This chapter illustrates the function and physiology of stomata to clarify the role stomata plays in determining carbon assimilation. Stomata are small adjustable pores found in large numbers on the surface of most aerial parts of higher plants. Leaf gas exchange is controlled by the stomata. The diffusion rate of gases into or out of the leaf, or any other plant part, depends on the concentration gradient and the diffusive resistance of the pathway. The resistance of the stomatal pathway depends on the geometry of the pores as well as their frequency. Stomatal behavior directly modifies the CO2 assimilation rate and transpiration rate and consequently affects plant water and carbon status. The chapter also describes the effects of several environmental factors such as CO2, humidity, light and temperature on stomatal movements and the consequences for photosynthesis. A change in the concentration of CO2 leads to a change in the aperture of the stomata. The transpiration rate increases linearly with leaf-to-air vapor pressure difference (VPD) caused either by changes in air vapor pressure or by leaf temperature affecting the vapor pressure inside the leaf. A change in light intensity may simultaneously change photosynthetic rate and leaf temperature that modifies transpiration rate and leaf water status. Finally, the chapter illustrates several examples of modern techniques for studying stomatal physiology.

This chapter discusses the complex processes at the interface between plant and environment along with the multifunctional hydrophobic coverage of land plants. The plant cuticle covers all primary parts of vascular plants (except roots) and many bryophytes as a thin extracellular membrane. The cuticle represents a natural composite that consists mainly of two hydrophobic components, the insoluble biopolyester cutin and soluble lipids. One of the major functions of the cuticle is the limitation of uncontrolled water loss via evaporation. Thus, selective pressure acted strongly on plants to develop an outer hydrophobic envelope functioning as a compromise between the contrary demands of desiccation avoidance and free gas exchange. The multiple constraints on the evolution of such an outer envelope (plant cuticle), found on fossil and recent plant material, can be summarized by transport phenomena across the cuticle, interaction with biotic and abiotic factors, and biomechanical requirements. In addition to the hydrophobic nature of the cuticle, the presence of epicuticular wax crystals often leads to water repellency instead of wetting. This is because of their hydrophobic nature and microroughness. The plant cuticle can also be seen as the first mechanical barrier against microorganisms and herbivores. From a mechanical point of view, the location at the outer perimeter of plant organs and its rigid appearance, at least in succulents, indicates that the cuticle may function as external structural element that possibly adds mechanical support for tissue integrity and impacts on morphogenetic processes.

This chapter discusses the evolutionary origin of the angiosperm ethylene biosynthesis pathway. Responsiveness to ethylene appears to be present throughout the plant kingdom. Evolution of ethylene responsiveness was driven by the natural connection between ethylene generation and plant stress. When cells are damaged, oxidative breakdown of cell constituents—particularly membrane fatty acids—generates ethylene, albeit in amounts that are orders of magnitude lower than those generated physiologically. As land plants evolved, they acquired increasingly complex physiological and biochemical responses to the stresses to which they were exposed. Among the biochemical responses were betaine synthesis, for the reduction of water potential during drought, and lignifications for the limit predator and pathogen attack. The synthesis of both betaine and lignin depends upon the availability of S-adenosylmethionine (SAM). This is an abundant metabolite in all organisms and plays a key role in transmethylation reactions. It is derived from methionine by the action of SAM synthetase. SAM is also the precursor of 1-aminocyclopropane-l-carboxylic acid (ACC) that arises by the action of ACC synthase on SAM. ACC is detectable in representatives of all major groups of land plants. In the earliest land plants, cell damage arising from stress leads to the formation of ethylene by chemical breakdown of cell constituents. In ferns and allied non-seed plants, stress leads to the increased expression of SAM synthetase for lignin and betaine synthesis, enhanced ACC synthase activity, and the accumulation of ACC. Ethylene is produced from ACC with the help of the enzyme ACC oxidase.

