photosynthesis • Photo means ‘light’ and synthesis means ‘to make’ • Process in which plants convert carbon dioxide and water into sugars using solar energy • Occurs in chloroplast
Photosynthesis: 6 CO2 + 6 H2O C6 H12 O6 + 6 O2 carbon dioxide + water = sugar + oxygen • photosynthetic products often stored as starch • Starch = glucose polymer STARCH Tracking atoms
Chlorophyll • Absorbs red & blue light • Reflects green light
Rubisco • Ribulose bisphosphate carboxylase oxygenase • (fixes CO2 & O2) • Enzyme in Calvin Cycle (1st step) • Most abundant protein on Earth • Ca. 25% total leaf protein
Photorespiration • When rubisco “fixes” O2, not CO2 • Lose 1/2 C as CO2; costs 2.5 extra ATP • Take up O2 • Only occurs in light • Occurs 1 out of 4 reactions under today’s atmospheric [CO2] • Rate increases with temperature
Types of photosynthesis • C3 • The majority of plants • C4 • CO2 temporarily stored as 4-C organic acids resulting in more more efficient C exchange rate • Advantage in high light, high temperature, low CO2 • Many grasses and crops (e.g., corn, sorghum, millet, sugar cane) • CAM • Stomata open during night • Advantage in arid climates • Many succulents (e.g., cacti, euphorbs, bromeliades, agaves)
Global Environmental Change & Photosynthesis: C3 vs. C4 vs. CAM • Increasing CO2 • Increasing chronic and acute temperatures • Increasing N (vs. decreasing C:N from increasing CO2) • Changes in water
CO2 effects on photosynthesis • C4 > C3 at low CO2 • But, C3 > C4 at high CO2
*At high CO2, C3 more efficient than C4 at all temps. (photosynthesis only, not other processes)
Photosynthetic N-use efficiency • C4 plants need (have) less leaf N than C3 • Photosynthesis higher per unit N in C4 • Humans are increasing global N, which benefits C3 more than C4 • Increasing CO2 decreases leaf N content, more in C3 than C4
Photosynthetic water-use efficiency • C4 plants use less water than C3 • (cause stomates open less) • Water availability may increase or decrease in the future.
Predicting the future for plants • How will increases in CO2, N, and chronic and acute heat stress affect photosynthesis? • Who will win or lose? C3? C4? • How will pollution (eg, ozone) interact? • Current research in my lab an example.
Hypothesis Elevated CO2 • High CO2 effects greater in C3 than C4 and CAM species. • High CO2 effects greater on induced than basal thermotolerance. Increased leaf C:N Decreased thermotolerance Decreased Heat-shock proteins (Hsps)
Heat stress decreased Pn in all species (not the result of stomatal closure). Elevated CO2 had negative effects on Pn of C4 species, and positive effects on C3 species. Pre-heat shock has a positive effect on Pn.
Heat shock decreased Фet of all C3 and C4 species There was negative CO2 effects on all species, except for wheat There was positive Pre-HS effects on all species