Transcript Document
Upcoming Seminars: • EECB seminars – 4:00 Thurs in OSN 102 – • Thurs Feb 5: Jed Sparks (Cornell) “Vegetation-level process controls on troposhperic chemistry” Geography seminar – 4:00 Wed in Mackay Science 215. – Wed Feb 4: Alan Taylor (Penn State) “presettlement Forests, Fire Regimes, and Climatic Influences in the Tahoe Basin” Outline 1. 2. 3. 4. 5. 6. Radiation: heat, light, and photosynthesis Energy budgets Energy capture and dissipation Photosynthesis: Calvin cycle, photorespiration Alternative photosynthetic pathways Carbon accumulation, ecological implications Radiation (electromagnetic waves) • Short wave (solar radiation) <4um wavelength – UV 10-400nm (@ Earth’s surface 292-400) • – Potentially damaging to biological systems Visible 400-700nm • – Near infrared 0.7-4.0um (and far red visible) • • • Source of most energy for life on earth “shade detector” pigment phytochrome absorbs 660nm and 730nm Can cause inhibition of germination etc. Long wave: far IR; heat. – Heat is energy from objects (molecular motion). Energy budgets • • • • • Energy cannot be created or destroyed Net=met+solar+thermal+conv+cond+LE+storage Plants absorb PAR and reflect or transmit non-PAR Use only a fraction of energy absorbed (2%); must dissipate the rest Angle of incidence determines energy intercepted. – – • • Paraheliotropic=leaf parallel to sun’s rays Diaheliotropic=leaf perpendicular to sun’s rays Sun VS shade leaves: different morphology Albedo, laminar flow, reduced leaves Energy effects • Light or temperature mediated plant responses: – – – – – Germination/seed dormancy (light) Thermoperiodism (buds, seeds) temp differential induces dormancy or activity Dormancy – end of dormancy triggered by photoperiod or temperature Temperature stratification of seeds (hot or cold) Physical rupture of seed coat (fire) Energy capture: photosynthesis • • • • • Leaves absorb light between 400 and 700nm Energy is captured through an electron transfer chain (“light reactions”) Carbon dioxide is fixed into carbohydrate (“dark reactions”) using energy captured in light reactions Calvin cycle (C3) prone to photorespiration (lightmediated release of CO2) Level of photorespiration depends on CO2:O2 Electron transport Calvin cycle C4 Photosynthesis • • • • Means to concentrate CO2 and reduce photorespiration. Requires energy (ATP) and special anatomy (Kranz) CO2 fixed in mesophyll cells to form a 4-carbon acid (oxaloacetate) C4 chain diffuses to bundle sheath cells, releases CO2, C fixation occurs ‘normally’. Higher light compensation point than C3, and greater water use efficiency. CAM Photosynthesis • • • • • • Temporally separates light and ‘dark’ reactions Succulents At night stomata open; plants fix CO2 to malic or isocitric acid (4-carbon). Uses ATP. Acid stored in large cell vacuole. During day stomata close, CO2 released, and fixed in “C3 type fixation” Plants with this pathway are not always obligate CAM – can switch to C3. Environmental differences • • • • • CO2 compensation lower for C4 than C3 (can have higher stomatal resistance and still fix C) Light saturation higher for C4 than C3 (reduced photorespiration) Light use efficiency highest for C4 monocots. C4 uses energy; less efficient at lower light and temperatures Water stress: C4 more efficient than C3, CAM can switch to C3 (less energy) when water available Ecological implications • • • • Timing of rainfall and nutrient availability affect composition. How? What are differences in radiation impacts in forest VS rangeland? What would effects of global climate change and CO2 enrichment be? How might plants with different photosynthetic pathways be distributed? Ecological implications • • • • Timing of rainfall and nutrient availability affect composition. N availability affects photosynthetic rates. Herbivory and “overcompensation” – dependent on N availability? What are differences in radiation impacts in forest VS rangeland? What would effects of global climate change and CO2 enrichment be?