Study of Plant Responses

There are two general approaches to the study of plant responses. One approach is the carefully designed greenhouse, field or pot experiments, where the observation of plant responses to soil factors are made. The alternative approach is the study of soils and plants in their natural environment. There are a number of drawbacks to the greenhouse experiments, for instance, reduction of time-scale in experiments creates uncertainty as to field relevance although reliable qualitative indications may be obtained (Folkeson et al., 1990). The responses of plants under artificial environmental conditions are usually not representative of the natural conditions (Irgolic and Martell, 1985). 

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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. 

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. 

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). 

Research into the effects of air pollutants on plant growth and metabolism has moved towards the study of plant responses to low levels of pollutants ("chronic injury") and the interactions with environmental conditions. Plant growth and crop yield are known to be affected after exposure for long periods to low pollutant levels (1). For example, exposure to SO2 and NOx resulted in increased leafiness and reductions in root growth (2,3,4), while in barley overwinter reductions in crop growth have been reported (5,6). These effects on plant growth have been related to photosynthesis and the distribution of photoassimilate, the processes that sustain dry matter production in plants (7). 

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. 

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. 

Nowadays the genome of many plant species have been sequenced. This amount of information may be useful in the studies regarding plant responses to HS. For example, mutant or transgenic plant lines may be used in order to discover new aspects on this area. 

Numerous laboratory studies of the response of plants of both types to a change in the quantity CA (Bazzaz, 1986) testify to the wide range of quantitative estimates of photosynthesis variations for the C3 type. On average, plants respond to a change in C02 concentration after a 1-month delay. Doubled C02 concentration causes a doubling of the rate of photosynthesis. Further increase of CA up to 400% leads to the effect of photosynthesis saturation for some plants (i.e., there can be a 20% addition to the rate of photosynthesis), and in some cases (e.g., Setaria lutescens) photosynthesis is suppressed. In fact, plants of the C4 type even with the present quantity of CA are in a state of photosynthesis saturation. 

Thus far LPS preparations used for the analysis of plant responses and for structural studies have been derived from bacteria grown in culture. We know almost nothing about the alterations in LPS that occur when bacteria are within plants, although this may be highly relevant to signalling. Changes could occur in both the size distribution of LPS (alteration in the ratio of LOS to LPS) and/or in decoration of LPS with saccharide, fatty acid, phosphate or other constituents. Increases in the sensitivity of mass spectrometric methodologies may allow development... 

The complexity of plant response to toxicants is even more apparent as biochemical information regarding toxicosis has become available. However, the biochemical information has arisen from individual studies and reflects the interests of many investigators and many points of view. The appreciation of and recognition of the basis of biochemical lesions as responses to air pollutants is essential if we are to progress in this important field. 

An increased awareness of the complexity of plant response also developed as considerations of the response of populations of plants exposed to a toxicant were initiated. Detailed descriptions of acute responses and even chronic responses were the result of chamber studies of individual plants held under relatively constant experimental conditions with only dosimetric variables. As investigators became more adept at recognizing symptomatology, whole fields of damaged plants were investigated, and the population aspects of the response became evident. 

The study of plant population toxicology is mandatory if we are to understand and eventually to ameliorate the problem of toxicant-mediated changes to the world around us. Rather than just being an interesting phenomenon to be studied by a few plant pathologists, the fact that toxic air can threaten our food supply stimulates us to greater concern. We still depend upon plants to exist. Phytotoxic response among many individual plants and plant populations is a fact, and adverse effects on food production are a matter of record. The papers in this volume address themselves to the problem. 

The birth of drug discovery is closely connected to the study of plant natural products and was shaped by two seminal events, the isolation of morphine 1 from opium by the pharmacist Serturner in 18171 and the introduction in the clinics of Antipyrin 6 (phenazone) 70 years later, in 1887.2 The obtaining of a pure compound responsible for the medicinal properties of a crude drug marked the beginning of medicinal chemistry, triggering the transition from botanical extracts to pure molecules and eventually leading to the isolation of the active principle of most heroic drugs. 

Two different lines of phytochrome research can be visualized in these early investigations (1) The study of physiological responses to various light treatments in intact plants and (2) the isolation of phytochrome and the study of its molecular properties. Both lines have been followed up since by numerous workers so that much and detailed information is available along these lines. However, there is still a gap if these lines are to be combined in the question of the mechanism of phytochrome action. This question is discussed in section E. 3 after the discussion of the molecular properties of phytochrome. 

Initial studies of proteome responses to environmental chemicals in soil-dwelling animals or plants are currently under way. Kuperman et al. (2004) have used the approach to identify differentially expressed proteins in earthworms exposed to chemical warfare agents. Toxicological studies also have been undertaken. Vido et al. (2001) analysed yeast cells exposed to an acute cadmium stress 54 proteins were induced and 43 repressed. Finally, Bradley (2000) used two-... 

By studying the spectrum of the root in a transparent box, the proposal of studying the ecology of plant responses to different climates and soils was introduced. It has been suggested that the function of the roots is serve as a communication network in the soil. The authors [58] state that, during the life of the plant, as much as 15 % of the total fixed carbon will be diverted into sugars, proteins and other small molecules that are surrendered to the plant. The role of these exudates was discussed, the authors suggesting an interdisciplinary approach to the study of plants, soils and microbes in the field. 

A follow-up design is given in the GT-MHR (Modular Helium Reactor) (see also section 4.7.2.) with a higher power output of 600 MW(th). A standard plant is planned consisting of four of those units. Helium inlet/outlet temperatures are 485 and 850 °C, respectively. The cycle efficiency is predicted to be 47 % [51]. Follow-on evaluations which need to be done include the study of transient response of plant components to normal and off-normal events, impact of turbine contamination, and confirmation of plant efficiency [47]. 

Most hydroponic studies of the response of wetland plants to anaerobic root environments have been based on the purging of nitrogen to remove oxygen from solution. Such treatments lead to a redox potential of 330-400 mV (DeLaune et al., 1990). This value is much higher than redox values found in flooded soils that may reach as low as -250 mV. Artificial redox buffer (titanium citrate) used to... 

It must be emphasized, however, that stress resistance or susceptibility is unlikely to reside in a single factor. Studies of the responses of plants to environmental stresses suggest rather that resistance results from the possession of a number of characteristics. Attempts to explain susceptibility or resistance of plants to environmental stresses in terms of single factors are therefore unlikely to result in plausible theories of environmental adaptation. Changes in nitrogenous compounds can only be regarded as components of the resistance or tolerance mechanisms. 

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