Intro to Plant Biology and Photosynthesis

Plant ecophysiology

"An experimental science that seeks to describe the physiological mechanisms underlying observations” Lambers and Oliveira (2019)

Ecophysiologists address questions in

forestry, horticulture, agriculture, and environmental sciences through
mechanistic understandings of plants (physiology, biochemistry, biophysics, and more), Lambers and Oliveira (2019).

Spectrum of sunlight

• Composed of visible light spectrum, UV, and infrared

• Blue light and UV have higher energy

• Infrared and red light have less energy

How do leaves go from absorbing and utilizing mainly 2 wavelengths in the
light spectrum to most of the full visible light spectrum?

1. Protien complexes

2. Accessory pigments

3. Light Scattering

LHCs

• Protein complexes

• Called Light Harvesting complexes (LHCs)

• Transfer energy to photosynthesis

Accessory Pigments

• Carotenoids take on the excess energy from light wavelengths outside of the peak wavelengths

• Excess energy is passed off as heat with the conversion of the carotenoid

• Component but not entirety of LHCs

Light Scattering

• Air spaces and water vapor within the spongy mesophyll allow for the light to scatter.

• This increases the amount of light absorptance of certain wavelengths by 25 – 30% (Vogelmann et al., 1996)

Sun leaves vs Shade Leaves

Shade leaves are more efficient with sun light at low levels

Sun leaves are able to photosynthesize much more than shade leaves

Assimilation curve

LCP- light compensation point, which is where leaf begins to be a benefit to
the plant instead of a drain of resources.

Φ- is the initial slope of the line and is called the quantum yield.

• Steeper slope indicates faster capacity to
  utilize more light. This type of slope would
  indicate a higher photosynthetic
  efficiency.

Rd is the rate of dark respiration.- the amount of carbon it is using itself
Θ- is the curvature of the assimilation curve

Light harvesting complex

Catch wavelengths the photosystems cannot.

Then pass the energy on to the photosystem at the level the photosystems can handle

Photosystems

• Outcome of LHC’s funneling light energy into photosystems

• Photosytems 1 and 2 generate NADPH and ATP that are used
  in the Calvin-Benson cycle

• Called the light process, because light is required

Calvin-Benson cycle

ATP and NADPH from the photosystems come into play in step 2

Called dark process, because light is not necessarily required

This is where C3 comes from

C4 photosynthetic pathway

starts by CO2 binding to a 4-carbon compound within one of the mesophylls.

Photorespiration

Rubisco carboxylate RuBP, but it also can oxygenate it

Why did this extra step (C4) evolve?

• This step evolved and increased the efficiency of CO2 transfer into the Calvin
  cycle.

• C4 processes also stop photorespiration from occurring.

• This evolutionary step occurred multiple times in different areas during a period when atmospheric CO2 was low and O2 was high.

• About 5% of global plant species are C4

CAM plants make up about

10% of global plant species

CAM process

process allows for some of the photosynthetic process to occur at
night and the other processes to occur during the day

This is particularly beneficial in really arid (dry) regions where a lot of water
would be lost during the day if the plant were a C3 or C4 plant.

CAM plants use

the CO2 that comes off of the malic acid in the Calvin cycle

The creation of malic acid results in

nocturnal acidification, due to the creation of malic acid

During the day, the plant becomes more basic as

it processes the CO2 that was bound to malic acid.

Xanthophyll cycle

is a protective cycle to prevent the damage with excessive light

Violaxanthin is converted to

Antheraxanthin is converted to

Antheraxanthin

Zeaxanthin with excessive light

Reverse takes places in the

dark

Plants acclimated to high light tend to have

a higher concentration of xanthophyllsthan low light acclimated plants

In cases where there are not enough Xanthophyll or mutants that can’t
produce the correct compounds

ROS is formed

ROS (reactive oxidative species) are formed when

oxygen takes the excess energy instead

ROS are highly reactive and tend to cause damage to

the chemical structures within plants

Some herbicides target Xanthophyll cycle.

Average irradiance decreases exponentially going from

the top of the leaves at the top of the canopy to the understory of an area with trees

The amount of photosynthetically active radiation (PAR) or light that
passes through a leaf (transmitted) is

less than 10%

Far – red light transmits at a much higher rate than

other wavelengths within PAR

Sunflecks

brief bursts of high sunlight energy that can approach or be the same
as direct sunlight

Sunflecks are important for shaded leaves because

they give a brief burst (seconds to minutes) of increased energy

Sunflecks make up 9-46% of

understory plant total carbon gain. Actual percentage
depends upon duration and frequency

Sunflecks occur due to

fluttering leaves, wind, and the movement of the sun across
the sky

Photosynthesis needs to be induced when

plants are in the dark or heavy shade

This can take up to 1 hour for plants to get used to the new light
state
This impacts the efficiency of plants at utilizing sunflecks

Types of plants that acclimate or don’t

Shade avoidant- ave a high Amax/chl and don’t acclimate much

Shade tolerant- Typically have lower Amax/chl overall, but acclimates quickly to light changes

Fast growing herbaceous species that have evolved with variable or
high canopy cover over them- have Amax/chl that strongly decrease with
declining sun

Woody shade adapted species that don’t compete for space in the canopy- have low Amax/chl that does not acclimate with increased light

Why is photosynthesis induction necessary?

• Rubisco and a couple other elements within the Calvin Cycle need to be activated by light
• Rubisco is critical to the Calvin cycle as it is the first step
• Rubisco may react with other compounds, but is deactivated by plants when light levels are low. This protects Rubisco.

