Plant Ecology Chapter 2 Photosynthesis Light Photosynthesis Light

Plant Ecology - Chapter 2 Photosynthesis & Light

Photosynthesis & Light Functional ecology how the biochemistry and physiology of individual plants determine their responses to their environment, within the structural context of their anatomy and morphology

Photosynthesis & Light Functional ecology closely related to physiological ecology, which focuses on physiological mechanisms underlying whole-plant responses to their environment

Photosynthesis & Light Photosynthesis is a “package deal” How much light Competitors Limitations (pollution, pathogens) Herbivores Plants must cope with multiple items at same time

Process of Photosynthesis Biochemical process to acquire energy from sun, carbon from atmosphere 2 parts Capture of energy (light reactions) Storage of energy into formed organic molecules (carbon fixation)

Process of Photosynthesis Reactions take place in chloroplasts Light reactions on thylakoid membranes Carbon fixation (Calvin cycle) within the stroma

Process of Photosynthesis Light reactions involve pigment molecules Many forms of chlorophyll Accessory pigments (carotenoids and xanthophylls in terrestrial plants)

Process of Photosynthesis Pigment molecules arranged into two molecular complexes Photosystems I and II Capture energy (form ATP, NADPH) plus generate oxygen

Process of Photosynthesis Energy captured from light reactions powers the Calvin cycle Captured energy ultimately stored in chemical bonds of carbohydrates, other organic molecules

Rates of Photosynthesis Gross photosynthesis total amount of carbon captured Cellular respiration organic compounds broken down to release energy Net photosynthesis gross photosynthesis minus respiration

Rates of Photosynthesis Basic limiting factor - amount of light energy reaching thylakoid membranes Darkness - loss of energy due to respiration - giving off CO 2 Low light - respiration plus some photosynthesis - giving off and taking up CO 2 Compensation point

Rates of Photosynthesis Strong light - respiration plus photosynthesis giving off and taking up CO 2, up to a point Maximum rate of photosynthesis, despite further increase in light energy

Rates of Photosynthesis Different plants have different photosynthetic responses to same light intensity Some do better under low light, others strong light Habitat - shade vs. sun Some can shift light compensation point to deal with changes in light availability (lots in spring, less in summer in shade)

Quality of Light quality (availability of different wavelengths) can limit rate of photosynthesis Blue and red wavelengths are captured preferentially Green wavelengths are discarded (green plants)

Global Light Availability Tropical latitudes - day and night equal Polar latitudes continuously light at midsummer, continuously dark at midwinter Maximum sunlight energy greater in tropics than polar regions

Global Light Availability Maximum sunlight energy greater at high altitudes than at sea level Damaging UV-B radiation greater in tropics than polar regions, high elevations vs. low elevations Biochemical protection: flavonoids to absorb, antioxidant and DNA repair enzymes

CO 2 Uptake Limitations CO 2 diffusion from surrounding air into leaf and into chloroplast Leaf conductance rate at which CO 2 flows into the leaf Mostly under control of stomata

CO 2 Uptake Limitations Stomata open, close to maintain water balance (seconds, minutes) Stomata change as leaf morphology, chemistry change (days, months) Natural selection modifies (100 s, 1000 s of years)

CO 2 Uptake Limitations Controlling water loss is main reason why plants restrict their CO 2 uptake Huge amount of air required for photosynthesis - 2500 L air for each gram of glucose produced

CO 2 Uptake Limitations Stomata can be very dynamic, opening and closing constantly to regulate CO 2 and water loss Much variation even within same leaf Patchy closure also common in stressed plants

Variation in Photosynthetic Rates: Habitats Photosynthetic rates vary within and among habitats Correlated with species composition, habitat preferences, growth rates

Variation in Photosynthetic Rates: Habitats Photosynthetic rates may be unrelated to species distributions, populations processes Other important components of photosynthesis: total leaf area, length of time leaves active, maintained

Photosynthetic Pathways Carbon fixation done using 3 different pathways C 3 C 4 CAM (crassulacean acid metabolism)

Photosynthetic Pathways C 3 and C 4 named for 3 carbon and 4 -carbon stable molecules first formed in these pathways CAM named after plant family Crassulaceae where it was first discovered

Photosynthetic Pathways Most plants use C 3 photosynthesis, and plants that use it are found everywhere C 4 and CAM are modifications of C 3, and evolved from it

Photosynthetic Pathways C 3: CO 2 joined to 5 -carbon molecule with assist from the enzyme Ru. BP carboxylase/oxygenase rubisco Rubisco probably most abundant protein on earth, but does its job very poorly

