Plant Physiology: Canopy Light Interception
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Questions and Answers

What is the primary reason for the yellowing of leaf tips during senescence?

  • Reduced photosynthesis leading to a decline in chlorophyll production
  • Loss of water from the leaves due to transpiration
  • Increased production of anthocyanin pigments
  • Breakdown of chlorophyll due to nitrogen mobilization (correct)

    • What is the relationship between Leaf Area Index (LAI) and light interception?

  • Higher LAI always leads to 100% light interception.
  • Lower LAI leads to more efficient light interception due to less competition between leaves.
  • LAI has no direct effect on light interception.
  • Higher LAI leads to greater light interception, but complete interception is unlikely. (correct)

    • What is the definition of the critical leaf area index (LAIcrit)?

  • The LAI at which the plant has optimal photosynthesis rates.
  • The LAI at which the plant reaches maximum height.
  • The LAI at which the plant produces the most seeds.
  • The LAI at which 95% of incident PAR is intercepted. (correct)

    • How does temperature influence leaf area index (LAI)?

    <p>Temperature influences multiple aspects of plant growth and senescence, thus impacting LAI. (C)</p> Signup and view all the answers

    Which of the following factors does NOT directly impact the leaf area index (LAI)?

    <p>Soil pH (C)</p> Signup and view all the answers

    What is the primary consequence of a high LAI in terms of photosynthesis?

    <p>Reduced photosynthetic efficiency due to self-shading. (C)</p> Signup and view all the answers

    What is the significance of leaf senescence in the context of LAI?

    <p>Senescence decreases LAI by removing leaf area. (D)</p> Signup and view all the answers

    How does LAI change over time in a crop?

    <p>LAI typically increases initially and then decreases as the crop matures. (C)</p> Signup and view all the answers

    What is the primary factor affecting the rate of leaf production in day-neutral crops?

    <p>Temperature (C)</p> Signup and view all the answers

    How does photoperiod affect long-day annual crops that emerge into decreasing photoperiods?

    <p>Extended vegetative growth period (B)</p> Signup and view all the answers

    What is the role of nitrogen nutrition in leaf expansion?

    <p>It helps determine leaf size (A)</p> Signup and view all the answers

    What is a direct effect of water stress on leaf expansion?

    <p>Results in smaller leaf size (A)</p> Signup and view all the answers

    What is the significance of axillary leaf production in plants?

    <p>It is crucial for establishing competition for light (C)</p> Signup and view all the answers

    Which type of crops tends to produce axillary leaves more quickly?

    <p>Annual species (B)</p> Signup and view all the answers

    What happens to leaf expansion rates with increasing temperature?

    <p>They increase (A)</p> Signup and view all the answers

    What primarily controls the processes of leaf senescence and death?

    <p>Genetic factors (D)</p> Signup and view all the answers

    What primarily influences the maximum yield of a crop under optimal conditions?

    <p>The availability of light intercepted as PAR (A)</p> Signup and view all the answers

    Which of the following factors does NOT influence the canopy size of a growing crop?

    <p>Color of soil nutrients (B)</p> Signup and view all the answers

    How does an increasing photoperiod affect the growth of long-day plants?

    <p>Delays vegetative growth (A)</p> Signup and view all the answers

    What relationship exists between the amount of light intercepted and crop dry matter produced?

    <p>A strong linear relationship (C)</p> Signup and view all the answers

    What is affected by the photoperiod at the planting location?

    <p>The time of crop emergence (A)</p> Signup and view all the answers

    Which of these factors is mainly under genetic control?

    <p>All of the above (D)</p> Signup and view all the answers

    Which crop characteristic is most likely to lead to increased biomass production?

    <p>Prolonged vegetative growth (B)</p> Signup and view all the answers

    Which factor impacts the duration of the vegetative growth stage in annual crops?

    <p>Time of flowering (A)</p> Signup and view all the answers

    What is the primary factor determining the value of LAIcrit for different crop species?

    <p>Leaf angle (B)</p> Signup and view all the answers

    How does the arrangement of leaves in a crop canopy affect light interception?

    <p>Leaves at the top of the canopy receive more light than leaves below. (B)</p> Signup and view all the answers

    What does the extinction coefficient (k) quantify in relation to light interception?

    <p>The efficiency of light transmission through the canopy. (D)</p> Signup and view all the answers

    What is the relationship between the extinction coefficient (k) and leaf orientation?

    <p>Crops with horizontal leaves have a higher k value than crops with erect leaves. (B)</p> Signup and view all the answers

    How does the leaf angle of alfalfa change within its canopy?