Oxygen is one of the first products from a sunfleck

so after a sunfleck the amount of O2 released diminishes rapidly

However, CO2 uptake continues for awhile after due to the energy
lingering in

the form of NADPH and ATP from the light burst

Understory plants tend to be more efficient at

sunfleck utilization

Due to more Calvin-Benson cycle metabolites maintained relative to
other plants of similar size

Additionally, tend to keep Rubisco in an induced state longer and have
faster capacity to reinduce Rubisco.

Shade leaves like understory plants often have more Calvin cycle metabolites ready to mobilize when photosynthesis is induced

This allows shade leaves to take advantage of sunflecks as well,
even within a plant that has sun leaves

High intensity blue and/or red light

can cause chloroplasts to move within the cells that they are occupying

High intensities cause chloroplasts to

line up along the walls and parallel to the light direction, instead of perpendicular to light

Movement of light is often to

prevent photoinhibition

Are plants limited to one photosynthetic system (C3, C4, CAM)?

No,

• Some plants can switch between CAM and C3 depending upon the growing conditions

• There are some intermediate species where C4 and C3 grasses have hybridized

• Some plants even have CAM photosynthesis in some parts of the plant and C3 in others (Frera indica)

UV-A (315 – 400 nm) cause

photo-oxidative damage to plants

UV-B (280 – 315nm) causes

photolesions

Both UV-A and UV-B can damage

the plants DNA. Additionally, they can inhibit the xanthophyll cycle

UV damage can be identified by

reduced enzymatic activity, lower photosynthetic yield, and stunted growth

Photoprotection measures

Additionally, thickening the lignin composition of secondary cell walls and the increased presence of phenlypropanoids in the cuticle waxes can protect plants from UV-B (Bulut et al., 2025)

Midday photosynthesis slump

• Often, during midday, you may notice leaves have altered their leaf angle to get minimal sunlight
• This protects the leaf from UV-A and UV- B exposure.
• It is also caused by too little water, excessive heat, and high light from the
sun being directly overhead
• Photosynthesis usually recovers later in the afternoon

Plant photosynthetic capacity over short periods of time. Newly formed leaves have

lower photosynthetic rates and quantum yields as they lack the biochemical components

Leaves that are recently fully developed tend to be at

peak capacity for photosynthesis

Plants vary by

species and evolutionary life history in their leaves ability to do photosynthesis over time

Damage to the photosynthetic systems in the upper mesophyll palisade layer are

common and are a part of the reason for this trend

Plant photosynthetic capacity over seasons

Varies by plant and evolutionary history

Some plants are more consistent in

their leaves photosynthetic capacity (c). Others vary on shorter (a) or longer time frames (b)

Some plants ramp up photosynthetic rates when

it is time to collect energy for fruit creation

Are all photosynthetic systems the same in their full capacity to do photosynthesis?

No

Plants have an optimum curve for photosynthesis in response to

temperature

There are a couple critical points

cold limit, optimal temperature, and heat limit

Cold limit is where photosynthesis begins or ends depending upon

whether it is approached from warmer temperatures or colder temperatures

Optimal temperatures vary by plant speciesOptimal temperatures vary by plant species

• C3 tend to be in the 15 - 30°C range
• C4 max photosynthetic rates can go much higher than C3 plants.
However, they are very cold sensitive and so are limited in the spread of
their range by both altitude and latitude.
• CAM plants have a high optimum temperature phase during the light
processes and a much cooler optimum temperature (20 °C less) in their
night phase.

Temperatures between 50 - 60°C range result in

damage to enzymes and important membrane structures

Ratio of CO2 to O2 decreases (more O2 in leaves) as

temperature increases

After high heat, photosynthesis may be impaired for a long time as the

chemical components may need to be largely regenerated

There are two critical points for photosynthesis and water content

1) Threshold between full photosynthetic capacity and reduced capacity
of photosynthesis
2) The zero point for gas exchange – where total or near total closure of
the stomata occurs

Plants vary in these points depending upon

their evolutionary life history

On average, C3 and C4 photosynthetic pathway plants are not that different from each other

• CAM plants are very efficient with water, much more than C3 and C4
• This is because CAM plants open their stomata at night

When water limitation occurs, CAM plants close their stomata at night as well.

They remain alive through recycling CO2 from their respiration through malate.

Water Use efficiency (WUE) of photosynthesis

photosynthesis/transpiration

C3 plants have a lower water-use efficiency than C4 plants because

the lower carboxylation efficiency relative to C4

WUE is influenced by

temperature, humidity, plant age, flowering, fruit development, and season.

Given all of the factors that effect short term water use efficiency

it is more practical to consider water use efficiency in dry matter production

Water use efficiency of productivity (WUEp)

Organic dry matter production/ water consumption

This value can be applied to single plants or large areas of plants

Elements needed in photosynthesis

N, S, Mg, Fe, Ca, Mn, P, Cu, K, Zn, Mo, B, and Cl

• Some elements play a more minor role, while others are needed in higher concentrations
• All of these elements make up the “essential elements” for plants
list. Some fall in the macronutrients and others fall in the micronutrients.

Nitrogen

• Important for the creation of enzymes, proteins, thylakoids, and chlorophyll
• Considered a macronutrient
• Soils are frequently deficient in this element due to the amount that plants use this element

Photosynthetic nitrogen use efficiency

photosynthesis/N content per leaf unit area

Phosphorus

• Considered a macronutrient, but note difference between P and N
• Soils are frequently deficient in this element due to the amount that plants use this element. N vs P deficiency related to soil age

Importance of Phosphorus

• Needed for energy rich compounds (ADP, ATP, and more).
• Necessary for Calvin cycle functioning (NADPH and ATP) and transport of metabolites and assimilates out of the cells.
• Deficiency results in build up of sucrose and starch, which decreases photosynthetic rates even when conditions are good

Mineral deficiencies

•General outcome of a mineral deficiency for plants is that they will begin to behave like sciophytes
• Even if they are not sciophytes