Environmental factors influencing Growth of Vegetables

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Environmental Factors Influencing the Growth of Vegetables HORT- 503

Transcript of Environmental factors influencing Growth of Vegetables

Page 1: Environmental factors influencing Growth of Vegetables

Environmental Factors Influencingthe Growth of Vegetables

HORT- 503

Page 2: Environmental factors influencing Growth of Vegetables

CLIMATIC FACTORS

Growth and development of plants depend on climatic as well assoil factors. Weather is the state of atmosphere with respect to temperature, moisture, solar radiation, air movement, and other meteorologicalphenomena over a short period of time. Climate is the average course of weather at a specific location over aperiod of many years and is the integrated effect of weather. The kinds or plant species that will or can be grown in a given regionIs predicated by the climate.The growth of an individual plant is directly dependent on theWeather during its life cycle.

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TEMPERATURETemperature has profound effects on all living organisms, favoringor limiting growth and thereby influencing the distribution of bothplants and animals. The source of this energy is the sun.Each minute about 2.0 g cal/cm2 is transmitted to the earth.

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Global Temperature Distribution

Although the world is a rotating sphere and each latitude receivesits portion of the light intensity.The temperature of any particular point on the earth is governed byits proximity to other land masses and to air and ocean currents.Thus, a world map of temperature is not uniform around the earth. The equatorial zone is consistently warmer, and polar regionsconsistently colder than the mid-latitudes. At high latitudes, the difference in light intensity is very large,resulting in four seasons: winter, spring, summer, and autumn.If it were not for the tilt of the earth, there would not be distinctiveseasons because temperature and climate would vary little and theday length would be uniform.

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The range of air temperature variation is much larger over thecontinents than over or near the oceans or seas because ofmoderation provided by the large heat capacity of water ascompared to soil.Fluctuations within 24 hours of a day occur due to the rotation ofthe earth, as solar radiation is not received at night, and fromradiation losses which occur from the surface at night. Thus, each day has a temperature maximum which occurs shortlyafter noon and a minimum which occurs at sunrise.Temperatures also decrease with increase in altitude, usually adecrease of about 6° C occurs with each 1000 m in elevation.

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Temperature Effects on Plants

Cardinal TemperaturesLength of Growing SeasonDiurnal Change (Thermoperiodicity)VernalizationFreezing and Chilling InjuryHardeningHigh-Temperature Injury

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Cardinal Temperatures

The cardinal temperatures are minimum, maximum, and optimum.Minimum or maximum temperature is where growth ceases, andOptimum is where growth is most favorable and/or rapid. The cardinal temperatures are not the same for all plants and varywith different families, genera, and species. Plant breeders select and develop plants for tolerance to lowtemperature in cool regions and for high-temperature tolerance inregions where that is appropriate.

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Length of Growing SeasonOne of the oldest and useful measures of evaluating and predictingwhen crops can be grown is the number of frost-free days. This is the average period between the last killing frost in the springand the first killing frost in autumn.

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Heat Units or Degree-DaysThe heat unit or degree-day concept is based on the theory thatplant development is dependent on the amount of heatexperienced during growth.It is calculated by subtracting the minimum threshold temperaturefrom the average temperature for a given day. The minimum threshold temperature varies with different crops,but usually ranges from 5°C to 15°C.The daily average is obtained by adding the minimum and maximumtemperature and dividing by two. For each day, every degree of difference above the threshold has avalue of 1. The units are accumulated for a desired period, usually from plantingto harvest.

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For example, if the average temperature for a day is equal to or lessthan the threshold temperature, the degree-day value is 0. If the average were 5°C above the threshold value, the degree-dayvalue would be 5.This concept is useful for crop scheduling and harvest predictionand is generally more accurate than reliance on previous croppinghistories, such as days from planting to harvest. For some vegetables such as peas and sweet corn, the degree-dayconcept functions very well. For others, clearly established thresholds with regard totemperature response are unclear or are yet to be identified.

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Diurnal Change (Thermoperiodicity)Generally, a large diurnal range is favorable for net photosynthesis.Night temperatures play an important role for some crops. High night temperatures increase respiratory rates, which result inan increased utilization of carbohydrates produced during the day.In such situations, the plant makes little, if any, progress in growthor development.

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VernalizationThe exposure of certain plants to low temperatures induces oraccelerates bolting and/or flowering. This inductive stimulus iscalled vernalization.The required length of low-temperature exposure to achievethis effect varies with species and stage of plant development. With some species, devernalization, a reversal of vernalization, canoccur if the plant is immediately exposed to high temperatures,usually above 30°C, following low-temperature exposure. In some situations, reversal is partial or not observed.

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For some species, seedling and young plants are insensitive tolow temperature conditions that promote flowering in older plants.Such plants at that growth stage are considered juvenile ornonresponsive.With other species, even newly germinated seed can be vernalized.Some tubers, corms, and bulbs require chilling temperaturesbefore dormant buds become active, and others require lowtemperatures before resumption of growth.

