P. J. Edwards

Wound induced defences in plants and their consequences for patterns of insect grazing

SummaryThree scales of wound-induced chemical responses in plants are identified: (1) highly localised chemical changes associated with disruption of cell compartmentation; (2) changes induced in cells surrounding the damaged area, forming a kind of halo around the damage, and (3) more widely-dispersed changes which may affect an entire organ, branch or plant. A brief review of the literature reveals that such chemical responses are very widespread in plants, and many of the substances formed are known to affect adversely the growth, development, or reproduction of insects. It is argued that wound-induced changes in plant chemistry represent for insects a powerful selective pressure for the dispersal of grazing. Levels and patterns of invertebrate grazing in a range of herbaceous and deciduous woody plants sampled at the end of the growing seasons were examined. Leaves of many species exhibited a strikingly evident over-dispersion of grazing initiations, and in some cases the arrangement of holes appeared close to regularity. The pattern of damage between leaves was, in most cases, heavily biased towards a large proportion of leaves receiving a low level of grazing. These highly dispersed patterns of grazing damage are consistent with the hypothesis that wound-induced responses play an important role in determining patterns of insect feeding. They have important implications for the expected levels of insect exploitation of host plants and for the advantages to the plant of distributing grazing damage evenly through the canopy.

Wound-induced changes in the palatability of Betula pubescens and B. pendula

SummaryLeaves of Betula were damaged artificially in April, June and August 1982. Palatability of damaged and adjacent undamaged leaves was assessed against controls in bioassays using the polyphagous Lepidoptera Spodoptera littoralis and Orgyia antiqua. Assessments were carried out at intervals from six hours to five months following each damage date. Palatability (relative proportions of leaves consumed) was significantly lower than controls in damaged and adjacent leaves after six hours and remained detectable for up to two months but this was less clear in the June-damaged samples and undetectable in the August group. Adjacent leaves were significantly affected whether distal or proximal to the damaged leaves. Marked and significant changes occurred in levels of soluble tannins in the damaged and adjacent leaves but the relationship between crude tannin levels and changed palatability was not simple cause and effect. No significant effects of damage on aphid (Euceraphis punctipennis) reproduction could be detected among birches in a growth room experiment in which half the trees were artificially damaged and half were controls. The results are discussed in the light of earlier work on induced defence in birch and five areas of significant new information represented by the results in this paper are identified.

Induced plant defences against insect grazing: fact or artefact?

Recent evidence for the ecological effects of damage-induced changes in leaves is reviewed and a framework proposed for the types of evidence needed to separate coincidental effects of wound repair from an evolved defence response. The ecological evidence presented is: the overdispersion of grazing damage within and between leaves at the end of the season; the induction of unpalatability and chemical changes following damage; negative associations between insect grazers on plants; grazer foraging behaviour. The framework of types of evidence which help to distinguish wound-repair processes from defence is: the induced chemical substances are not always directly involved in repair; chemical changes may be induced in tissues remote from the site of damage; the speed and magnitude of the changes are sufficient to affect grazer behaviour or fitness; plant species exhibiting induced changes show other general ecological similarities.

Wound-induced changes in the acceptability of tomato to larvae of Spodoptera littoralis: a laboratory bioassay

Abstract. 1. Leaves of tomato (cv. Moneymaker) were artificially damaged and offered to Spodoptera larvae at a range of intervals following damage. Grazing levels on these leaves were compared with those on undamaged leaves on the same or different plants.

The ecological significance of rapid wound-induced changes in plants: insect grazing and plant competition

SummaryThis paper investigates the hypothesis that a rapidly induced phytochemical response to grazing damage, such as that seen in tomato, serves to deflect insect herbivores away from leaves soon after damaging them (the grazing dispersal hypothesis). As a result, grazing damage is more dispersed than it otherwise would be, and young leaves, which may be of particular importance to a plant in competition for light, are not damaged excessively. In the first experiment, artificial removal of c. 15% of leaf area led to a significant reduction in plant performance compared with undamaged controls, but only when the plants were grown together in competition for light. The second experiment demonstrated that the distribution of grazing damage within the plant was an important factor in the outcome of competition; in those plants in which grazing was applied to the lower leaves there was no effect of damage upon performance compared with undamaged controls, whereas grazing to the upper leaves significantly reduced plant performance. A third experiment provided some insight into how this interaction between damage and competition comes about. It was shown that damage to leaves led to a rapid drop in the rate of extension growth of the main shoot, especially when the upper leaves were damaged, and normal rates of growth were not resumed for at least 3 days. It is argued that in a rapidly growing canopy, such an effect may mean that a damaged plant loses its position in the height hierarchy. The final experiment showed that previous damage to plants can affect the distribution of subsequent grazing by larvae of Spodoptera littoralis, apparently through a wound-induced reduction in leaf palatability. Plants which had been artificially damaged 48 h previously were grazed significantly less than controls, and the avoidance effect was greatest in the young leaves. These results are consistent with the grazing dispersal hypothesis, and suggest that rapid wound-induced responses may be of greatest significance in species characteristic of fertile environments where competition for light is particularly intense.

