© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Different Photosynthetic Mechanisms When sufficient water is available, transpiration and photosynthesis work together. –In times of drought & high light, the two may conflict. In many areas, sufficient rain in the growing season makes a compromise between CO 2 intake & water loss is a non-issue. –Plants evolved here are called C3 plants, because CO 2 that enters the leaf is directly used to generate the 3-carbon molecule phosphoglycerate.

© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Different Photosynthetic Mechanisms Plants in more arid regions have adaptations that help conserve water —but not inhibit photosynthesis. These are C4 and crassulacean acid metabolism (CAM) plants. The C4 mechanism is found in tropical grasses such as corn, sorghum, warm-season turf- grasses, and other species.

© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Different Photosynthetic Mechanisms In C4 plants, CO 2 temporarily attaches to a 3-carbon organic acid, making a 4-carbon organic acid. –The acid shuttles to the bundle sheath cells and the CO 2 is released and used to generate glyceraldehyde. Due to increased CO 2 capture efficiency, not as much CO 2 has to enter the leaf, so stomates can be more tightly closed to reduce water loss.

© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Different Photosynthetic Mechanisms CAM plants are water-storing desert plants such as succulents and cacti. –Stored water absorbs heat & keeps plants cool in the day when the temperatures are hot, but stomates are closed. CAM attaches CO 2 to a three-carbon organic acid at night when stomates can open with less water loss. CO 2 is held on the acid until light is available & released to follow the normal photosynthetic pathway.

© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Plant Development & the Source–Sink Relationship Plant growth significantly influences net & total rate photosynthesis, in single leaves & the total canopy. –Provided all other environmental factors are constant. As single leaves develop, photosynthetic rates rise in step with the expansion of the leaf. –Chlorophyll content & rate of net photosynthesis per square centimeter of leaf area both rise to maximum values just after the leaf reaches full expansion.

© 2011, 2007, 2002, 1988 Pearson Education, Inc. Pearson Prentice Hall - Upper Saddle River, NJ Practical Horticulture 5 th edition By Margaret J. McMahon, Anton M. Kofranek and Vincent E. Rubatsky tab Chapter 11 - Photosynthesis and Respiration Plant Development & the Source–Sink Relationship When the leaf reaches full expansion, it is called a source leaf because the carbohydrate synthesized in that leaf is in excess of what the leaf itself needs. –Excess is exported to parts of the plant that are actively growing—roots, fruits, seeds—often called sink tissues.