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Rest and Dormancy in Seeds and OrgansMost mesophytic plants go through a phase of little or no growth atsome stage in the life cycle. Seeds, buds, tuberous roots, tubers, rhizomes, bulbs, and cormsoften show this phenomenon. Under natural conditions, this period usually coincides withunfavorable environmental conditions such as low or hightemperature or a lack or excess of moisture; sometimes,photoperiod is involved.However, even when the environmental conditions are favorable,seeds and vegetatively reproductive organs are said to be at restwhen the organ shows no signs of growth resumption. This rest period is also called internal or innate dormancy.

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Seeds and organs that have the potential to germinate or resumegrowth but do not because of unfavorable environmentalconditions are termed dormant. Other terms describing this state are external, imposed, andquiescent dormancy.Actually, the phases of rest and dormancy are not abrupt, but occurgradually as changes in inhibitor and/or hormonal concentrationsoccur over time. The rate of change is strongly influenced by temperature.Some plant physiologists describe several progressive stages suchas early rest (predormancy) ~ rest (mid-dormancy) ~ after rest(postdormancy).

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Normally, when the organ is at rest, it may be extremelydifficult to induce a resumption of growth. However, sometimes, high temperature treatment at early rest mayresult in resumed growth.Following a period of rest, there is a transition period when itbecomes progressively easier to induce growth. Temperature at which the organ is maintained can influence therate of these changes; at high temperatures, each stage is shorter

induration than at low temperatures.

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Freezing and Chilling InjuryMany cold-climate plants may be frozen by low temperatureswithout incurring injury. However, most vegetable plants are injured by temperatures at or slightly below freezing, unless they can be acclimated to toleratethese levels.Many tropical and subtropical plants can be damaged at nonfreezing temperatures below 10 oC, this type of damage is knownas chilling injury. Vegetables of tropical origin are sensitive to such injury whenexposed to prolong periods of less than 10 oC.

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A few hours at less than 10 oC but above freezing usually is notharmful; generally, no harm occurs if warm temperatures soonfollow the low temperature exposure. These kinds of injury have a time and temperature interaction.Susceptibility to cold and chilling injury varies with species andthere can be large differences among cultivars of the same species.Susceptibility also varies with the stage of plant development;Flowering and fruit development are highly susceptible periods.

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HardeningPlants can be somewhat modified (acclimated) and made relativelytolerant to cold temperature by subjecting the plant each day toLower and lower temperatures. This process, called hardening,causes an adaptation of cell protoplasm to low temperatures. Such a process takes place naturally in the autumn of the year.Hardening is not protection against freezing. Hardening also occurs when plants are subjected to gradual waterstress or nutrient deprivation. Both conditions are useful to acclimate plants for high- or lowtemperature stress.

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High-Temperature InjuryIn many arid and semiarid regions, high temperature may be alimiting factor in the economic production of some vegetables.Under high light intensity and high humidity, leaf temperatures 8°Cabove that of ambient air temperature have been recorded. When the temperature rise is too great, heat destruction ofprotoplasm results in cell death.This often occurs in the range 45-50°C.For example, in some field conditions, with air temperatures above38°C, tomato fruit temperatures of 49-52°C have been observed.Sun-exposed fruit in such conditions become damaged because ofsunburn or sun scald.

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As with cold resistance, plant's tissues, to a certain extent, cangradually become heat acclimated by slowly raising temperaturesand increasing the exposure period to such conditions each day.Transpiration from leaf stomata helps cool leaves. It has been calculated that transpiration can reduce heating byabout 15-25%.

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Thermo classification of VegetablesVegetables can be loosely grouped according to climatic

preference,such as cool season and warm season crops. This classifies vegetables according to their optimum-temperaturerequirements and tolerance for certain minimum and maximumtemperatures, and although not precise, it does have usefulness.The cool season crops can be further divided into a grouping thatincludes crops such as spinach, beet, and many brassica vegetablesthat have a growth optimum between 16°C and 18°C and sometolerance to freezing. Another grouping has a similar temperature optima, but the cropsare damaged by freezing temperatures. White potato, celery, lettuce, cauliflower, and Chinese cabbage aresome ofthe crops typical of this group.

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Another cool season grouping involves crops such as onion andasparagus that have a higher optimum-temperature range(18-30°C) and some frost tolerance.The warm season crops likewise can be separated into a grouphaving temperature optima between 18°C and 30°C andintolerance to frost.Most vegetable legumes, tomato, pepper, and many cucurbit cropsare representative of this group. Other crops such as sweet potato, yams, cassava and okra have anoptimum growth range between 21°C and to 35°C but will not growwell at temperatures less than 21°C. All can suffer chilling injury by exposure to extended periods oftemperature less than 10°C.