks before harvest cause premature ripening in ‘Bartlett’ pears. The premature ripening was linked with rising levels of abscisic acid. Pre-harvest temperatures also affect the rate of ethylene production during ripening.
High production rates of ethylene in ‘Bartlett’ pears were common in fruit produced in regions with lower temperatures before harvest. The ethylene-forming enzyme (EFE) activity develops earlier in apples exposed to low night temperatures as opposed to fruit that mature under warm night conditions.
Furthermore, the daily-hourly average (DHA) temperatures occurring during the last six weeks before harvest were found to influence the acid and sugar content of pears. Increased acid and sugar levels were reported in pears produced at 17.2°C and 13.9°C DHA temperatures, whereas in pears grown at 20.0°C and 11.7°C, the ripening capacity was low.
The maturity of fruit is highly affected by pre-harvest temperature in the field. For example, grapes on a particular bunch showed differential maturity time due to variation in exposure to sunlight.
Fruits borne on the exposed side of the sun-ripened faster than those borne on the shaded side. These fruits also contained higher sugars and lower acidity than shaded fruits. On the other hand, avocado fruits exposed to the sun on the tree took 1.5 days longer to ripen than those which were on the shaded side.
A higher flesh temperature of about 35°C might have slowed down the ripening process in avocado. The same fruits when ethylene-treated exposed fruit being firmer ripened slowly than shaded fruit.
Pre-harvest exposure of fruit and vegetables to direct sunlight leads to several postharvest physiological disorders among which, sunburn or solar injury is the most prevalent temperature-induced disorder in fruit and vegetables.
Photosynthetic activity of the fruit is also hampered by high fruit temperature. In severe cases or frequent exposure, it leads to browning or blackening of the skin of produce due to tissue failure. This situation results in the complete inactivation of the photosynthetic system.
Pre-harvest temperature also has a considerable effect on the quality of fruit during and after postharvest storage. Generally, a higher temperature in the field is associated with higher sugar and lower acidity in the fruit.
Grapes and oranges grown at high temperatures contain a higher level of sugar and lower level of tartaric and citric acids respectively than those grown at low temperatures. The firmness of the fruit is also affected by temperature prevailing during fruit growth.
Avocado fruits grown on the trees exposed to direct sunlight having temperature around 35°C showed fruits 2.5 times firmer than those grown on the shaded side (20°C) of the tree.
The colour development of fruit is also influenced by temperature. Generally, warm days and cool nights during growth are conducive for colour development in fruits.
Disorders associated with pre-harvest exposure of fruit to high temperature or direct sunlight.
FruitDisorderSymptomsReferenceAppleSunburnSkin discolouration, pigment breakdownBergh et al. (1980), Wünsche et al. (2000)AppleWatercoreWater soaking of fleshMarlow and Loescher (1984)AvocadoSunburnSkin browningSchroeder and Kay (1961)PineappleFlesh translucenceWater soaking of fleshPaull and Reyes (1996); Chen and Paull (2000)LimeStylar end breakdownJuice vesicle ruptureDavenport and Campbell (1977)CranberrySunscaldTissue breakdownCroft (1995)
Source: Woolf and Ferguson (2000)
Influence of Relative Humidity on Leaf Growth and Crop Yield
Relative humidity directly influences the water relations of plants and indirectly affects leaf growth, photosynthesis, pollination, the occurrence of diseases, and finally economic yield.
Leaf growth does not only depend on synthetic activities resulting from biochemical processes but also on the physical process of cell enlargement. Cell enlargement occurs as a result of turgor pressure developed within the cells.
When RH is low, transpiration increases causing a water deficit in the plant. Water deficits cause partial or full closure of stomata and increase mesophyll resistance, blocking the entry of carbon dioxide.
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Importance of Light (Sunshine) in Fruit Development and Ripening
Light is essential in the production of chlorophyll for photosynthesis, the process by which plants manufacture food in the form of sugar (carbohydrate).
Other plant processes that are enhanced or inhibited by this climatic factor include stomatal movement, phototropism, photomorphogenesis, translocation, mineral absorption, and abscission.
Three properties of light that affect plant growth and development are light quality, light intensity, and day length or photoperiod. Light quality refers to the specific wavelengths of light; light intensity is the degree of brightness that a plant receives; day length is the duration of the day concerning the night period.
Increased exposure to light increases fruit size, total soluble solids, and flesh firmness as a result of high photosynthetic rates and carbohydrate reserves.
