Physiology deals with the functions of plants. Its development as a subdiscipline has been closely interwoven with the development of other aspects of botany, especially morphology. In fact, structure and function are sometimes so closely related that it is impossible to consider one independently of the other. The study of function is indispensable for the interpretation of the incredibly diverse nature of plant structures. In other words, around the functions of the plant, structure and form have evolved. Physiology also blends imperceptibly into the fields of biochemistry and biophysics, as the research methods of these fields are used to solve problems in plant physiology.
Plant physiology is the study of plant function and behaviour, encompassing all the dynamic processes of growth, metabolism, reproduction, defence, and communication that account for plants being alive (Salisbury & Ross, 1992; Baluška et al., 2006; Scott, 2008). Considering that most of these processes take place at the level of cells, tissues, and organs, there is, because of the close association between structure and function in plants, also a close association between plant physiology and plant anatomy. Moreover, within the living cell, much of the metabolic activity is at the molecular level; therefore, a full understanding of a plant’s physiology requires an essential background in chemistry and physics.
Many plant physiological insights into basic processes were gained from research based on a relatively small number of convenient experimental or model plants (e.g. beans, lettuce, maize, wheat, and in more recent times, Arabidopsis thaliana (L.) Heynh., the thale cress) (Pitzschke, 2013). Knowledge thus gained is then extrapolated to other plants as it is assumed that such processes operate similarly. For example, the vast majority of green plants have never been chemically analysed for the presence of chlorophyll, yet we assume that the green colour of their leaves is due to the presence of this pigment.
Although few indigenous southern African kalanchoes have been the subjects of physiological studies, a lot can be inferred about their physiology from work conducted on other members of the genus and other representatives of the Crassulaceae. Of particular significance is the fact that a few mainly Madagascan species of Kalanchoe, especially K. blossfeldiana, K. daigremontiana, K. fedtschenkoi, and K. tubiflora, have served as model experimental plants in many plant physiological studies, especially ones aimed at elucidating the mechanism of photosynthesis in succulents and how flowering is initiated. Kalanchoe fedtschenkoi is an emerging molecular genetic model for the study of crassulacean acid metabolism (see below) in the eudicots (Yang et al., 2017).
we will be highlighting crassulacean acid metabolism (CAM), a specific type of photosynthesis first discovered in a species of Kalanchoe and subsequently found to operate in most Crassulaceae and many other groups of succulents. Kalanchoes are known for their flowering that peaks during the winter months. We will explain this reproductive behaviour in some detail and show how knowledge of the underlying processes is being used to manipulate a selection of kalanchoes, especially K. blossfeldiana and hybrids of which it is one parent, for lucrative commercial purposes.
The leaves and stems of indigenous kalanchoes are not always green; sometimes, they are variously infused with red (anthocyanins) with the southern African Kalanchoe sexangularis being a prime example, and/or their outer surfaces are often substantially covered in a powdery, whitish deposit (epicuticular wax) as in, for example, the group of southern African kalanchoes with club-shaped inflorescences and large, paddle-shaped leaves. We will briefly explain the nature of these two modifications and their possible functions. Finally, some noteworthy anatomical features of the kalanchoes will be briefly mentioned.
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