Plant Physiology 2- Photosynthesis

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

photosynthetic products often stored as starch Photosynthesis: 6 CO2 + 6 H2O C6 H12 O6 + 6 O2 carbon dioxide + water = sugar + oxygen photosynthetic products often stored as starch Starch = glucose polymer features same as in Chlorophyta (green algae) 3. similar photosynthetic storage product: starch aerobic photosynthesis more efficient than anaerobic Fig. 10.3 Photosynthesis STARCH Tracking atoms

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Chlorophyll Absorbs red & blue light Reflects green light

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Fig. 10.17

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)

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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 High CO2 effects greater in C3 than C4 and CAM species. 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

SoyFACE: CO2 & ozone