PBL 1: Environment I, The Atmosphere

Question 1

Light and Plants: OVERALL

Answer 1

UV light+shorter wavelengths of visible light promotes formation of anthocyanins (plant pigments). Can be involved in the inactivation of certain hormonal systems important from stem elongation and phototropism.

-Although epidermis reduces the amount of UV entering tissue/cells, an increase in UV exposure can damage leaf cells, inhibit photosynthesis/growth, promote mutations. Halocarbons can affect/destroy the ozone layer.

-Infrared light energy influences hormones involved with germination, plant’s response to changes in day length, etc.

-Chlorophyll photoreceptors are most absorptive of violet-blue and orange-red light. (green+yellow light not as useful, unable to be absorbed. most green is reflected back, making plants appear green)

*Light wavelengths that chlorophyll absorbs best correspond roughly to the wavelengths at which photosynthesis is most efficient.

Question 2

Characteristics of Visible Light Exposure: Quality

Answer 2

Quality: the relative amounts of the colours that make it up (varies)
-Most of direct sunlight at the earth’s surface is at the centre of the visible light spectrum
-The diffuse light from the sky (i.e. shade of a building) is relatively higher in blue and violet light.
-Light quality=photosynthetic efficiency (different portions of the light spectrum can be used for photosynthesis more efficiently than others)

Question 3

Canopy species and light quality

Answer 3

-In some cropping systems canopy species remove most of the red+blue light, leaving primarily transmitted green+far-red light

-Light quality can therefore become a limiting factor for plants under the canopy, even though the total amount of light may be adequate

Question 4

Characteristics of Visible Light Exposure: Intensity

Answer 4

-The total energy content of all the light that reaches a leaf surface is the intensity of that light
-Light intensity can be expressed in Langley (cal/cm2), watt(J/s), and Einstein (6x1023 photons)

Question 5

Saturation point

Answer 5

At very high light intensities, photosynthetic pigments become saturated: additional light does not increase photosynthesis.
-Excessive light can lead to degradation of chlorophyll pigments and harm plant tissue.

Question 6

Compensation point

Answer 6

Low levels of light can bring a plant to the level of light intensity where photosynthate amount=respiration amount. When light intensity<compensation point, energy balance for the plant is negative.
-If negative balance is not offset by a time period of active photosynthesis and energy gain, the plant may die.

Question 7

Characteristics of Visible Light Exposure: Duration

Answer 7

-The length of time that leaf surfaces are exposed to sunlight each day can impact photosynthetic rates as well as long-term plant growth+development

Photoperiod: Total number of hours of daylight, is an important aspect of the duration of light exposure.

Question 8

Duration of exposure to excessive levels of light in regards to time

Answer 8

-Duration of light exposure is an important variable in how light intensity/quality can affect a plant.

-Exposure to excessive levels of light for a short time can be tolerated, a longer-time can be damaging.

OR

-Short period of intensive light could allow the plant to produce an excess of photosynthate, can then allow for tolerance of a longer period below the light compensation point.

Question 9

Determinants of Variations in the Light Environment

Answer 9

-Seasonality
-Latitude
-Altitude
-Topography
-Air quality
-Structure of Vegetation Canopy

Question 10

Light: Seasonality

Answer 10

-Except at the equator, daylight hours are longest during the summer and shortest during the winter

-Since the angle of the sun in relation to the surface is much lower towards the poles during the winter, the available sunlight has to pass through more atmospheres before it reaches the plant, making that sunlight much less intense.

-Intensity and duration of light are affected by seasonality, plants are adjusted to the seasons

Question 11

Light: Latitude

Answer 11

The closer to either of the poles, the greater the seasonal variation in day length.

Question 12

Light: Altitude

Answer 12

-Higher elevation=higher light intensity, because the thinner atmosphere absorbs and disperses less light. Thus, plants are subject to light saturation and face greater danger of chlorophyll degradation.

-More UV as well= protection needed against damage. Many high elevation plants have evolved to reduce the amount of light penetrating the leaves (i.e. reflective coloration, protective hairs/scales on leaf cuticles).