    <p>Leaf angle is more vertical at the top of the canopy and more horizontal at the bottom. (A)</p> Signup and view all the answers

    Why is green leaf area important for maximizing biomass yield?

    <p>Green leaves have a higher concentration of chlorophyll, which is essential for photosynthesis. (C)</p> Signup and view all the answers

    Which of the following factors can influence the leaf area of a crop?

    <p>Temperature (B), Rainfall (C), All of the above (D)</p> Signup and view all the answers

    What is the key to maximizing biomass yield in a crop?

    <p>Maximizing the amount of green leaf area in the canopy. (C)</p> Signup and view all the answers

    Flashcards

    Light Interception

    The absorption or reflection of PAR by the leaf canopy.

    PAR

    Photosynthetically Active Radiation; light used in photosynthesis.

    Canopy Size Impact

    Canopy size influences light interception and crop yield.

    Leaf Area Expansion

    The increase in leaf area that affects light capture.

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    Photoperiod

    The length of day that affects plant growth stages.

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    Time of Crop Emergence

    The date of sowing impacts the emergence and growth duration.

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    Vegetative Growth Duration

    Length of time a plant grows before flowering.

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    Flowering Time in Crops

    Timing of flowering affects biomass accumulation.

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    Plant Population Density

    Number of plants per area affecting light interception efficiency.

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    Water Supply Effect

    Impacts leaf size and how long they last.

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    Environmental Stresses

    Factors like frost or disease affecting leaf area.

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    LAIcrit

    Critical leaf area index for effective light interception.

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    Leaf Orientation Impact

    Arrangement affects how much light is captured.

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    Extinction Coefficient (k)

    Measures light transmission through the canopy.

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    Leaf Angle Variation

    Leaf angle changes from top to bottom in canopies.

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    Maximizing Green Leaf Area

    Key to boosting biomass yield in crops.

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    Phyllochron

    The rate of leaf appearance in annual crops, influenced by photoperiod and temperature.

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    Leaf Appearance Rate

    The frequency at which new leaves are produced on a plant.

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    Day-Neutral Crops

    Crops that do not depend on photoperiod for leaf production; driven by temperature instead.

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    Leaf Expansion

    The growth of existing leaves, crucial for increasing photosynthetic area.

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    Axillary Leaf

    The first leaf produced from the axil of a stem, significant for establishing a canopy.

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    Water Stress Effect

    Limited water availability that can reduce leaf size and expansion rate.

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    Nitrogen Nutrition

    The availability of nitrogen, crucial for leaf growth and size.

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    Leaf Senescence

    The aging process of leaves, controlled by genetics and environment.

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    Senescence

    The process of aging in plants leading to decreased photosynthetic potential.

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    Leaf Area Index (LAI)

    A measure of the leaf area per unit ground area in a crop canopy.

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    Critical Leaf Area Index (LAIcrit)

    The LAI value at which 95% of available light is captured.

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    Photosynthesis Maximization

    To achieve the best photosynthesis, LAI must exceed 1.0.

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    Chlorophyll Breakdown

    The process that causes yellowing in leaves during senescence.

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    Nitrogen Mobilization

    The movement of nitrogen from older leaves to younger parts of the plant.

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    Light Interception Efficiency

    LAI below 1.0 results in less than 50% light capture due to canopy gaps.

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    Factors Influencing Leaf Area

    Temperature and nitrogen status affect crop leaf growth and senescence rates.

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    Study Notes

    Plant Physiology: Interception of Solar Radiation by Canopy

    • Light interception is crucial for crop yield. Optimal temperature, nutrient, and water availability, along with weed and insect control, are essential factors.
    • Photosynthetically Active Radiation (PAR) is either absorbed or reflected by surfaces it first encounters.
    • Leaf canopy interception of PAR is key. If light isn't intercepted by active leaves, that energy is not used for plant growth.
    • Leaf area in the canopy directly influences how much PAR is captured and subsequently affects photosynthesis and overall crop yield. A strong linear relationship exists between the amount of light intercepted and crop dry matter produced.

    Leaf Area Expansion

    • The size of the canopy at a specific time is determined by several factors:
      • Time of crop emergence
      • Phyllochron (rate of leaf appearance)
      • Leaf expansion rate and duration
      • Rate of axillary leaf production
      • Rate of leaf senescence
    • Each of these factors can be influenced by genetics, environmental conditions, or agricultural management.

