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

Photosynthesis: 6 CO H 2 O C 6 H 12 O O 2 carbon dioxide + water = sugar + oxygen Tracking atoms STARCH photosynthetic products often stored as starch Starch = glucose polymer

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Chlorophyll Absorbs red & blue light Reflects green light

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Rubisco Ribulose bisphosphate carboxylase oxygenase (fixes CO2 & O2) Enzyme in Calvin Cycle (1 st 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 –CO 2 temporarily stored as 4-C organic acids resulting in more more efficient C exchange rate –Advantage in high light, high temperature, low CO 2 –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.

Elevated CO 2 Increased leaf C:N Decreased Heat-shock proteins (Hsps) Decreased thermotolerance High CO 2 effects greater in C 3 than C 4 and CAM species. High CO 2 effects greater on induced than basal thermotolerance. Hypothesis

Heat stress decreased P n in all species (not the result of stomatal closure). Elevated CO 2 had negative effects on P n of C 4 species, and positive effects on C 3 species. Pre-heat shock has a positive effect on P n.

Heat shock decreased Ф et of all C 3 and C 4 species There was negative CO 2 effects on all species, except for wheat There was positive Pre-HS effects on all species

SoyFACE: CO2 & ozone