Photosynthesis: Physiological and Ecological Considerations

1Lincoln Taiz and Eduardo ZeigerPlant PhysiologyFourth EditionChapter 9Photosynthesis:Physiological andEcologicalConsiderationsLectured by L-F Chien2009 Physiological and Ecological Considerations2 Physiological consideration:In the intact leaf, three major metabolic stepshave been identified as important for optimalphotosynthetic performance:• Rubisco activity• Regeneration of ribulose bisphosphate (RuBP)•Metabolism of the triose phosphates Ecological consideration:Farquhar and Sharkey (1982): supply and demand1

9.1 Light, Leaves, and Photosynthesis3• Three light parameters in the measurement of light:(1) spectral quality(2) amount(3) directionFIGURE 7.1 Light is a transverse electromagnetic wave,consisting of oscillating electric and magnetic field.9.2 Units in the Measurement of Light4With collimated lightFlatWith 45lightSphericalFIGURE 9.2 Flat and spherical light sensors.2

56•Light can be measured as energy, and the amount of energythat falls on a flat sensor of known area per unit time isquantified as irradiance.•Unit: watts per square meter (W m 2 ) (1 W = 1 Joule s 1 )•PAR: photosynthetically active radiation3

7•Light can be measured as the number of incident quanta(singular quantum).•Unit:(a) moles per square meter per second (mol m 2 s 1 )(b) microEinstein (Einstein, one mole of photons) per squaremeter per second (E m 2 s 1 )•PPFD: photosynthetic photon flux density8• Light can be measured omnidirectionally (from all directions) asfluence rate.• Unit:(a) W m 2(b) mol m 2 s 14

9.2.1 Light anatomy maximizes light absorption9Sun: 1.3 kW m 2FIGURE 9.3 Conversion of solar energy intocarbohydrates by a leaf.•The sun leaf is much thicker than the shade leaf.10FIGURE 9.1 Leaf anatomy from a legume (Thermopsis montana) grown indifferent light environments5

11FIGURE 9.4 Optical properties of a bean leaf.9.2.2 Plants compete for sunlight12FIGURE 9.5 The spectral distribution of sunlight at the top of a canopy and underthe canopy.6

9.2.3 Leaf angle and leaf movement can control lightabsorption13FIGURE 9.6 Leaf movement in sun-tracking plants.9.2.4 Plants acclimate and adapt to sun and shade14• Acclimation:a process whereby the newly produced leaf has a setof biochemical and morphological characteristics• Adaptation:a process by which an organism becomes fitted to itsenvironment•Sun and shade leaves:(a) Shade leaves have more total chlorophyll perreaction center, have a higher ratio of chlorophyll b tochlorophyll a, and are usually thinner than sun leaves.(b) Sun leaves have more rubisco and a larger pool ofxanthophyll cycle components than shade.(c) shade leaves PSII: PSI = 3:1sun leaves PSII: PSI = 2:17

9.3 Photosynthetic Responses to Light bythe Intact Leaf9.3.1 Light-response curves reveal photosynthetic properties15FIGURE 9.7 Response of photosynthesis to light in a C 3 plants.16Fluorescence•Photosynthetic activity:(a) Photosynthetic O 2 evolution(b) Photosynthetic CO 2 assimilation(c) Chlorophyll fluorescence8

17(triangle orache)(wild ginger)Dashed line has beenextrapolated from themeasured part of thecurve.FIGURE 9.8 Light-response curves of photosynthetic carbon fixation in sunand shade plants.18Barbara J. Collins, Ph.D.California Lutheran UniversityThousand Oaks, California, 91360Copyright © 2000-2003 by Barbara J.Collins and Lorence G. Collins.Scientific Name: Atriplex triangularisCOMMON NAME: Spearscale(triangle orache)Scientific Name: Asarum caudatumCOMMON NAME: Wild-ginger9

19(California sunflower)(Redscale)FIGURE 9.9 The quantum yield of photosynthetic carbon fixation in a C 3 plantand in a C 4 plants as a function of leaf temperature.20Scientific Name: Atriplex roseaCOMMON NAME: RedscaleScientific Name: Encelia californicaCOMMON NAME: California sunflower10

21(triangle orache)FIGURE 9.10 Light-response of photosynthesis of a sun plant grown undersun or shade conditions.22Picea sitchensis (Sitka spruce)FIGURE 9.11 Changes in photosynthesis (expressed on a per-square-meter basis)in individual needles, complex shoot, and a forest canopy of Sitka spruce (Piceasitchensis) and a function of irradiance.11

23鍚 特 卡 雲 杉The Sitka Spruce (Picea sitchensis) is a large coniferous evergreen tree growingto 50-70 m tall, exceptionally to 90 m tall, and with a trunk diameter of up to 5 m.It acquires its name from the community of Sitka, Alaska.(http://en.wikipedia.org/wiki/Picea_sitchensis)9.3.2 Leaves must dissipate excess light energy24FIGURE 9.12 Excess light energy in relation to a light response curve ofphotosynthetic evolution.12

THE XANTHOPHYLL CYCLE25An epoxide isa cyclic etherwith only threering atoms.FIGURE 7.35 Chemical structure of violazanthin, antheraxanthin, and zeaxanthin.•energy dissipation: As the amount of light incident to a leafincrease, a greater proportion of violaxanthin is converted toantheraxanthin and zeaxanthin.26FIGURE 9.13 Diurnal changes in xanthophyll content as a function ofirradiance in sunflower (Helianthus annuus).13