    Time of Crop Emergence

    • The date of sowing influences the time of seedling emergence.
    • Crop emergence and subsequent growth are strongly influenced by the photoperiod at the planting location.
    • Long-day plants that emerge during a decreasing photoperiod will have extended vegetative growth and develop a larger canopy.
    • In contrast, crops emerging into an increasing photoperiod will become reproductive sooner and thus develop a smaller canopy.
    • In annual crops, the time of flowering impacts biomass accumulation. Later-flowering crops typically produce more biomass than early-flowering ones. However, the time of flowering depends on the sowing date and photoperiod.

    Phyllochron

    • The rate of leaf appearance (phyllochron) and subsequent expansion on the main stem affect canopy expansion.
    • Leaf emergence rates vary depending on the planting time and prevailing photoperiod.
    • For example, long-day annual crops emerging into decreasing photoperiods develop at slower leaf production rates but with an increased vegetative growth duration.
    • Short-day crops exhibiting the reverse trend.
    • Photoperiod does not affect leaf production rate in day-neutral crops; temperature is the dominant factor in this case. The rate of leaf production increases with increasing temperature.

    Leaf Expansion

    • The speed of leaf expansion is directly connected to the increase in photosynthetic leaf surface area.
    • Each plant species exhibits a distinct ontogenetic trend in leaf size.
    • Leaf numbers on the main stem are affected by cultivation parameters.
    • Water stress negatively impacts leaf expansion rate, resulting in smaller leaves. Nitrogen nutrition is also crucial in determining leaf size.

    Axillary Leaf

    • Leaf appearance rates determine establishment success and competitiveness among species.
    • The time needed for the first axillary leaf production in the seedling's development stage is a key phase.
    • It signifies when leaf expansion begins to accelerate and the plant begins forming a canopy suitable for light competition.
    • Plants that reach axillary leaf production sooner gain a competitive advantage due to additional light interception. Annual plants typically develop axillary leaves faster than perennials.

    Leaf Senescence

    • Leaf senescence and death are integrated processes, primarily controlled by genetics but also affected by available nutrients and environmental conditions.
    • Leaf senescence involves a decrease in photosynthetic potential.
    • The visible signs of senescence include leaf tip yellowing due to chlorophyll breakdown as nitrogen is redistributed to younger leaves or storage areas like grains.
    • Senesced older leaves become dead material at the base of the canopy.

    Leaf Area Index (LAI)

    • LAI describes the leaf area per unit ground area.
    • LAI is zero at the beginning of crop growth (no leaves).
    • LAI gradually increases as seedlings emerge and develop leaves for light interception.
    • A higher LAI is needed to maximize photosynthetic activity, typically exceeding 1.0.
    • A LAI of 1.0 implies that less than 50% of incoming PAR will be captured, with significant light loss from the canopy.
    • At a critical LAI (LAI crit), 95% of the incoming light can be captured, but herbage accumulation is minimal because of high senescence.

    Factors Influencing LAI

    • Temperature—influences crop emergence, leaf and branch production, leaf expansion, and senescence
    • Nitrogen status—determines leaf size and longevity
    • Plant population density—influences the efficiency of light interception.
    • Water supply—influences leaf size and longevity.
    • Environmental stresses (frost, high temperature, herbivory, disease)—constrain leaf growth.

    Canopy Architecture and Light Interception

    • The value of LAI crit varies across different crops and species due to leaf orientation towards the sun; erect leaves enhance interception compared to horizontal ones.
    • Leaf arrangement and angle determine how PAR is intercepted and subsequently distributed within the canopy.
    • Some leaves are exposed to direct sunlight while others receive light transmitted or reflected from the canopy.
    • The effectiveness of light interception is measured by the extinction coefficient (k).
    • K values range from 0.2 for erect leaves to 1.0 for crops with horizontal leaves.
    • Leaf arrangement and angle play a crucial role in determining light interception patterns. The relationship between LAI crit, k, and crop growth rate is complex but can be understood by considering that higher LAI often correlates to more photosynthetic leaf available per unit ground area. The plant material intercepting the radiation should be green and photosynthesizing.

    Practical Applications

    • Manipulating crop canopies to maximize green leaf area is key for maximizing biomass yield.
    • Crop management aims at maximizing initial ground cover and maintaining healthy leaves. Early, rapid canopy development is vital.
    • Avoid premature leaf senescence.
    • Small-sized plants often possess smaller canopies as this trait is sometimes controlled genetically.
    • Crop management recommendations often encourage higher plant populations to improve solar radiation interception, thereby increasing competition amongst plants.
    • Nitrogen fertilization and irrigation are vital for healthy leaf growth and canopy development.

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