Question 13

Light: Topography

Answer 13

-Slope and direction of the soil surface can create localised variations in the intensity and duration of exposure to sunlight

-Temperature difference can occur, and slope orientation may cause variation (i.e. a topographic feature=hill shadows over vegetation)

Question 14

Light: Air quality

Answer 14

-Smoke, dust, other pollutants, can interfere with photosynthetic activity either by: reducing the amount of light energy that reaches the leaf, or by coating the leaf+cutting down the amount of light that penetrates the cuticle

-Air quality problems are most common in urban/industrial regions.

Question 15

Light: Vegetation Canopy structure

Answer 15

-Depending on canopy structure, leaves will overlap: adds to the density of the canopy and reducing the quantity+quality of light that reaches the soil surface.

-Sunlight may pass between leaves/spaces that become available between leaves as wing moves the canopy and sun moves across sky.

Question 16

Relative Rate of light transmission

Answer 16

(of a canopy) is the average amount of light that is able to penetrate the canopy as a percentage of the total incident light available at the top of the canopy or on the surface of an adjacent area free of vegetation.

-Change in average light penetration depends on the density of the foliage and leaf arrangement: another way of determining the potential for light absorption is to measure leaf area index (LAI).

Question 17

Leaf area index (LAI)

Answer 17

-Found by calculating total SA of leaves above a certain area of ground (identical units, m2)= LAI is unitless. (i.e. if LAI=3.5, that’s the number of layers that light will have to travel through before reaching the ground)

-Both total light intensity and quality are reduced as we enter deeper into the vegetative cover.

Question 18

Photosynthetic Rate

Answer 18

Determined by three sets od factors:
1.) plant’s developmental stage
2.) environmental conditions surrounding the plant
3.) type of photosynthetic pathway (C3, C4, CAM) used by the plant

2.) Impacted by:
-temperature
-light intensity
-light quality
-duration of light exposure
-availability of CO2, moisture, wind
All of these: plant has maximum and minimum tolerances, and optimum condition

Question 19

Other forms of response to light

Answer 19

1. Germination
2. Growth and Development
3. Establishment
4. Plant Growth
5. Phototropism
6. Photoperiod

Question 20

Other forms of response to light: Germination

Answer 20

-Seeds of many plants require light to germinate; when buried beneath the soil they do poorly
-A light-sensitive hormone controls the response

Question 21

Other forms of response to light: Growth and Development

Answer 21

Light intensity or duration of light exposure can control the plant’s response, either as a: stimulus for the response or as a limiting factor.

Question 22

Other forms of response to light: Establishment

Answer 22

-Early seedling establishment an be affected by light levels, especially when seed germination or seedling establishment takes place under the canopy of established plants

-Some seedlings are less shade tolerant than others, and have more difficulty establishing when there is a lack of sufficient light.

Question 23

Other forms of response to light: Plant Growth

Answer 23

-Amount of light reaching leaves can become limiting when competition for light begins to occur. Competition for light is especially likely in same-species plant populations or in plant communities made up of similar species with similar light needs.

-Stem+leaf growth can be limited if a plant is completely shaded by neighbours. If some part of the plant is able to reach full sunlight, photosynthesis in that part may be able to compensate for shading on the rest of the plant.

-Many plants develop anatomically different leaves depending on the level of shading or sun.

*Shade leaves are:
-thinner and have larger SA,
-thinner epidermis
-less photosynthetic pigment
-spongier leaf structure
-more stomata than sun leaves
-important that these leaves are protected from too much light.

Question 24

Other forms of response to light: Phototropism

Answer 24

-Light can induce a plant to synthesise chlorophyll and anthocyanin, which stimulate growth in certain plant parts (leaf petiole/flower peduncle), causing the plant to grow toward or away from light

-Leaves can be oriented towards the sun to capture more light, or away from the sun in high-light environments.

