Precipitation and the transfer of water, nutrients and pollutants in tree canopies.

The functioning of a forest community depends upon the routes, rates and transformations of material flowing through it. The canopy's interaction with precipitation can result in the interception of rainfall and the scavenging of water from clouds and fog. But this movement of water through the canopy also mediates the transfer of mineral nutrients and the deposition of some atmospheric pollutants. The past decade has seen the development of increasingly complex models of transfer processes in the canopy and new methodologies for studying them. One impulse for these advances has been the need to understand not only the movement of water, nutrients and pollutants but also their reciprocal effects. Another has been the need to evaluate the nature of pollutant-induced effects with respect to the costs and effectiveness of possible remedies.

Effects of airborne saline particles on vegetation in relation to variables of exposure and other factors.

Saline particles are a heterogeneous group of chloride(Cl)-containing airborne materials of natural as well as anthropogenic origins. They are usually a local problem of air pollution in terms of source and dispersion, but within these areas their effects on agricultural, ornamental or natural species of plants can be of substantial practical concern. These effects include the accumulation of Cl, the production of foliar lesions, and changes in the plant's levels of mineral nutrients and metabolites, physiological processes, and growth and reproduction. Some quantitative exposure-effect relationships have been formulated for foliar Cl, foliar lesions, and changes in growth and yield. These relationships are sensitive to various factors, such as flux, duration and frequency of exposure, species and stage of development of the plant, size and chemical composition of the particle, and light, temperature, relative humidity and precipitation during or after exposure. The interactions of these factors affect the response of the plant to saline particles by determining three major sets of processes: collection and retention of particles by the foliage; penetration of material from superficial deposits into the foliar tissue; translocation of absorbed Cl (or other ionic components) and susceptibility of tissue to it within the leaf.

Responses of plants to simulated saline drift as affected by species and conditions of exposure.

In exposures to simulated saline drift generated from a 0.6% (w/w) chloride (Cl)-solution, under controlled environmental conditions, the median effective doses for the occurrence of any salt-induced foliar injury (expressed as microgCl cm(-2) deposited in six hours) were: 2.9 for Canada hemlock (Tsuga canadensis [L.] Carr.); 10.3 for white flowering dogwood (Cornus florida L.); 43.5 for potato (Solanum tubersum L. cv Superior); 44.2 for northern red oak (Quercus rubra L.); 65.1 for sweet corn (Zea mays L. cv Golden Cross Bantam); and, 123 for bush bean (Phaseolus vulgaris L. cv Pinto). Response of bean was a function of total deposition and independent of its rate with multiple (one per day) 6-h exposures but not with exposures less than 6 h where toxicity (per mass of Cl) increased with an increase in the rate of deposition. Toxicity of particles increased with an increase in the concentration of Cl in the solution (1.6 or 5.0% w/w) from which they were generated with bush bean but not with hemlock. Post-exposure periods that cycled between 50 and 855% relative humidity (RH) produced a greater incidence of salt-induced foliar injury than did a regime of constant 85% RH.

Acid-soluble nucleotides of pinto bean leaves at different stages of development.

Acid-soluble nucleotides of unifoliate leaves of Pinto bean plants (Phaseolus vulgaris L.) were determined at young, mature, and senescent stages of development. At least 25 components could be distinguished on the basis of inorganic phosphorus determinations and 37 or more fractions on the basis of (32)P labeling, with adenosine di- and triphosphates accounting for 60% of the total moles of nucleotide. The total nucleotide P and inorganic P, on a fresh weight basis, decreased about 44% between each stage of leaf development, but decrements in the levels of individual nucleotides varied from this over-all pattern.Minor changes in the relative abundance of the individual nucleotides accompanied aging although the percentage of purine-containing nucleotides decreased with age. Total (32)P activity per leaf in the nucleotide pool increased about 3-fold between the young and mature leaves and decreased slightly as leaves became senescent. In general, the specific activities of the nucleotides increased with increased age and adenosine-, guanosine-, uridine-, and cytidine triphosphates and adenosine diphosphate accounted for approximately 90% of the total activity. The changes in the relative sizes and energy status of the nucleotide pools were not so obvious as the changes in other metabolites that have been reported to accompany aging in leaf tissue.