Robert W. Hess

Abstract: Introduction How to use this book The dictionary Appendix Acknowledgement of sources Abbreviations and symbols (used in this book).

Abstract: Contents Summary 341 I. Introduction 342 II. What is C4 photosynthesis? 343 III. Why did C4 photosynthesis evolve? 347 IV. Evolutionary lineages of C4 photosynthesis 348 V. Where did C4 photosynthesis evolve? 350 VI. How did C4 photosynthesis evolve? 352 VII. Molecular evolution of C4 photosynthesis 361 VIII. When did C4 photosynthesis evolve 362 IX. The rise of C4 photosynthesis in relation to climate and CO2 363 X. Final thoughts: the future evolution of C4 photosynthesis 365 Acknowledgements 365 References 365 Summary C4 photosynthesis is a series of anatomical and biochemical modifications that concentrate CO2 around the carboxylating enzyme Rubisco, thereby increasing photosynthetic efficiency in conditions promoting high rates of photorespiration. The C4 pathway independently evolved over 45 times in 19 families of angiosperms, and thus represents one of the most convergent of evolutionary phenomena. Most origins of C4 photosynthesis occurred in the dicots, with at least 30 lineages. C4 photosynthesis first arose in grasses, probably during the Oligocene epoch (24–35 million yr ago). The earliest C4 dicots are likely members of the Chenopodiaceae dating back 15–21 million yr; however, most C4 dicot lineages are estimated to have appeared relatively recently, perhaps less than 5 million yr ago. C4 photosynthesis in the dicots originated in arid regions of low latitude, implicating combined effects of heat, drought and/or salinity as important conditions promoting C4 evolution. Low atmospheric CO2 is a significant contributing factor, because it is required for high rates of photorespiration. Consistently, the appearance of C4 plants in the evolutionary record coincides with periods of increasing global aridification and declining atmospheric CO2. Gene duplication followed by neo- and nonfunctionalization are the leading mechanisms for creating C4 genomes, with selection for carbon conservation traits under conditions promoting high photorespiration being the ultimate factor behind the origin of C4 photosynthesis.

Abstract: Trichomes occur in a multitude of forms and sizes. Although they have been used widely for taxonomic purposes, their adaptive significance has been all but ignored by the evolutionist and ecologist. It is clear that trichomes play a role in plant defense, especially with regard to phytophagous insects. In numerous species there is a negative correlation between trichome density and insect feeding and oviposition responses, and the nutrition of larvae. Specialized hooked trichomes may impale adults or larvae as well. Trichome may also complement the chemical defense of a plant by possessing glands which exude terpenes, phenolics, alkaloids or other substances which are olfactory or gustatory repellents. In essence, glandular trichomes afford an outer line of chemical defense by advertising the presence of \"noxious\" compounds. In some groups of plants, protection against large mammals is achieved by the presence of stinging trichomes. Intraspecific variation for trichome type and density is known in many species, and often is clinal in accordance with ecographic parameters. The presence of such correlations does not imply that differences in predator pressure are the causal factors, although this may indeed be the case.