CHANGES IN DORMANCY LEVEL IN SORGHUM HALEPENSE SEEDS INDUCED BY WATER STRESS DURING SEED DEVELOPMENT

Dormancy in Sorghum halepense seeds as affected by intermittent water stress imposed on the mother plant during seed development was investigated. The drought treatment was imposed in cycles within the maturation period by with-holding water for 5 days, rewatering at the end of each drought cycle and withholding water again. The results showed that two sources of dormancy exist in S. halepense seeds: one is inherent in the caryopsis itself and is not affected by water stress during seed development (...)

Wound-induced changes in the acceptability of tree-foliage to Lepidoptera: within-leaf effects

Leaves of alder, hawthorn and birch were artificially damaged with a single hole and offered to larvae of Spodoptera littoralis Boisd. (Noctuidae) at a range of intervals following damage. Grazing levels were compared between the damaged and undamaged sides of each leaf at two or three different spatial scales. Bioassays conducted in May revealed that within 1 d, acceptability of the damaged side of alder and birch leaves was significantly reduced compared with the undamaged side. (The most extreme reductions in acceptability were apparent in the immediate vicinity of the damage in both species). These effects persisted for at least seven days in birch and at least 14 d in alder. Within 6 h, grazing levels on the damaged side of hawthorn leaves were significantly lower than on the undamaged side. These effects also persisted for at least 14 d. Bioassays conducted in August revealed significant reductions in grazing on the damaged side of alder leaves 14 d after damage and of hawthorn leaves after two days. Possible explanations for reduced acceptability surrounding a hole are discussed with particular reference to chemical defences. The ecological significance of these changes are discussed in relation to insect behaviour and patterns of natural grazing.

Hydroxamic acid glucosides in honeydew of aphids feeding on wheat

DIMBOA glucoside (2-O-/gb-D-glucopyranosyl-4-hydroxy-7-meth-oxy-1,4-benzoxazin-3-one), the main hydroxamic acid (Hx) in intact wheat plants, was detected in the honey dew ofRhopalosiphum padi feeding on seedlings of six wheat cultivars that differed in their concentration of Hx, suggesting that the chemical circulates in the phloem. Neither the aglucone (DIMBOA) nor its main breakdown product were found in any of the honeydew samples. Honey dew production by aphids caged on seedlings of the wheat cultivars and DIMBOA glucoside concentrations in the honeydew followed biphasic curves when plotted against Hx concentration, suggesting passive ingestion of the chemical from the phloem at low Hx concentrations and limited ingestion due to feeding deterrency by Hx in mesophyll cells at high Hx concentrations. The presence of plant toxins such as Hx glucosides in the phloem sap, the main ingesta of aphids, and in the mesophyll cells, has major implications for plant defense, through a feeding deterrent effect during stylet penetration, and deterrency (antixenosis) along with antibiosis during feeding.

Wound-induced changes in tomato leaves and their effects on the feeding patterns of larval lepidoptera

Several studies have shown changes in the patterns of damage from feeding insects associated with changes in palatability and overall consumption as a result of wound-induced chemical changes in plants. This paper describes how the pattern of feeding damage made by the larvae of Spodoptera littoralis Boisd. (Lepidoptera: Noctuidae) on tomato is affected by changes in palatability of the leaves. Two sorts of responses to leaves from plants that had received prior damage were observed. Larvae offered a choice of leaves tended to take fewer meals on leaves from previously-wounded plants than on control leaves, frequently rejecting the former after sampling them. On wounded plants this rejection behaviour was associated with a shift in feeding site towards the base of the plant. However, starved larvae offered only a single excised leaf readily ate leaves from wounded plants but took shorter meals on these leaves than on controls. Although it was not directly tested it is possible that this difference in response reflected changes in food selectivity with a differing level of satiation. The results are considered in relation to the adaptive significance of the plant of changes in within-plant distributions of herbivore damage.

Temporal and spatial variation in palatability of soybean and cotton leaves following wounding

SummaryLeaves of soybean (Glycine max (L.) Merr.) and cotton (Gossypium hirsutum L.) were mechanically damaged with a single hole and offered to Spodoptera littoralis Boisd (Lep., Noctuidae) larvae in laboratory bioassays at intervals of between 0 and 7 days from damage. The subsequent within-leaf grazing patterns of damaged and undamaged areas were compared using an image-analysing computer, and estimations were made by eye of percentage, areas grazed at three spatial scales. Reduction in palatability of damaged areas of both plant species was detected, at time intervals ranging from 0 to 7 days after damage. This effect was strongest for the longer time intervals and the effect became weaker with increasing distance from the site of damage. These results are discussed in relation to possible defensive roles of wound-induced changes.