Combinations of feeding deterrents are additive in their effects on the feeding behaviour of Locusta migratoria. The effect has been shown within and across different chemical classes. A mixture of fourteen different phenolic compounds from Sorghum bicolor tested in naturally occurring concentrations is deterrent although the individual levels of deterrence are not measurable. The significance of the findings in relation to plant resistance is discussed. RÉSUMÉ EFFET DE DIFFÉRENTES COMBINAISONS DE SUBSTANCES PROVOQUANT L'INAPPÉTENCE SUR LE COMPORTEMENT ALIMENTAIRE DE LOCUSTA MIGRATORIA Dix substances provoquant l'inappétence, appartenant à sept classes chimiques différentes, ont été examinées isolément et en diverses combinaisons quant à leur effet sur Locusta migratoria. L'inappétence obtenue avec différentes combinaisons a indiqué que les effets des substances chimiques individuelles s'additionnaient. Un groupe de quatorze composés phénoliques dérivés des feuilles de sorgho a été éprouvé à des concentrations existant dans la nature. Bien que ces composés n'aient pas causé individuellement une inappétence mesurable, le mélange de tous les composés, aux měmes concentrations, a réduit considérablement l'ingestion d'aliments.

The present study describes the effects of simulated acid rain (pH 2.5–5.6) on foliar histology of an arctic herb, Artemisia tilesii Ledeb., which is remarkably tolerant to naturally occurring atmospheric acidity at Smoking Hills, N.W.T. Plants were exposed to simulated acid rain twice weekly for 4 weeks in exposure chambers in the greenhouse. Droplets as acidic as pH 2.5 caused limited macroscopic foliar damage. However, much greater damage was observed when sectioned leaf tissue was examined microscopically. On leaves having no injury visible to the unaided eye, small lesions consisting of one to three collapsed epidermal cells were observed in scanning electron micrographs and in cleared leaves after exposure to rain of pH 3.0 and 3.5. Stomata remained open in damaged areas of acid-sprayed leaves. Lesions most commonly developed from an initial collapse of a few adaxial epidermal cells, followed by progressive injury of underlying tissues. Palisade and spongy mesophyll cells underwent hypertrophic (abnormal cell enlargement) and hyperplastic (abnormal cell division) responses in the region adjacent to severely collapsed tissue, causing reduced intercellular spaces. These effects isolated the injured areas from adjacent healthy tissues, and resembled wound periderm responses to fungal pathogens and to mechanical irritation. This response may be one mechanism of limiting acid rain damage.

Summary In order to compare the ability of leaf surfaces of different species to neutralize acidic rain, acidic droplets of identical size were placed on leaves of Artemisia tilesii, Spinacea oleracea and Phaseolus vulgaris. The pH values over time were measured using a micro‐pH electrode. The pH of rain droplets initially of pH 3.0, 3.5 and 5.6 increased after contact with leaves of these species. However, for droplets of pH 2.5 the pH of droplets actually decreased with time on leaves as the drops evaporated, despite the very large quantity of hydrogen ions neutralized by the leaf surface. Neutralization of acidic droplets was greatest in A. tilesii plants, which also showed the lowest sensitivity to foliar injury from simulated acid rain, Spinacea oleracea leaves were the least able to neutralize acid droplets and also were most sensitive to foliar damage. In each species the more acidic the rain, the greater the quantity of hydrogen ions neutralized by the leaf surface. This ability to neutralize acidic droplets on the leaf surface may be an effective means of reducing damage by acid rain, especially since rain events with a pH < 3.0 are very rare. Neutralization of acidic droplets may be brought about when cations are leached from exchangeable pools or by dissolution of salts deposited on the leaf surface through guttation. Large increases in Ca especially, but also in Mg and K in droplets collected from leaves of A. tilesii, suggest that these cations may play an important role in increasing the pH of acidic droplets.