Three relevant aspects of radiation are (i) day length, (ii) its influence on photosynthesis and dry matter accumulation in crops, and (iii) its effects on evapotranspiration. Radiation levels may also be important in the drying and ripening of crops.
The vegetative growth of most plants increases linearly with solar radiation up to a limit beyond which no further increase occurs. In many tropical areas, water shortages rather than radiation limit growth and the radiation-limited potential is not attained.
However, marked seasonal effects on yields may be evident. In temperate countries, radiation is one of the most dominant growth-limiting factors in the winter months. Sunshine influences the pollination, development, and colouring of fruit.
1. Day length: Day length affects photoperiod-sensitive cultivars of crops such as rice, influencing floral initiation and the onset or length of vegetative and reproductive phases of growth and development.
The interaction of day length with water availability or temperature can sometimes prove class determining at the project level (e.g. in influencing the flowering of sugarcane, flowering and fruiting of mangoes, and in the bulbing and ripening of onions, etc.).
The influence of day length on plant development has been extensively studied, and virtually all crop plants as well as many weed species have been classified according to their response to light and the initiation of reproductive growth.
2. Wind: Wind serves as a vector for transferring pollen from one flower to another, thus aiding in the process of pollination. It is therefore essential for the development of fruits and seeds in wind-pollinated flowers, as observed in many grasses.
Moderate winds favour gas exchange, but strong winds can lead to excessive water loss through transpiration as well as the lodging or toppling of plants.
When the rate of transpiration exceeds that of water absorption, partial or complete closure of stomata may occur, which restricts the diffusion of carbon dioxide into the leaves. As a result, the rate of photosynthesis, plant growth, and yield will decrease.
During the growth period, wind may also cause damage to fruits and vegetables. Damage caused by wind can be grouped into two categories: damage from less frequent severe storms, and damage from frequent winds of intermediate strength.
High-velocity winds can damage leaves and cause defoliation in leafy vegetables, leading to a reduction in product appearance and market value. In fruit crops, defoliation often results in smaller fruits and poor fruit colour development, especially in citrus.
Mild winds may cause wind scarring disorders as fruits rub against twigs. The injured fruits develop tan-to-silvery patches, which enlarge as the fruit matures. The use of windbreaks is therefore recommended for fruit and vegetable production in areas exposed to excessive wind.
3. Air: Air is a mixture of gases in the atmosphere. Approximately 75% of air is located in the troposphere the lowest layer of the atmosphere which extends about 17 km above sea level at the equator and about 8 km over the poles. Around 99% of the clean, dry air in the troposphere is composed of 78% nitrogen and 21% oxygen.
The remaining portion includes argon (less than 1%), carbon dioxide (0.036%), and trace amounts of other gases. Oxygen and carbon dioxide are especially important to plant physiology.
Oxygen is required for respiration, which generates the energy used in plant growth and development. Carbon dioxide is a key raw material in photosynthesis.
Air also contains suspended particles of dust and various chemical pollutants such as carbon monoxide (CO), carbon dioxide (CO₂), sulfur dioxide (SO₂), sulfur trioxide (SO₃), nitrogen oxides, methane (CH₄), propane, chlorofluorocarbons (CFCs), soot, asbestos, lead, and ozone.
These pollutants especially ozone, sulfur dioxide, fluoride, and nitrogen oxides can severely damage crops and reduce the quality of fruits and vegetables.
During the summer, when high temperatures and solar radiation are common, ozone levels in the atmosphere usually increase due to elevated nitrogen and the emission of volatile organic compounds. Ozone enters plant tissues through stomata and causes cellular damage by increasing membrane permeability.
This may lead to plant injury. High ozone concentrations also disrupt photosynthesis and respiration, which affects postharvest quality in terms of appearance, colour, and flavour, and also accelerates the turnover of antioxidant systems.
Elevated ozone levels may cause yellowing or chlorosis in leafy vegetables, blistering in spinach, alteration in sugar and starch content in fruit and tuber crops, and a reduction in fruit size due to decreased biomass accumulation.
Fluoride causes discolouration in peach fruits. Similarly, high nitrogen dioxide levels lead to marginal and interveinal collapse of lettuce leaves.
In addition to air pollutants, heavy metal ions such as silver, cadmium, cobalt, magnesium, manganese, nickel, and zinc may enter the plant system through soil amendments, runoff, or contaminated irrigation water, which can also deteriorate the quality of fruits and vegetables.
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