Plants responses

Treshow, M., "Environment and Plant Response." McGraw-Hill, New York, 1970. 

One of the products of a nuclear power plant PSA is a list of plant responses to initiating events (accident starters) and the sequences of events that could follow. By evaluating the significance of the identified risk contributors, it is possible to identify the high-risk accident. sequences and take actions to mitigate them. 

These six major steps require consideration of a) the occurrence frequency of fires, b) the physical effects of fires, and c) the response of the plant. Crucial is the plant response as it is affected by components damaged by fire, and by components unavailable for other re isons u-.e., random failures, maintenance, and fire-fighting activities). 

Accident progression scenarios are developed and modeled as event trees for each of these accident classes. System fault trees are developed to the component level for each branch point, and the plant response to the failure is identified. Generic subtrees are linked to the system fault trees. An example is "loss of clcciric power" which is analyzed in a Markov model that considers the frequencies of lo,sing normal power, the probabilities of failure of emergency power, and the mean times to repair parts of the electric power supply. 

In recent years the extended controversy concerning the appropriate terminology to use in studies of plant responses to stressful environments (e.g. Kramer, 1980 Levitt, 1980 Harper, 1982) has often detracted attention from the identification and understanding of underlying principles. Despite this it is useful at this stage to outline the main concepts involved and attempt to provide a generally acceptable common framework for further discussions. 

In many cases, where one is concerned with the effects of specific environmental factors it is appropriate to replace the general term stress by the appropriate quantitative measure (e.g. soil water content or water potential) together with an appropriate measure of the plant response (e.g. growth rate). 

Are there common features of different stresses and plant response ... 

There are several aspects of different environmental stresses that either have common features or the plant responses or adaptations to those stresses may have common components or indicate general principles. It is an objective of this volume to identify such features where they exist so as to help in the development of stress-tolerant crop plants by making the best use of the newer techniques of molecular biology. Particular examples will be discussed in more detail in succeeding chapters. 

On the other hand, there is substantial evidence that plant responses to... 

Some other examples of common plant responses that are not discussed in great detail elsewhere in this volume, but that are nevertheless worth highlighting include ... 

Hi) Poly amines. In many respects the role of poly amines in plant functioning is still mysterious after many years work. They are almost certainly involved in the control of growth and development through their interactions with nucleic acids and membranes (Smith, 1985). There is increasing circumstantial evidence for their involvement, especially of putrescine, in plant responses to a wide range of stresses including pH, Mg deficiency, osmotic shock, cold, SO2 pollution, and cadmium and ammonium toxicity (Smith, 1985). It remains to be determined, however, how, and indeed whether, putrescine accumulation in response to these diverse stresses is beneficial. 

The use of stress terminology has been discussed in Chapter 1, where it was pointed out that the value of the term stress in indicating some adverse force or influence lies in its extreme generality, without the need for a precise quantification. Nevertheless it is appropriate that a scientific discipline should be concerned with definable quantities. This will be the starting point for this paper, which will follow the example of Levitt (1972) who applied the concepts and terminology of mechanical stress (force per unit area) and strain (a definable dimension change) to the study of plant responses to the environment. This approach will be developed here in an attempt to incorporate the philosophies behind stress effects into a general treatment of the responses of ecosystems to adverse environmental conditions. 

Whole-plant responses to stress in natural and agricultural systems... 

After describing some of the main implications of Plant Strategy Theory for the study of stress responses, brief accounts are provided of two additional dimensions of variation in plant response to stress these consist of stored growth and resistance to mechanical stress. 

The search for a predictive model of plant responses to stress... 

A quite different approach to the study of plant responses to stress has been explored by those ecologists who have followed the example of Harper (1977) in applying to plants techniques originally deployed in investigations of animal populations. Here the methodology has been demographic and the resulting data have allowed responses to stress to be analysed in terms of fluctuations in the rates of mortality and recruitment of either plant populations or plant parts (e.g. leaves, inflorescences). 

This clearly overstates the potential of demographic study to provide a mechanistic understanding of plant responses to environments and, if implemented, would lead to unnecessary delay in the development of generalising principles. The remainder of this chapter is founded on the assumption that the most direct route to a coherent predictive theory of plant responses to stress is likely to involve a synthesis of insights derived from plant population biology, ecophysiology, and many other fields of botanical endeavour. 

In addition to the mechanisms of stress response so far considered, there are several others which have attracted the attention of plant ecologists. These include innate or environmentally determined forms of dormancy in seeds, spores, and vegetative buds, many of which represent adaptive responses restricting plant growth and development to favourable seasons or sites. Dormancy has been the subject of numerous publications and will not be considered here. Instead, opportunity will be taken to refer to two forms of plant response to stress which until recently have received only scarce attention. The first is the phenomenon of stored growth whilst the second involves the response of the developing shoot to mechanical impedance. 

Farquhar, G.D. (1988). Models relating subcellular effects of temperature to whole plant responses. In Plants and Temperature, ed. S.P. Long and F.I. Woodward. Society for Experimental Biology Symposium 42 (in press). 

Hsiao, T.C. (1973). Plant responses to water stress. Annual Review of Plant Physiology, 24, 519-70. 

So far this presentation has dealt exclusively with responses to water potential perturbations at the cellular level. In concluding we wish to consider one example of a response at the whole-plant level in the light of this analysis. This example is developed further by Yeo Flowers (Chapter 12) who also refer to further examples of the complexity of whole-plant responses. 

Physiological aspects of particular stresses and plant responses... 

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