This study examines the condition of fine-root systems of healthy and declining sugar maples (Acersaccharum Marsh.) at two sites in Central Ontario, one moderately declining and the other severely declining. Roots are frequently sensitive indicators of soil nutrition, thus ingrowth cores (soil transplants) were used to assess the effect of bulk surface (F-layered Ah horizon) soils collected from beneath healthy or declining trees on fine-root chemistry and growth. Soil at each site was collected from healthy and declining trees and reciprocally transplanted (buried) in mesh bags, and roots were allowed to grow into the soil cores for 8 weeks. In addition to transplants of untreated soils, additional transplants of "healthy" and "decline" soils treated by either (i) steam sterilization to remove pathogens or (ii) fertilization with superphosphate were made. Root dry weight of healthy and declining trees was reduced 25–70% in untreated decline soils compared with healthy soils (p ≤ 0.007). This was consistent with lower Ca, Mg, Mn, and Fe in the soil solution of declining trees at the severely declining site and a lower Mn concentration and Ca/Al and Mg/Al ratios in the soil solution at the moderately declining site. Compared with roots of healthy trees, roots of declining trees had significantly lower Ca concentrations for both stands, and in addition, significantly lower concentrations of P, Mg, S, K, Mn, Al, Fe, and Zn in the more severely declining stand. The experimental soil transplants established that lower nutrient concentrations in soils from declining trees are reflected in the chemistry of roots growing in them. On the whole, the fertilizer treatment significantly increased root growth and the steam sterilization treatment had no effect on root growth. While this study shows that reduced nutrient availability in the rooting substrate is an important factor in decline, it is not known how recently this nutrient depletion has occurred, nor what is the cause. Although other mechanisms may be involved, reduced nutrient availability in the rooting substrate is consistent with the speculation that deposition of acidic salt solutions of sulphate and nitrate to these forest soils has caused accelerated soil base cation leaching losses that are reflected in nutritional deficiencies and growth decline.

A study was made of the ability of leaf surfaces of sunflower, radish, cabbage and sugar beet to neutralize acid rains (pH 3-0) of varying duration. Sprayings with simulated rain of 5, 15, 45 and 90 min duration were made weekly. The species were selected for their contrasting leaf surfaces (e.g. wettability, droplet retention, hairiness and thickness of epicuticular waxes). Following acidic sprays, the pH of raindrops on leaves was measured at intervals until drops dried. Leaves of cabbage and sugar beet, which had smaller areas of contact with raindrops than those of radish and sunflower, were markedly less able to neutralize acidic droplets. Droplets on the cotyledons of all species had higher pHs than droplets on the true leaves. DiflFerences in age of true leaves, however, did not significantly affect droplet pH. The duration of rain spraying had a significant effect on leaf surface neutralization. In 2-and 3-week-old plants, long exposures to rain (i.e. 45 and 90 min) markedly increased neutralization in comparison with short exposures. In 6-week-old plants, the effect was the reverse; sprays of 45 and 90 min significantly decreased foliar neutralization compared with sprays of 5 and 15 min. Significant decreases in both leaf and root dry weights were found for cabbage and sugar beet treated with rain of pH 3 0 for 45 and 90 min, compared with both unsprayed plants and those treated with the same pH for shorter periods. Radish showed a similar but non-significant trend.Chemical analysis of raindrops (initially pH 3-0) left on foliage for 75 min showed an increase in the concentrations of Ca^"^, Mg^"*^ and K^, sometimes up to 50-fold. The elemental composition of drying droplets collected on 4-week-old plants was not affected by the duration of previous rain exposures. In contrast, the Ca^"^ and Mg^"*" concentrations of drying droplets on 7-week-old plants exposed to 45 and 90 min rains were significantly lower than on those exposed to 5 and 15 min rains. Drying droplets differed sharply in their cation concentrations in different species. There was, however, a poor correlation between the degree of droplet neutralization and its acquired cation status. Results from a subsequent experiment using cabbage, radish, broccoli, brussel sprout and tomato, in which droplet pHs were measured over acid rain-induced lesions and over uninjured leaf tissue, showed significantly higher pHs in the droplets over the foliar lesions.