Plant injury, physiology

Howell, R. K. Phenols, ozone, and their involvement in pigmentation and physiology of plant injury, pp. 94-105. In M. Dugger. Ed. Air Pollution Effects on Plant Growth. ACS Symposium Series 3. Washington. D.C. American Chemical Society, 1974. 

Phenols, Ozone, and Their Involvement in Pigmentation and Physiology of Plant Injury... 

The nonvisual or subtle effects of air pollutants involve reduced plant growth and alteration of physiological and biochemical processes, as well as changes in the reproductive cycle. Reduction in crop yield can occur without the presence of visible symptoms. This type of injury is often related to low-level, long-term chronic exposure to air pollution. Studies have shown that field plantings exposed to filtered and unfiltered ambient air have produced different yields when no visible symptoms were present (5). Reduction in total biomass can lead to economic loss for forage crops or hay. 

Desiccation tolerance and injury avoidance The remarkable tolerance to prolonged anhydrobiosis in resurrection plants suggests they are able to maintain essential structure and physiological integrity in the dry state or are able to repair dehydration-induced damage rapidly following rehydration. 

Larkin injected several peroxidases at 0.1-10 ppm into one-half of a tobacco leaf and found some protection. He suggested that peroxidase, which is often associated with plant stress conditions, may be important in physiologic resistance. It is doubtful that any one mechanism of action exists. It is important that we understand the mechanism of ozone injury and resistance in plants, so that we can determine better what chemicals may play a role in protecting plants against oxidants. 

The complex interaction between climatic factors, toxicant concentration, exposure duration, soil conditions and physiological characteristics determines the susceptibility of plant tissue and, to a large extent, influences the characteristics of the injury symptom syndrome produced. Distinct and relatively uniform symptoms may be produced repeatedly on susceptible members of the plant community if they are exposed under controlled-standardized conditions. But the extent of injury and a concomitant alteration in injury appearance usually occurs when there is a deviation in toxicant dosage, plant species or environmental condition. 

The toxic effects of ozone in plant systems have been studied for some time, yet the actual mechanisms of injury are not fully understood. In addition to visible necrosis which appears largely on upper leaf surfaces, many other physiological and biochemical effects have been recorded ( ). One of the first easily measurable effects is a stimulation of respiration. Frequently, however, respiration may not increase without concomitant visible injury. Furthermore, photosynthesis in green leaves as measured by CO2 assimilation, may decrease. It is well known that ozone exposure is accompanied by a dramatic increase in free pool amino acids ( ). Ordin and his co-workers ( ) have clearly shown the effect of ozone on cell wall biosynthesis. In addition, ozone is known to oxidize certain lipid components of the cell ( ), to affect ribosomal RNA (16) and to alter the fine structure of chloroplasts (7 ). 

Most plants suffer damage, both physiological and biochemical by exposure to temperatures higher or lower than optimal for growth [99]. The results of these injuries, which are reflected in most metabolic processes may be a reduced growth capacity of the crops and therefore lower commercial yield [100]. It has been demontrated that thermal stress induces the production of phenolic compounds [3,4,101]. 

Haagen-Smit, A.J., E.F. Darley, M. Zailin, H. Hull and W. Noble (1952) Investigation on injury to plants from air pollution in the Los Angeles area. Plant Physiology 27 18. 

Sagisaka, S. (1985). Injuries of cold acclimatized poplar twigs resulting from enzyme inactivation and substrate depression during frozen storage at ambient temperatures for a long period. Plant and Cell Physiology 26, 1135-45. 

Steponkus, P.L. (1984). Role of the plasma membrane in freezing injury and cold acclimation. Annual Review of Plant Physiology 35, 543-84. 

Excessive concentrations of some metals in soils may produce toxic symptoms in plants. Levitt (1980) suggested that metals in the plant environment operate as stress factors in that they cause physiological reaction change (strain) and in so doing can reduce vigour, or in the extreme, totally inhibit plant growth. Sensitivity describes the effects of a stress which result in injury or death of the plant. 

When the cumulative load of fluoride in the plant reaches a threshold concentration, a number of characteristic symptoms may appear, culminating with death of the plant. The levels of fluoride in a plant may be high relative to the background amounts in the ambient air. The concentration at which injury symptoms appear seems to depend largely upon the plant species and to a certain extent upon a series of complex interacting environmental conditions which in turn affect the physiological state of the plant. 

If beans could be affected by trifluralin as described then it seems reasonable to assume that similar physiological and morphological injuries can occur in weeds making them more susceptible to mycoherbicides such as that described by Yu et ai. (31). In fact, concentrations as low as 10% of recommended field rates could have affected hypocotyls and foliage of treated plants, including both host plants and weeds. 

It is therefore necessary to provide suitable safeguards to prevent or minimize the injuries that can occur to workers in industrial plants and to the general public. There is a need to understand the ways by which these chemicals enter the human body and their physiological effects. Preventive measures should be exercised to avoid this absorption. 

The research available to date presents a partial view of the impacts of acid rain on woody plants. Many of the impacts are still only potential impacts, as simulation studies versus field studies present a conflicting view. However, one thing appears quite clear - more research is needed. As many researchers have found, the effect of acid rain is not going to be one of simple cause and effect, but rather one of a multiple factor interaction. Thus, future work should be statistically designed to test the inter-action(s) rather than main effects. Work needs to be done over both the short and long term to assess injury. Basic physiological work across disciplines with the standardization of techniques used (e.g. one set type of simulator for all researchers to produce simulated acid rain) must be employed in order for different experimental results to be comparable. If we can discover how plants will react to given combinations of stresses, only then will we be able to propose an appropriate course of action. 

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