Question 25

Other forms of response to light: Photoperiod

Answer 25

-Plants have developed adaptive responses to the changing light/dark regimes over time: correlation of hours of light/dark with other climatic factors (temperature)

-“Short-day” response: flowers start when nights get longer
-“Long-day” response: flowers start when nights get shorter

Question 26

Photoperiod: Phytochrome

Answer 26

-Pigment that is the major photoreceptive agent responsible for regulating these light responses.

-2 forms: 1. absorption peak for red light, 2. absorption peak for far-red light.

-In daylight, the red light form is rapidly converted to the far-red form, and in the dark, the far-red form slowly converts back to the red form.

-The far-red phytochrome is biologically active and responsible for the basic responses of plants to the number of hours of light/darkness.

-In the morning, after a few minutes of light exposure, the far-red phytochrome becomes the dominant form and remains throughout the day. This dominance is maintained into the night, since the conversion back to red phytochrome during darkness is slow.

-Therefore, far-red phytochrome stays dominant in relatively short nights.

Question 27

Managing the light environment in agroecosystems: crop selection

Answer 27

-One aspect of managing the light environment is to match the availability of light in the system to the plants’ response to light.

-C4 plants require high light intensity and long duration of light exposure for optimal production. They are not well adapted to areas with cooler, moisture conditions.

-C3 plants will not grow well in the light conditions favoured by C4 plants.

Question 28

Managing the light environment in agroecosystems: Carbon partitioning and Sustainability

Answer 28

-a relatively small percentage of the carbon that gets fixed by photosynthesis into carbohydrate form eventually gets transformed into biomass

-increasing the proportion of carbon partitioned into harvestable material isn’t necessarily always positive (i.e. stems are used for animal food)

-Possible impacts of this “loss” of organic matter on such agroecosystem ecological components like:
1. soil organic matter maintenance
2. soil aggregate stability
3. biological activity in the soil
4. nutrient inputs that are essential for the long-term sustainability of the agroecosystem

Question 29

Temperature

Answer 29

-Temperature range and degree of temperature fluctuation in an area can set limits on the crops species and cultivars that a farmer can grown

-can cause variations in quality and average yield for the crops that are grown

-When selecting crops, it is necessary to consider the range of temperature conditions that might occur form day-day, between day and night and from season to season.

-one must be concerned with both aboveground and belowground temperature

Question 30

Patterns of Temperature Variation on the Earth’s Surface

Answer 30

1.Latitudinal Variation
2. Altitudinal Variation
3. Seasonal Variation
4. Maritime vs. Continental Influence
5. Topographic Variation

Question 31

Patterns of Temperature Variation on the Earth’s Surface: Latitudinal Variation

Answer 31

-At/near the equator incoming radiation strikes the earth’s surface at a vertical angle. At increasing distances from the equator, the sun’s rays strike the surface at an increasingly shallow angle. (Shallower= incoming solar radiation spreads over larger SA of Earth)

-Sun rays must pass through an increasingly thick atmospheric layer at higher latitudes, resulting in a loss of energy to reflection and scattering by materials in the atmosphere (i.e. water droplets+dust)

-Overall effect is a regular decline in the intensity of solar radiation per square unit of surface as one moves away from the equator.

Question 32

Patterns of Temperature Variation on the Earth’s Surface: Altitudinal Variation

Answer 32

-At any latitude as altitude increases, temperature decreases. (On average, 100m elevation gain= 0.5C ambient temperature drop.)

-Increasing thinness of atmosphere at higher altitude results in greater heat loss from both soil surface and hair above it by reradiating at night.

-Contributes significantly to lower nighttime temperatures at elevations much above sea level.

Question 33

Patterns of Temperature Variation on the Earth’s Surface: Seasonal Variation

Answer 33

-Seasonal differences in temperatures over the earth’s surface are the result of changes in the orientation of the earth in relation to the sun as it revolves around the sun on its tilted axis.

Question 34

Patterns of Temperature Variation on the Earth’s Surface: Maritime vs Continental Influence

Answer 34

-Large bodies of water (especially oceans) greatly affect the temperature of adjacent land masses.

