Crop Growth and Light Interception

Radiation Spectrum Overview
- The radiation spectrum is quite wide, extending from gamma rays to long radio waves.
- The visible spectrum is a small segment located in the middle of this range.
- Photosynthesis is driven by the visible spectrum, which represents the light detectable by the human eye.
Wavelengths Relevant to Plants
- The spectrum of visible light ranges from approximately $400$ to $700\,nm$ .
- Chlorophyll is the primary molecule responsible for absorbing light for photosynthesis.
- Chlorophyll absorption peaks at the blue and red wavelengths of the spectrum.
- Plants appear green because chlorophyll does not absorb green light; instead, the green wavelengths are reflected.
Photosynthetically Active Radiation (PAR)
- PAR refers to the specific wavelengths between $400$ and $700\,nm$ that drive photosynthesis.
- Measurement focus is placed on the entire $400$ to $700\,nm$ range rather than isolating specific blue or red levels.
Total Solar Radiation
- Total solar radiation is the total amount of radiation hitting the earth after being dispersed by the atmosphere.
- This is commonly measured by weather stations.
- As a general estimate, PAR constitutes approximately half ( $50\%$ ) of total solar radiation.

The Photosynthetic Reaction
- Light energy enters the system to trigger two different reactions.
- The primary objective of the reaction is the production of carbohydrates (sugars).
- The chemical components of the reaction include:
  - Input: Carbon dioxide ( $CO_2$ ) and water ( $H_2O$ ) combined with light energy.
  - Output: Carbohydrates (sugars) and oxygen ( $O_2$ ).

Correlation Between Light and Growth
- Dry matter production is directly related to intercepted solar radiation.
- Calculations for crops like sugar beet, barley, and wheat demonstrate that plotting intercepted radiation against dry matter production results in a nearly straight line.
- A significant distinction must be made between total hitting radiation and "intercepted" radiation; light hitting bare ground does not contribute to crop growth.
- The fundamental principle is that as a crop intercepts more solar radiation, there is a commensurate increase in total dry matter accumulation.

Leaf Area and Leaf Area Index (LAI)
- Leaf area is the primary factor in radiation interception.
- Crops with greater leaf area, such as certain hybrid or conventional canola varieties, will intercept more light than those with sparse leaf area.
- Leaf Area Index ( $\text{LAI}$ ) is the specific metric used to measure leaf area to determine the scale of light interception.
Leaf Shape and Venation
- Leaf structures differ significantly between species:
  - Canola (Dicot): Features a complex venation pattern throughout the leaf.
  - Wheat (Monocot): Features parallel veins.
Leaf Angle
- The angle of the leaf determines how it catches incoming radiation.
- Horizontal Leaves: Common in crops like canola. These leaves are flat and intercept more radiation when light falls perpendicularly (straight down).
- Vertical Leaves: Common in crops like wheat. These leaves stand more upright. While they may not intercept light perpendicularly, they allow light to be spread down the entire leaf surface.
Light Saturation and Yield Potential
- Horizontal leaves reach a point of light saturation quickly.
- Vertical leaves are often bred into high-yielding lines (especially in rice) because they do not reach light saturation as fast, allowing for higher yield potential by distributing radiation more efficiently throughout the canopy.

Photosynthetic Pathways in Relation to Irradiance
- C3 Plants: The photosynthetic rate plateaus relatively quickly as irradiance increases. This plateau is referred to as being light saturated and is partly limited by the supply of oxygen available for photosynthesis.
- C4 Plants: These plants (e.g., maize, sorghum) continue to increase their photosynthetic rate at higher levels of radiation and do not saturate as easily as C3 plants.
Canopy Dynamics
- While individual C3 leaves saturate quickly, the behavior changes when they are part of a crop canopy.
- Certain crops like alfalfa (lucerne) and sunflower can reorient their leaves to track the sun, maximizing light use.
- Within a canopy, C3 crops such as wheat can still achieve high levels of photosynthesis comparable to C4 crops like maize due to vertical leaf structures and canopy-level light distribution.

Definition and Importance
- Leaf Area Duration ( $\text{LAD}$ ) refers to the length of time green leaves remain available to intercept light.
- Only green leaves contribute to growth; brown leaves may intercept light, but they do not facilitate photosynthesis.
The Stay-Green Trait
- Incorporated into species like sorghum and maize over the last decade.
- This trait allows plants to maintain green leaves longer, even under drought stress.
- While standard plants might undergo senescence (die-off) during drought, stay-green plants maintain their foliage, allowing them to resume growth immediately if it rains.
Impact of Disease
- Disease causes yellowing or necrosis in leaves.
- Infected crops (e.g., wheat with foliar diseases) suffer reduced growth because they cannot intercept as much light as a healthy, green crop.

Row Spacing Strategies
- Skip-Row Sorghum: This involves skipping Every other row or specific outer rows during planting to store moisture in the soil between rows. This is highly effective in low-rainfall environments where roots can explore the gaps over time.
- Solid-Row Sorghum: Standard planting where all rows are filled.
Yield Limitations of Skip-Row Systems
- In high-rainfall environments, skip-row systems limit yield potential because the large gaps in the canopy fail to intercept available solar radiation.
- If moisture is not the limiting factor, plant configurations should aim to maximize radiation interception by closing the canopy.

Environmental Profiles
- Australia (Southern New South Wales): Rice is grown in regions like the Murray Irrigation Area ( $\text{MIA}$ ) near Wagga. The climate is characterized by dry summers with very clear skies, long days, and high levels of total solar radiation.
- Japan and Asia: Rice is often grown in monsoonal climates during the wet season. These environments are characterized by heavy cloud cover.
Radiation as a Yield Driver
- Australian rice yields are frequently higher per hectare than those in Japan or other parts of Asia because the Australian crops receive significantly more light.
- Even when both crops are perfectly managed (free of weeds, pests, and disease), the Japanese crops cannot match the yield potential of Australian crops because the cloud cover reduces the amount of radiation hitting the plants.
- The total accumulation of intercepted radiation over the growing season is the primary driver of yield differences across these environments.