Botany: An Introduction to Plant Biology - Mauseth, James D. 2017

Plant Physiology and Development

Part Opener Image: Plants often develop in wonderfully diverse ways. The main branch here grows horizontally but all its branches grow upward. Notice too that only axillary buds on the upper surface of the horizontal branch have grown out whereas all those on the sides and lower surface are still dormant. All axillary buds in the vertical branches are dormant. As these branches grow, they perceive and respond to conditions around themselves. Almost certainly the important environmental cue is either sunlight (which comes from above) or gravity (which pulls things downward): Can you design an experiment to determine which factor is controlling the physiology of these branches? Araucaria subulata (related to Norfolk Island pines).

When you are studying plant structure, you can often see the material directly with the naked eye, although light or electron microscopy may be necessary to see some structures. In plant physiology and development, however, the objects of our study—chemical reactions and metabolic pathways—cannot be seen at all. Instead, the results of experiments, measurements, and analyses are studied, and hypothetical reactions and pathways are set down on paper. From these, predictions are made and new observations planned to test the hypotheses. It is easy for us to study and memorize metabolic pathways, chemical formulas, and diagrams of physiological control mechanisms without appreciating that these have never been, and cannot ever be, seen directly. Every one is a theoretical model that is consistent with the majority of the available data and that is logically and internally consistent.

When experienced anatomists see an unusual structure, they may be able to recognize instantly that it is new to science or is at least a significant variation. For physiologists, it is not so easy: If an experiment on photosynthesis does not come out as expected, it may be that a new type of photosynthesis is being discovered; it may be that the current theories of photosynthesis are not completely accurate and have made an erroneous prediction, or it may be that an experimental error has occurred. It can be difficult for students to appreciate the tremendous amount of careful, ingenious work that must be done just to establish that a particular theoretical metabolic pathway truly represents the reactions that occur in certain plants. Whereas it is relatively easy to determine that natural selection has resulted in the evolution of many types of leaves, stems, roots, flowers, xylem, phloem, and so on, it is much more difficult in physiology. Numerous differences in microhabitats, water availability, heat, cold, soils, pests, and plant diseases have resulted in diverse types of structures that are selectively advantageous under various conditions. It is logical to expect the same to be true of metabolism; we do know that there are several varieties of photosynthesis and respiration, and there may be others that have not yet been discovered.

As you study the material in this section, keep in mind that, just as is true for structure, organization is of fundamental importance for metabolism. The chemical and physical reactions that constitute plant metabolism are highly ordered and not at all random. This orderliness is maintained by the input of energy (see Chapters 10 and 11) acting on materials brought into plants from the environment (see Chapters 12 and 13). There are many types of order, and the information necessary to establish the proper reactions acting on the proper material is stored in the genes, both in the nucleus and in the plastids and mitochondria. The mechanisms by which the genes control the interaction of energy and matter, such that a plant of a particular species results, are discussed in Chapters 14 and 15.