Photosynthesis-Environmental Influences

EFB530 Plant Physiology

Photosynthesis-environmental effects

What is limiting to plant growth?

At the ecosystem level: water, nutrients, light & temperature

Through the day: light for some times, then CO₂ and/or photosynthetic capacity

High light and photoinhibition

at low light levels, light is limiting, but the PS apparatus can become saturated
in order to maintain the flow of electrons through the photosystems, they must be supplied with NADP⁺, which is regenerated by the activity of the C3 cycle
if too much light energy hits the photosystems, they can begin to react with substrates other than the normal electron carriers - these reactions can generate very damaging reactive oxygen species, like hydrogen peroxide, singlet oxygen radicals, and superoxide
carotenoids function to dissipate the excess energy as heat or in non-damaging chemical reactions (see Fig. 7.36 and discussion of non-photochemical quenching)
in high light, the core proteins of PSII will begin to breakdown, reducing the capacity for photosynthesis
conditions that reduce the activity of the C3 cycle can make the PS apparatus more susceptible to photodamage; i.e. low temperature
under drought stress, when internal CO₂ concentration will drop, the C2 cycle will have higher activity - this may actually protect the plant by dissipating energy under high light conditions

Light quality

plants can vary the ratio of PSII:PSI in order to optimize photosynthetic efficiency
under the canopy, the light is shifted to the far-red, therefore understory plants increase the PSII:PSI ratio (because PS I is preferentially excited more than PS II)

Photosynthesis vs. light

the amount of light where photosynthetic activity balances respiratory activity is the light compensation point
the point where the photosynthesis fails to increase with increasing light is the light saturation point=indicates that the C3 reactions or CO₂ are limiting

Plants can adapt to different light levels.

Sun leaves vs. shade leaves

sun leaves have less light harvesting capacity per reaction center, so light saturation occurs at a higher light level (at light saturation, CO₂ levels and/or photosynthetic enzyme levels are limiting)
shade leaves have more light harvesting capacity per reaction center, so light saturation occurs at a lower light level
photobleaching (or photoinhibition or photooxidation) can occur when the light harvesting array captures more light energy than can be released by electron transport, so instead this energy is released during the formation of toxic radical compounds, like singlet oxygen, which can damage the proteins and lipids of the thylakoid
changes in leaf morphology are associated with sun leaves producing more photosynthetic machinery (prominent palisade layer), shade leaves have minimal palisade layer
shade leaves have lower respiratory activity

The entire canopy never reaches light saturation; there is always sufficient capacity even in the brightest light (of the entire canopy)

Photosynthesis vs. [CO₂]

the [CO₂] where photosynthetic CO₂ uptake balances respiratory CO₂ release is called the CO₂ compensation point
the [CO₂] where photosynthesis does not increase with increasing [CO₂] is called the CO₂ saturation point

C3 vs. C4 plants

C4 plants have a very low CO₂ compensation point, because there is almost no photorespiration
C4 plants have a lower CO₂ saturation point than C3 plants
C4 plants have lower photosynthetic efficiency than a C3 plant in low O₂
since C4 plants can function at lower [CO₂], they can keep their stomata closed more and conserve water

Photosynthesis vs. temperature

the light reactions are insensitive to temperature, while the C3 reactions are highly sensitive to temperature
photosynthesis in C3 plants becomes less efficient at higher temperatures due to increasing photorespiration
C4 plants generally grow better than C3 plants in dry, arid climates; while C3 plants grow better than C4 plants in cool, moist climates

Ultimately, water is limiting to growth