Pollution crop plants

The preparation of soils for crops, planting, and tilling raises dust as a fugitive emission. Such operations are shll exempt from air pollution regulations in most parts of the world. The application of fertilizers, pesticides, and herbicides is also exempt from air pollution regulations, but other regulations may cover the drift of these materials or runoff into surface waters. This is particularly true of the materials are hazardous or toxic. 

Host plant preferences of insect pests are often Influenced by environmental conditions. The effects of plant stress on susceptibility to Insect attack have been obseirved in many crop plants. Stress factors affecting the physiological state of the plant Include drought, disease, chemical pollutants, and high salt concentrations. Profound... 

Use has also been made of the metal content of crop plants in the assessment of contaminated soils. Kabata-Pendias et al. (1993) suggest that legumes are promising as bioindicators of metal pollution since they have in general a relatively higher tolerance to and uptake of metal than monocotyledons. Kovacs et al. (1993) have... 

Tisible symptoms of acute injury have been the principal means of identifying the effect of air pollutants on plants for well over a century. They have served as major factors in assessing the impact of man s activities on the total environment and have served as the basis for numerous estimates of economic damage to agricultural crops. Such estimates are admittedly crude because the total effect of air pollutants on growth and development is not indicated by symptoms of acute injury. Nevertheless, such evaluations are essential since adequate controls historically develop only after economic pollutant damage is well documented. 

Air pollutants cause plant damage which varies both in type and degree. A few crop, ornamental, and forest species have shown a variability in the degree of damage so that some varieties within these species may be classed as resistant and others as susceptible. This variability probably exists in many species in which it has not yet been demonstrated. Further, in a few species this variability is at least in part genetically based. Therefore, the opportunity exists to select those plants resistant to the toxicants and to breed new varieties which can be successfully grown in a polluted atmosphere. Obviously, this solves only part of the air pollution problem. Resistant varieties will alleviate some effect of air pollution on plants but will not eliminate the pollutants themselves. 

For example, if genetic variation exists in a crop plant population, it will become apparent under the environmental stress of a siege of pollution. Some individuals will survive the attack with no or minimum... 

The development of crop plant varieties with resistance to pests including Insects and diseases has several advantages compared with other approaches to pest control The application of pesticides may be decreased or eliminated, resulting in decreased costs of production and environmental pollution The use of resistant varieties also does not require the adoption of complicated technologies by the user. 

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). 

When incorporated in the soil, lead is of very low mobility. Hence once contaminated, a soil is liable to remain polluted with lead. This might have adverse consequences for soil fertility if the degree of contamination is great. In addition, plants grown on lead-rich soils incorporate lead and thus the concentration of lead in crop plants may be increased slightly. These questions are explored in the following sections. 

Plant microbial association S. meliloti S3 + S. plymuthica 57 +M sativa may be recommended for phytorecovery of soils contaminated with diesel fuel, cmde and industrial oil. AppHcation of the proposed plant-microbial consortium for phytoremediation of polluted soil could succeed only if hydrocarbon levels in soil don t exceed 1%. It should be noted that soil contamination is damaging to alfalfa seedlings, therefore crop plantings should be started with 1-month lag behind the pollution moment and seed doses should be increased by 20%. 

In recent years, an increasing number of investigation deals with the uptake and disposition of environmental contaminants (pollutants, crop protection products) and the following metabolic reactions of the plant. 

Injury to plants and vegetation is caused by a variety of factors, of which air pollution is only one. Drought, too much water, heat and cold, hail, insects, animals, disease, and poor soil conditions are some of the other causes of plant injury and possible plant damage (3). Estimates suggest that less than 5% of total crop losses are related to air pollution. Air pollution has a much greater impact on some geographic areas and crops than others. Crop failure can be caused by fumigation from a local air pollution source or by more widespread and more frequent exposure to adverse levels of pollution. 

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. 

Selective toxicity is also important in relation to the development of resistance or tolerance to pollutants from two distinct points of view. On the one hand, there is interest among scientists concerned with crop protection and disease control in mechanisms by which crop pests, vectors of disease, plant pathogens, and weeds develop resistance to pesticides. Understanding the mechanism should point to ways of overcoming resistance, for example, other compounds not affected by resistance mechanisms or synergists to inhibit enzymes that provide a resistance mechanism. On the other hand, the development of resistance can be a useful indication of the environmental impact of pollutants. 

Cultivation of soil has and will continue to be an important means of controlling weeds (1). However, extensive soil cultivation leads to various problems such as losses of soil, soil moisture and nutrients. This results in water pollution by both the soil itself and pesticides and nutrients associated with it (2, 3, 4). Minimum or no-till cropping systems can reduce these problems because various crop residues (i.e., mulch) are left on the soil surface with a minimum of soil disturbance in planting the crop. 

---