Photosynthesis and Light

Response of Photosynthesis to Light

• Photosynthesis is affected by the amount of incoming solar, and photosynthetic photon flux density (PPFD)
• The article will discuss:
  • Absorption of solar radiation and leaf traits that maximize photosynthetically active radiation (PAR) absorption
  • Comparison of sun and shade leaves
  • How leaf photosynthesis responds to changes in photosynthetic photon flux density
  • The impact of too much light and how plants dissipate excess energy

Why Examine Photosynthesis?

• The Earth receives 5.2 x 10^21 kJ year⁻¹ of solar radiation, with about 50% in wavelengths used for photosynthesis
• Only 0.05% of solar energy (3.8 x 10^18 kJ year⁻¹) is captured in organic molecules
• CO2 turnover is about 3 x 10^12 t year⁻¹, nitrogen turnover is 5% of that
• Marine organisms account for about 50% of global photosynthesis
• Photosynthesis produces 5 x 10^11 t year⁻¹ of organic matter annually
• Earth's organic matter is estimated at 10^11 to 10^12 t of dry matter
• Burning fossil fuels and deforestation contribute about 15% of the photosynthetic rate
• The atmospheric CO2 content (340 ppm) has a half-life of 10 years
• Oxygen turnover has a half-life of 6500 years; water, 3 million years
• Understanding photosynthesis can lead to:
  • Preventing environmental disasters
  • Enhancing food, fiber, and energy production
  • Solar energy conversion advancements
  • Electronic circuit designs
  • Medicine and drug development
• In maize fields, only 1-2% of solar energy is recovered as photosynthetic products; efficiency in sugar cane is 8%
• Agriculture optimizes solar radiation interception for maximum yields by understanding how plant design affects solar interception and environmental factors

Light and Photosynthesis

• Photorespiration reduces photosynthetic efficiency, especially at high temperatures
• Altering photorespiration through molecular biology could lead to more efficient plants.
• Photosynthesis research may lead to new crop strains that use sunlight more effectively, addressing levelling agricultural production

The Applications of Photosynthesis

• Chemists and physicists study photosynthesis to build man-made solar energy harvesting devices, by creating artificial photosynthetic reaction centers

Defining Light

• Radiant energy is transferred by photons, discrete bundles of electromagnetic energy, travelling at approximately 3 x 10^10 m s¯¹ in vacuum, behaving as both particles and waves
• Irradiance is the amount of energy on a known area per unit time (W m¯²)
• Quantum flux measures incident quanta (photons) striking a leaf (mol m¯² s¯¹) where one mol of light is 6.02 x 10^23 photons
• Photon energy is inversely related to wavelength: shorter wavelengths have higher energy

Describing Light for Photosynthesis

• Photosynthetic photon flux density (PPFD) expresses light for Photosynthesis

Quantifying Light

• On a sunny summer day in Pretoria, midday PPFD is approximately 2000 µmol m⁻² s⁻¹
• The maximum PPFD on May 2, 2019, was 1530 µmol m⁻² s⁻¹ in the open, and 350 µmol m⁻² s⁻¹ under a pecan tree due to light absorption

Why Plants are Green

• Chlorophyll absorbs blue and red light, reflecting green wavelengths, which is why vegetation is green

How Leaves Capture Light

• Only 5% of sunlight is converted into carbohydrates through photosynthesis because a large part of the radiation wavelengths are either too short or too long for photosynthetic pigments.
• Of the PAR that reaches a leaf:
• Some light is reflected, and some is transmitted
• Some energy is lost through heat and metabolism

Plant Adaptations

• Epidermal cells can act as lenses, focusing light into the leaf, increasing the light amount reaching chloroplasts
• Epidermal focusing is especially common in herbaceous plants and tropical understory plants

Leaf Adaptations for Light Capture

• Palisade cells are shaped like pillars in parallel columns, one to three layers deep, maximizing the "sieve effect"
• Light channeling moves incident light through central vacuoles and air spaces, transmitting light to the spongy mesophyll
• Interface light scattering in spongy mesophyll layers results from refraction and reflection, and photons change direction and increases path length

Leaf Structure and Light Absorption

• Incoming photons directly excite chlorophyll in chloroplasts
• Epidermis allows light to pass through layers until absorbed
• Epidermal cells redirect light to palisade cells
• Palisade cells act as light guides, allocating photosynthesis to the spongy mesophyll to conserve energy

PAR Interception by Canopies

• PAR interception by canopies are important to consider because biological yield has been increased in many crops by maximizing seasonal total light interception
• Better management and practices improve leaf growth, duration, and canopy display
• Subtle changes in leaf angle increase light penetration, with leaf angles decreasing with canopy depth
• There is a fundamental relationship between crop dry matter production and seasonal accumulated light interception

Light Interception in Orchards

• Approximately 50% light interception yields are linearly related
• Fruit yields vary when light interception is over 50% indicating that other factors than total light interception is more limiting
• Orchard design factors:
  • Planting System
• Tree Spacing
• Tree shape
• Tree Height
• Alley width
• Row Orientation
• Leaf Area Index
• Length of Growing Season

Light Environment Within Vegetation

• Leaves at the top of the canopy absorb most of the light, resulting in lower light and differing spectral quality for lower leaves, which thus have lower photosynthetic rates
• Most plants adapt for light interception through branching
• Leaves at different canopy levels differ in morphology and physiology
• Canopies absorb different wavelengths changing the light quality between the top and bottom
• Sunflecks can bring high light levels deep into the canopy, with available energy comprising half of the total during the day that provide a burst of energy and the leaves have mechanisms to take advantage of this

Optimizing Sunlight Interception

• Maximizing the amount of sunlight incident on a leaf depends on the its angle relative to the sun
• In tree leaves at the top of the canopy tend have steep angles, leaves often tend to decrease

Solar Tracking to Maximize Light

• Some plants maximize sunlight through solar tracking
• Species include cotton, soybean, bean, and lupine
• Plants can orient leaves vertically in the morning, tracking the sun throughout the day with accuracy up to 15 degrees
• Specialized regions in the leaf or stem is where sensing is most intense
• In legumes pulvinus senses and initiates this tracking

Responding to PAR levels

• Light response curves quantify a leaf's response to PAR or CO2 assimilation across varying PPFD levels
• Point A is the dark respiration rate
• PPFD increases photosynthesis until CO2 release and uptake is balanced. This is called the light compensation point (LCP) and is indicated by Point B
• Above the LCP light increases CO2 which increases photosynthesis

Too Much Light

• When plants can not utilize the absorbed light energy: Plants must be able to dissipate the surplus absorbed light to prevent damages of the photosynthetic apparatus
• Plants must dissipate the extra energy with the xanthopyll cycle

Acclimation vs Adaptation

• An acclimated phenotype has plasticity to change. An adapted genotype is adapted to the environment
• Plants can acclimate to sun or shade environments, changing leaves to a new enviroment

Sun vs Shade Leaves

• Sun leaves are thicker while Shade leaves are thinner with more surface area
• Sun: Higher LCP values/ Shade: Lower LCP values
• Sun: High respiration rates/ Shade: Low respiration rates
• Shade: More chlorophyll per reaction centre (with a higher ratio of chlorophyll b to chlorophyll a)

Excess Energy Dissipation

• Through non-photochemical quenching and the xanthophyll cycle