Photosynthetic Pathways Rubisco inefficient at capturing CO 2 Also takes up O 2 during photorespiration O 2 uptake favored over CO 2 uptake as temperatures increase Limits photosynthesis Plants must have HUGE amounts of rubisco, especially those in warm, bright habitats, to compensate for poor performance

Photosynthetic Pathways Increases in atmospheric CO 2 concentrations should allow C 3 plants to increase rates of photosynthesis

Photosynthetic Pathways C 4 photosynthesis contains additional step used for initial CO 2 capture 3 -carbon PEP (phosphoenol-pyruvate) + CO 2 = 4 -carbon OAA (oxaloacetate) Catalyzed by PEP carboxylate

Photosynthetic Pathways PEP carboxylate only captures CO 2 Higher affinity for CO 2 than rubisco Not affected by warmer temperatures Decarboxylation (CO 2 removal) process allows standard Calvin cycle (including rubisco)

Photosynthetic Pathways C 4 requires special leaf anatomy Spatial separation of C 4 and C 3 reactions Rubisco exposed only to CO 2, not O 2 in atmosphere like in C 3 plant

Photosynthetic Pathways C 4: Mesophyll cells for carbon fixation, bundle sheath cells for Calvin cycle - keeps O 2 away from Calvin cycle C 3: Mesophyll cells for carbon fixation and Calvin cycle - allows O 2 access to Calvin cycle

Photosynthetic Pathways C 4 plants generally have higher maximum rates of photosynthesis, and have higher temperature optima

Photosynthetic Pathways C 4 plants generally do not become lightsaturated, even in full sunlight Also have better nitrogen use and water use efficiencies because of reduced needs for rubisco (1/3 to 1/6)

Photosynthetic Pathways Requires additional energy to run C 4 pathway, but easily compensated for by photosynthetic gains at high light levels Very successful in warm, full-light habitats, e. g. , deserts

Photosynthetic Pathways CAM photosynthesis Crassulacean acid metabolism Uses basically same biochemistry as C 4, but in very different way Rubisco found in all photosynthetic cells, not just bundle sheath cells

Photosynthetic Pathways CAM uses temporal separation of light capture, carbon fixation rather than spatial separation as in C 4 CO 2 captured at night, converted into organic acids

Photosynthetic Pathways During daylight, organic acids broken down to release carbon, used normally in Calvin cycle Stomata remain closed during day

Photosynthetic Pathways CAM plants have thick, succulent tissues to allow for organic acid storage overnight Tremendous water use efficiency (stomata closed during heat of day)

Photosynthetic Pathways Some CAM plants not obligated to just CAM Can use C 3 photosynthesis during day if conditions are right, to achieve higher rates of photosynthesis CAM can’t accumulate carbon as fast as C 3 or C 4 plants, lowering rate of photosynthesis

C 3, C 4, and CAM C 3 plants most abundant (# of species, total biomass) More CAM species than C 4 species CAM plants less abundant than C 4 in biomass, worldwide distribution

C 3, C 4, and CAM Half of grass species are C 4 Dominate warm grassland ecosystems Warm, bright conditions where C 4 is favored

C 3, C 4, and CAM plants typically are succulents in desert habitats, or……

C 3, C 4, and CAM Epiphytes growing on trees in tropics or subtropics Both types experience severe water shortages

C 3, C 4, and CAM Phenology - seasonal timing of seasonal events C 3 plants typically more springtime, vs. C 4 plants being mostly summer

C 3, C 4, and CAM C 4 grasses are most common where summer temperatures are warm in N. America

C 3, C 4, and CAM C 3 grasses - cool, winter-moist C 4 grasses - warm, summer-moist

C 3, C 4, and CAM

Sun & Shade Leaves

Sun & Shade Leaves

Sun & Shade Leaves Higher light saturation levels Greater maximum photosynthetic rates

Species Adaptations-Sun Solar tracking increases light availability

Species Adaptations-Shade Velvety, satiny leaf surfaces, blue iridescence on leaf enhance available light

Species Adaptations-Shade species use brief sunflecks with high efficiency: stomata open + slow loss of photosynthetic induction

Species Adaptations. Ecotypes? Genetically distinct populations of same species adapted to low- and high-light conditions? Phenotypic plasticity

Daylength Flowering, seed dormancy, seed germination, other physiological responses of plants controlled by daylength (actually nightlength) More reliable predictor of seasonal change than temperature Ratio of two forms of phytochrome A controlled by length of dark period