-Water reflects a larger proportion of insulation in relation to land, loses heat readily through surface evaporation, has a high specific heat and readily mixes layers vertically, the temperature of large water bodies is slower to change than that of land masses

-Land heats up more during the summer because all the absorbed heat stays in the surface horizon and the atmosphere close to that surface, cools to a lower temperature during the winter because of reradiation and heat loss.

-Water masses: moderators of broad temperature fluctuation: lower temperatures in the summer, raise temperatures in winter (maritime influence). Distance from water is continental influence- more widely fluctuating.

Question 35

Patterns of Temperature Variation on the Earth’s Surface: Topographic Variation

Answer 35

-Slope orientation+topography introduce temperature variation especially at local level: e.g. equator-facing slope is significantly warmer than a pole-facing slope

-Valleys are subject to nighttime microclimate variation as well. Higher elevation slopes above a valley, leads to cold air drainage.

Question 36

Cold Air Drainage.

Answer 36

-reradiation occurs more rapidly; cooled air is heavier than warm air below, the cooler air begins to flow downslope.

-Often cooler air passes under warmer air, pushing warmer air and forming an inversion in which a warmer layer of air becomes “sandwiched” between two layers of colder air.

Question 37

Describing Temperature Variation

Answer 37

-Lowest annual minimum: Regardless of the date, what is the coldest possible temperature (or coldest ever recorded) at a location?

-Highest annual maximum: “”

-Highest daily maximum and lowest daily minimum

-Average daily maximum and minimum
-Magnitude of difference between average daily high and low

-duration of extreme heat or cold: for how many successive days may the temperature drop below a certain threshold at night?

Question 38

Effects of Climate Change on Temperature

Answer 38

-Temperature in general will increase in many areas of the world
-Areas closer to the poles are likely to see larger increases in average+maximum temperatures than those near the equator

-Weather and climate variability are likely to increase in most areas, meaning that extreme temperatures will increase in both frequency+magnitude.

Question 39

Responses of Plants to Temperature: Heat

Answer 39

-Heat stress causes a decline in metabolic activity (inactivation of enzymes/other proteins)

-Heat raises the rate of respiration. If rate of respiration>rate of photosynthesis= plant growth stops and plant tissue is killed.

-Excess heat can lead to:
1. high CO2 compensation, changes in leaf structure
2.White/grey leaves that reflect light/absorb less heat
3. leaf hairs (pubescence) that insulate leaf tissue
4. Small leaves, less SA to expose to light
5.Vertical orientation of leaves to reduce heat gain
6. Extensive roots/greater root-to-shoot ratio to absorb water/offset water loss
7. Thick/corky/fibrous bark that insulates cambium+phloem in plant trunk

Question 40

Responses of Plants to Temperature: Cold

Answer 40

-Dormancy occurs when temperatures drop below minimum required for growth.

-Wax/pubescense provides resistance to cold: allows leaves to endure without freezing interior tissue/smaller cells in leaf that resist freezing

-Temporary cold hardiness can be induced in some plants by short term-exposure to temperatures a few degrees above freezing/witholding water for a few days.

Question 41

Hardening

Answer 41

Limited resistance to extreme cold in plants.

Question 42

Responses of Plants to Temperature: Vernalization

Answer 42

Some plants need to undergo a period of cold (vernalisation) before certain developmental processes can take place.

Question 43

Microclimate (General)

Answer 43

The localised conditions of temperature, humidity and atmosphere in the immediate vicinity of an organism

Question 44

Microclimatic Profile

Answer 44

The specific microclimatic conditions along a vertical transect within a cropping system.

Within a cropping system, the conditions of:
-temperature
-moisture
-light
-wind
-atmosphere quality

vary with specific location.

Question 45

Canopy Vegetation

Answer 45

Trees/tall plants that create a canopy over the other plants in a system can greatly modify the temperature conditions under the canopy.

Shade from canopy
-helps soil retain moisture
-Reduces solar gain at soil surface

Question 46

Soil Surface Cover

Answer 46

-Changes in soil temperature microclimate can be induced by covering the surface of the soil.