Aluminum (Al) is ubiquitous in the environment as it is the most abundant metal and the third most common element in the Earth's crust. Al accumulation in plants is not only a very important and ecophysiologically highly interesting phenomenon, but it also provides useful information for systematic purposes at relatively high taxonomic levels. Al accumulation has independently been developed in several plant groups that are not necessarily closely related, but its occurrence is far from being randomly distributed. Its primitive status in the derived groups of the angiosperms is generally confirmed following recent molecular phylogenies. About 93% of all Al accumulators that have been recorded belong to the asteroids and rosids. The primitive status of Al accumulation in angiosperms is suggested on the basis of statistical correlations. Its primitive (plesiomorphic) or derived (apomorphic) nature; however, largely depends on the taxonomic level. Further, evolutionary trend of Al accumulation is complicated by the occurrence of numerous reversals (or losses) and parallel origins. Alternatively, the low incidence of Al accumulators in derived groups might also be correlated with their frequent herbaceous habit. Al accumulation is considered to be a process that depends partly on the influence of heredity and partly on ecological influence. Among ecological factors, the most important factor is soil acidity. The soil pH comprises the most important factor in Al uptake because the solubility and bioavailability of Al increases with decreasing pH.

This chapter describes the chemical methods that provide detailed molecular insight into the chemical composition of both extant and fossil resistant (bio)macromolecules. Numerous chemical extraction methods exist to obtain the most resistant organic molecules in plant remains. The various chemical methods that are used to study the resistant biomacromolecules can be subdivided into nondestructive and destructive techniques. Nondestructive techniques includes solid state 13C NMR and FTIR. The destructive methods include pyrolysis and chemolysis. The chapter discusses the macromolecular composition of outer coverings of modern and fossil plants, including algal cell walls, spore and pollen walls, and cuticular tissues in terms of their chemical similarities and differences and in relation to the physiological adaptations and evolution of land plants. The resistant molecules in cuticles and spore and pollen walls (sporopollenin) are all based on even carbon numbered long-chain aliphatic chemical building blocks, providing sufficient hydrophobicity to reduce water loss. These aliphatic moieties are largely similar to those present in the resistant walls of algae (algaenan) from which the land plants may have evolved. Apart from the aliphatic materials, sporopollenin and, to some degree, cutin and cutan from both modern and fossil examples also reveal the presence of cinnamic acids, which probably are responses to the enhanced levels of ultraviolet radiation on land. Finally, the chapter evaluates the chemical composition of water-conduction tissues of modern and fossil land plants in terms of their physiology but with specific emphasis on the biomacromolecule lignin.

This chapter explores the physiological basis for Metasequoia's success in the Eocene high-Arctic and the ecophysiological attributes that imparted adaptive value to trees inhabiting this unique environment (Eocene high-Arctic). The environment that has been advanced for Eocene high-latitude forests provided a unique combination of light, temperature, and water regimes that offers an unparalleled set of adaptive challenges to plant species. The temperate continuous light (CL) regime offered both substantial physiological challenges and the potential benefit of high growth rates for species able to overcome those physiological hurdles. Among the unusual aspects of Metasequoia is a collection of characteristics that do not fit contemporary models of shade-adaptation or sun-adaptation. Metasequoia possesses an aggregation of characteristics not generally associated with either adaptive strategy, but competitively adaptive at high latitudes. While the photosynthetic rates of Metasequoia under light intensities typical of the growing season at middle latitudes are clearly much lower than those of sun-adapted conifers, its resource allocation to photosynthetic systems (leaf-level) is close to optimum for the moderate light intensities of the Arctic lowland forests, or cloudier lower palaeolatitudes. Metasequoia could rapidly produce canopies with extensive leaf area, not only efficiently capturing incident light for photosynthesis, but minimizing the transmitted light that would be available for competitors. Production of low-density stem wood permitted Metasequoia rapidly to overtop potential competitors that were establishing concurrently. The importance of photorespiration and alternate pathways for photoenergetics in Metasequoia and other species growing under CL regimes are yet to be explored.