CHLOROPLASR MOVEMENTS & LEAF MOVEMENTS27FIGURE 9.14 Chloroplast distribution in photosynthesizing cells of the duckweed Lemna.9.3.3 Absorption of too much light can lead tophotoinhibition28FIGURE 7.3414

•Two types of photoinhibition29FIGURE 9.15 Changes in the light-response curves of photosynthesis caused byphotoinhibition.9.4 Photosynthetic Responses to Temperature9.4.1 Leaves must dissipate vast quantities of heat30FIGURE 9.16 The absorption and dissipation energy from sunlight by the leaf.15

Leaves must dissipate vast quantities of heat:•Radiative heat loss•Sensible heat loss•Latent heat loss31Sensible heat loss Bowen ratio Evaporative net loss•This concept was developed by Ira S. Bowen (1898–1978), anAmerican astrophysicist.•When the evaporation rate is low, because water supply is limited,the Bowen ratio tends to be high. Thus, the Bowen ratio is about(a)10 for deserts,(b) 2-6 for semi-arid regions,(c) 0.4 to 0.8 for temperate forests and grasslands,(d) 0.2 for tropical rain forests and(e) 0.1 for tropical oceans.9.4.1 Photosynthesis is temperature sensitive32(Honeysweet)FIGURE 9.17 Changes in photosynthesis as a function of temperature.(After Berry and Björkman 1980)16

33Scientific Name: Tidestromia oblongifoliaCOMMON NAME: HoneysweetScientific Name: Larrea tridentata(or Larrea divaricata)COMMON NAME: Creosote Bush(http://www.laspilitas.com/plants/374.htm)34FIGURE 9.18 The relative rates of photosynthetic carbon gain17

9.5 Photosynthetic Responses to Caron Dioxide 359.5.1 Atmospheric CO 2 concentration keeps risingFIGURE 9.19 Concentration of atmospheric CO 2 from 420,000 years ago to the present.Web Topic 9.5Prehistoric Changes in Atmospheric CO 2The history of atmospheric CO 2on Earth has been one of slow but constant changes,far slower than the rate of today’s atmospheric CO 2changes. The last million year’sworth of history of atmospheric carbon dioxide is trapped in the ice core bubbles.36•These ice core data show thatatmospheric CO 2levels changedbetween 180 ppm (glacial periods)and 280 ppm (interglacial periods)as Earth moved in and out of iceages.•As of January 2007, the CO 2concentration in Earth’s atmospherewas about 0.0383% by volume, or383 ppmv.Web Figure 9.5.A Air bubbles trappedinside frozen ice. Courtesy ofNewScientist.com:http://www.newscientist.com/data/images/ns/cms/dn8369/dn8369-1_500.jpg•ppm, part per million(a) by mass: 1 mg/kg(b) by volume: 1 mL/10 3 L (cm 3 /m 3 )18

THE GREENHOUSE EFFECT37FIGURE 9.19 Concentration of atmospheric CO 2 from 420,000 years ago to the present.9.5.2 CO 2 diffusion to the chloroplast is essential tophotosynthesis38FIGURE 9.20 Points of resistance tothe diffusion of CO 2 from outside theleaf to the chloroplasts.19

9.5.3 Patterns of light absorption generate gradientsof CO 2 fixation39FIGURE 9.21Distribution ofabsorbed light inspinach sun leaves.40Tidestromia oblonfifolia (Honeysweet)Larrea divaricata (Creosote bush)10 15FIGURE 9.22 Changes in photosynthesis as a function of intercellular CO 2concentrations.20

411 Pa = 1 Newton per meter 2 (N m 2 )= 1 kg m s 2 m 2 42C 3 plants versus C 4 plants1. (a) In C 4 plants, photosynthetic rates saturate at c ivalues of about 15 Pa, reflecting the effective CO 2 -concentrating mechanisms operating in these plants.(b) In C 3 plants, increasing c i levels continue tostimulate photosynthesis over a much broader CO 2 .2. (a) In C 4 plants, the CO 2 compensation is zero ornearly zero, reflecting their very low levels ofphotorespiration.(b) In C 3 plants, the CO 2 compensation point is about10 Pa, reflecting CO 2 production because ofphotorespiration.21

Ehleringer et al. 199743FIGURE 9.23 The combination of atmospheric CO 2 levelsand daytime growing season temperatures.9.6 Crassulacean Acid Metabolism44Cactus(Nabors 2004)22

45CO 2 assimilationH 2 O evaporationStomatalconductanceFIGURE 9.24Photosyntheticcarbon assimilation,evaporation, andstomatalconductance9.6.1 Carbon isotope ratio variations reveal differentphotosynthetic pathways9.6.2 How do we measure the carbon isotopes of plants?4613CO13 2C / 12 C ratio (R) of plant : R 12CO2Carbon isotope ratio ( 13 C) of plant :13 C%oR Rsamplestandard1 1000where the standard represents thecarbon isotopes contained in afossil belemnite from the Pee Deelimestone formation of SouthCarolina.(http://en.wikipedia.org/wiki/)23

472814[singular: taxon]FIGURE 9.25 Frequency histograms for observed carbon isotope ratios in C 3 and C 4 taxa. 13 C of C 3 plant = 28 ‰ 13 C of C 4 plant = 14 ‰There is less 13 C in atmosphericCO 2 than is found in the carbonatedof the belemnite standard.48•C 3 plants: wheat, rice, potatoes, sugar beet,beans, and etc.•C 4 plants: corn, sugarcane, sorghum, and etc.•CAM plants: cactus, pineapple, and etc.24