-Growing a cover crop is a well-recognised method of modifying soil temperature

Question 47

Cover Crop

Answer 47

-shades soil, reduces soil temperatures, positive impacts on soil organic matter content, seed germination, moisture conservation.

-Living mulch: a cover crop that is planted in-between active crop plants. Makes soil surface less reflective and raises temperature immediately above the crop. Can also have the opposite effect by increasing evaporation off of vegetation.

-Nonliving mulch: effect depends on colour, texture, thickness of material. (e.g. straw from wheats/oats used as a dry mulch).
Plant derived mulches eventually get incorporated into the soil.
Recent times, Nonplant mulch include: newspaper, cardboard, cloth, plastic sheeting.

-No-till system lets mulch accumulate naturally, prevents moisture loss

-Changes the colour of the soil surface to alter the amount of solar energy it absorbs. Burning crop residue: burning darker absorbs more heat.

Question 48

Greenhouses and Shade Houses

Answer 48

-often used to conserve/trap heat.

-light energy penetrates glass/plastic and inside it is absorbed and reradiated as long-wave heat energy, which is trapped inside.

Question 49

Methods of Preventing Frost Damage

Answer 49

-Mulching and row covers

-Raising soil moisture with irrigation when frost is expected may raise temepratures close to the ground because evaporation transfers heat from soil to vapor, which surrounds the plants. (increased atmospheric moisture also provides protection)

-Water: freezing to protect the plant: ice is always 0 degrees

-Smudging: a fuel is burned to generate heat-trapping smoke/create air turbulence to keep cold air from settling in depressions during a calm night.

Question 50

Relative humidity

Answer 50

ratio of water vapour content of the air to the amount of water vapour the air can hold at that temperature.
(i.e. at 50%, the air is holding 50% of the water vapour it could hold at that temperature)

-Can change as a result of chanciness in either absolute amount of water vapour or temperature changes.

-If absolute amount of water vapour in the air is high, small temperature variations can influence humidity greatly.

-When 100% reached, vapour begins to condense into water droplets, shows up as dew. This temperature is called DEW POINT.

Question 51

Precipitation

Answer 51

Primary natural source of water for agroecosystem: usually in the form of rain or snow

-occurs when droplets of water in cloud become heavy enough to fall. Usually occurs when moisture-containing air rises and begins to cool.

-As air cools, its ability to hold moisture in vapour form or as small cloud droplets decreases, resulting in more condensation/aggregation of droplets.

Question 52

Hydrological Cycle

Answer 52

a global process moving water from the earth’s surface to the atmosphere and back to earth.

-the core of the cycle is made up of evaporation and condensation (includes precipitation).

-when 100% relative humidity reached, condensation begins to occur, small water droplets form and aggregate to create clouds.

Question 53

Evapotranspiration

Answer 53

-Evaporation occurs at the earth’s surface, as water evaporates from soil, bodies of water, wet surfaces

-Transpiration is part of the mechanism by which plants draw water from the soil into their roots

Collective term for all of these is evapotranspiration

Question 54

Describing Rainfall Patterns

Answer 54

Each region of the earth has its characteristic patterns of precipitation

Important determinants of agriculture in a particular region:
-total amount of precipitation received in a typical year
-total yearly distribution, -intensity+duration of precipitation events
-regularity+predictability of precipitation patterns

Question 55

Average total annual rainfall

Answer 55

Total amount of precipitation that falls in an area during an average year

-good indicator of the moistness of the area’s climate

Question 56

Distribution and periodicity

Answer 56

how rainfall is spread out through the year, both on average and during a specific year

-in many parts of the world, rainfall is distributed to create predictable wet+dry periods

Question 57

Intensity and duration (mm/h)

Answer 57

how intense the rainfall is, what length the rainfall occurs

Question 58

Availability

Answer 58

how much of the rainfall becomes available as soil moisture

Question 59

Predictability

Answer 59

Every region has a characteristic degree of variability in its rainfall patterns.

-the higher the variability, the less predictable the rainfall for any particular time period.