This chapter presents a discussion on the evolution of plant biochemistry and the implications for physiology. Integrating physiological and biochemical knowledge can gain a functional understanding of plants. Plant cells are usually treated as unusual but only in so far as they have walls and chloroplasts. This emphasis on the commonality of the basic metabolism of organisms is helpful to those learning the subject but it does tend to limit conceptual approaches to biochemistry. The intimate connection between the physiology and biochemistry of plants is well established in the case of some aspects of physiology, especially where there are dramatic morphological and anatomical adaptations that make the links clear. Biochemical evolution overlaps with molecular evolution but can extend to consider more than one enzyme in a pathway and the control of that pathway. To achieve cost saving by inducible defence, three extra elements are needed: (1) a sensing system(s) that can detect the conditions that are an accurate indicator of a need for defence, (2) a regulatory step in the chemical production capacity, and (3) a linkage mechanism between the detection system and the regulatory system. Evidence is accumulating that plants possess all the three abilities, and evolution may have provided multiple ways of linking the three elements.

This chapter maps the stress-related physiological traits onto a robust phylogeny for modern charophycean algae and bryophytes. Trait mapping suggests that early phenolics could have been preadaptive to the development of stable plant–microbe relationships. As in modern plants, phenolic compounds may have controlled microbial behavior, allowing microbes to live in close proximity to algae and early land plants without becoming pathogenic. The chapter also compares the aspects of phenolic chemistry among charophyceans, bryophytes, and pteridophytes and estimates the extent to which nonvascular plants could have contributed to carbon sequestration prior to the origin of vascular plants. Thioacidolysis was used as an assay for lignin-specific β-O-4 phenolic linkages in representative green algae and early-divergent land plants. Selected green algae and bryophytes were surveyed for the presence of resistant biomass and the percentages of resistant cell wall biomass were quantitatively determined. The amount of resistant organic carbon that might have been generated by early non-vascular land plants was also estimated. Adaptive utility for high levels of wall phenolics might include (1) resistance to attack by pathogenic bacteria, protists and fungi, (2) increased stability of cell walls, contributing to the ability to achieve increased height, (3) UV-damage resistance, and (4) desiccation resistance.

This chapter discusses the link between the delayed evolution of megaphyll leaves and atmospheric CO2. A model of leaf biophysics and physiology was used to investigate the functional consequences for Devonian megaphylls of the evolutionary relationship between stomatal density and atmospheric CO2. The model mathematically describes the key interactions between photosynthetic CO2 fixation, stomatal conductance, transpiration, and the leaf energy balance. It explicitly accounts for the influences of temperature, light, humidity and CO2 on these processes, and their feedbacks operating via CO2 concentration in the sub-stomatal cavities, leaf temperature and the leaf-air vapor pressure deficit. Model simulations show that hypothetical large megaphyll leaves would have provided no selective advantage over branched axes for photosynthesis in the Early Devonian high CO2 atmosphere. This is because the exceptionally low stomatal densities observed for this period would have restricted transpiration rates in these leaves, and so dissipated very little absorbed solar energy as latent heat, with a high associated risk of lethal overheating. Simulation results suggest that as stomatal density rose in response to falling CO2 levels through the Devonian and Early Carboniferous periods, large laminate megaphyll leaves increasingly gained a selective advantage in terms of carbon gain by photosynthesis, without the penalty of high temperature damage.

This chapter explores the current understanding of the origin of the spore wall, the function(s) of the spore wall in early land plants, and mode of spore wall formation in early land plants. The embryophytes are one of the major kingdoms of eukaryotice life. The sporopollenin wall surrounding charophycean zygotes and embryophyte spores/pollen grains is homologous, and the spore/pollen wall is an embryophyte synapomorphy that evolved as an adaptive response to the invasion of the land. It is considered that the primary function of the spore/pollen wall involves protection in the harsh subaerial environment. Regarding spore wall development, ultrastructural studies have demonstrated that structural elements present in the spore walls of extant plants can be recognized in the fossil spores of early land plants, suggesting that spore wall development was similar in extant and ancient land plants. Studies of molecular genetics of spore wall formation are in their infancy but have the potential to solve many unanswered questions regarding spore wall homologies and developmental processes—particularly when such studies are commenced on more “primitive” land plants and their extant sister group.