2.photosynthesis (1).pdf

1/15/24 A) The Photosynthetic Process PHOTOSYNTHESIS 6CO2 + 6H2O + light = C6H12O6 + 6O2 Key enzyme: Rubisco - very N-expensive (why we eat leaves!) 50% of leaf N is photosynthetic machinery, largely Rubisco Key Points Light Reactions CO2 uptake and water loss Light compensation point Shade toleranc...

1/15/24 A) The Photosynthetic Process PHOTOSYNTHESIS 6CO2 + 6H2O + light = C6H12O6 + 6O2 Key enzyme: Rubisco - very N-expensive (why we eat leaves!) 50% of leaf N is photosynthetic machinery, largely Rubisco Key Points Light Reactions CO2 uptake and water loss Light compensation point Shade tolerance Shade and forest ecology Alternative photosynthetic mechanisms Read Chapter 2 1 (capture light, generate energy, release O2) Calvin Cycle (uses energy to reduce / fix CO2 into organic molecules) 2 Photosynthetic rates B) Photosynthetic requirements: CO 2 and water (the ecological outcome) CO2 is scarce (~410 ppm 2019) 2500L air / g glucose! CO2 enters stomata (stomates) down diffusion gradient Flux = conductance x concentration difference: C fixed into carbohydrates by Calvin Cycle But C also is lost by respiration; what matters to the plant is net fixation Flux = gleaf x (Cair – Cinternal) Net Photosynthesis = Fixation – Respiration Conductance mostly depends on stomata (higher when they're open) Gradient maintained by CO2 fixation inside leaf As CO2 enters, water leaves (transpiration) Units: C exchange (µMol m-2 s-1) Carbon Balance: + when plant is adding C (as carbohydrates) - when losing C (as CO2) (footnote: growth ≠ C uptake; also needs sufficient water, adequate cell turgor, adequate temperature, nutrients, etc.) 3 4 1 1/15/24 How do you lower LCP? C) Photosynthetic requirements: Light More efficient photosynthesis can reduce LCP (but expensive; plants are already pretty optimized) Light Compensation Point: light level at which net photosynthesis = 0 Alternative: lower respiration can reduce LCP (since net photosynthesis = gross photosynthesis – respiration) (but this usually means reduced growth) - Small Sugar Maple saplings in forest shade can be decades old - In shade, slow growth, suppression of reproduction lower LCP - Means they still meet respiratory needs despite low light - But flexible: when light is available, increase respiration, grow fast! Sugar Maple - Acer saccharum Source: USDA PLANTS database All else being equal, low LCP is good – lets plant survive with less light 5 6 Succession by trees is often driven by LCP D) Shade Tolerance and Forest Ecology Trembling Aspen (Populus tremuloides) replaced by Sugar Maple (Acer saccharum) Light on forest floor may be 1-5% of light in field Disturbance creates high light environment; favours fast growth Mature trees may reach full sun, but seedlings are in shade As trees colonize, light declines, only low LCP seedlings survive Lower LCP = better shade tolerance, but at cost of slow growth High LCP: fast growing Cost: Shade-intolerant Early successional colonist 7 Low LCP: shade tolerant Cost: slower growth Late successional dominant 8 2 1/15/24 Forest floor (understorey) plants Light sources after canopy closure - Plants in continuous low light must have low LCP, but deciduous forest species often can adapt to seasonally changing light availability Sunflecks Canopy Gaps - Key source of light after canopy closes - Require rapid response to changing light levels - Holes in the forest canopy created by dead trees - Create high-light environment - What maples are waiting for Maple is like Viola – low LCP when in dark, but fast growth when in sun Allium = wild leek – a spring ephemeral! Viola = violet Tiarella = foamflower Source: USDA PLANTS database 9 Koffler Scientific Reserve 10 2) C 4 photo-synthesis (corn, many other grasses, several other smaller groups: >8000 species in 19 families; evolved at least 65 times) E) Alternative Photosynthetic Mechanisms 1) C 3 photo-synthesis (most plants: 250,000 species) Problem: RUBISCO: requires high [CO2] - means stomata have to be kept open a lot to replenish internal CO2 - results in water loss - also can run backwards, especially at high T (photorespiration) - consumes O2 to release CO2, wasting reduced C - PEP Carboxylase takes up CO2, passes it on to RUBISCO - PEP Carboxylase has higher affinity for CO2, much more efficient at capturing it, doesn’t do photorespiration - initial uptake in mesophyll, fixation into carbohydrates in bundle sheath - CO2 uptake, carbohydrate production spatially separated - acts as pump, concentration mechanism (10X) - isolates RUBISCO from atmospheric O2, inhibiting photorespiration Wikipedia 11 12 3 1/15/24 Consequences C 4 - Leaf (Kranz) Anatomy C4 - sugar cane C4 (usually) has numerous advantages: - better water conservation (since stomata need to open less) - higher maximum photosynthesis (saturation) in bright light - higher temperature optimum; more tolerant of high T - lower N requirement (less RUBISCO) C3 - oats But: higher energy requirement (30 ATP / glucose vs. 18); less efficient in cool, wet sites 13 14 C3/C4 Grasses – Productivity C3/C4 Grasses – Species Diversity - More species in hotter, drier sites, reflecting higher T optimum, better water conservation - Fewer in cooler, wetter habitats where energy requirements outweigh advantages 15 Shortgrass Steppe, Colorado - In grassland habitats, relatively few species can dominate productivity - So patterns of biomass resemble patterns of diversity, but with a few differences - In prairie sites where both water and T can limit growth, dominant grasses reflect species that can use water when it is available - C3 grasses predominate where rainfall is in cooler months - C4 where rainfall is in hotter summer months 16 4 1/15/24 3) CAM (Crassulacean Acid Metabolism) photosynthesis (cacti, pineapples: >25,000 species in 25 families) - CO2 initially incorporated into organic acids (night) - Calvin cycle (day) temporally separated from uptake - Again, has advantages of using more efficient PEP carboxylase - Prevents photorespiration by allowing RUBISCO to work in even when stomata are closed in a high CO2 / low O2 environment - Saves water since stomata open at night (when it's cooler, humid) - Results in very high water use efficiency 3) CAM (Crassulacean Acid Metabolism) photosynthesis - But also has serious disadvantages - More energetically expensive than C3 (basically, it's a modified version of C4) - Photosynthetic rates limited by temporal separation of C4 and C3 cycles, and by how much organic acid can be stored during night - As a result, tend to have lowest rates of photosynthesis, growth Note: CAM = nighttime temperatures) Yamori et al. 2014 Photosythn Res 119: 101-117. See text Fig. 2.12 17 18 - As well, CAM plants are frost-sensitive - Reason is they need lots of water-filled vacuole space for dissolved acids in order to maximise photosynthesis - In eastern Canada, extreme southern Ontario is as far north as (most) cacti get! - Very high water use efficiency means CAM plants largely occur in dry sites, like deserts or as epiphytes - But scarce in really severely dry deserts - Reason is slow growth means that can't respond rapidly when rare rainfall events occur Saguaro cacti, Arizona Epiphytic bromeliad, Costa Rica Tree Aloes, South Africa Opuntia humifusa at Point Pelee 19 20 5