The germination of race 1 spores of Fusarium oxysporum f. sp. ciceri was significantly inhibited by the root exudate of the wilt‐resistant chickpea cvs CPS1 and WR315 compared to untreated spores and spores treated with root exudates from susceptible cultivars. The effect was concentration dependent, such that the exudate from 1 g of root in 2 ml of water almost completely inhibited spore germination, whereas the exudate from 1 g of root in 20 ml water did not do so. The inhibitory effects of the active exudates were negated when the apolar components of the exudates were removed by extraction with ethyl acetate. The root exudates of the susceptible cv. JG62 and the late wilting cv. H208 did not inhibit germination. The hyphal growth of germinated spores was also strongly inhibited by the concentrated exudates of CPS1 and WR315, and diluted exudates were less potent. The highest concentration of the exudate of the susceptible cv. JG62 showed some inhibition of hyphal growth, whereas none of the exudates of H208 were found to contain any antifungal activity. The effect of the exudates on the spores of race 2 was similar to that reported for race 1, except that the water‐soluble components of the crude root exudate of WR315 after ethyl acetate extraction were also found to inhibit germination significantly. Overall, the spores of race 2 appeared to be more susceptible to the effects of the exudates. The ethyl acetate fractions of the root exudates of CPS1 and WR315 strongly inhibited germination and hyphal growth of both race 1 and race 2, the effect being concentration dependent. The results suggest that the resistance of chickpeas to vascular wilt depends, at least in part, upon the antifungal activity of the root exudates. Differences in the expression of resistance in the field could depend upon the concentration or rate of production of constitutive antifungal components by the root.

A strong negative relationship was found between the concentrations of procyanidin, a condensed tannin, in the leaf bud petioles of seven genotypes of groundnut (Arachis hypoguea) and fecundity of the aphid Aphis craccivora on the same genotypes. Genotype EC 36892 contained the highest amount of procyanidin per weight of fresh petiole (ca 0.7%) and aphids feeding on this genotype produced significantly fewer offspring than aphids reared on genotypes with low procyanidin levels. It is proposed that testing for high procyanidin concentrations may provide plant breeders with a quick and relatively simple method to screen new groundnut genotypes for resistance against Aphis craccivoru.

No abstract

Increased activity of the brown planthopper, Nilaparvata lugens (Stål) on rice variety IR46 over that observed on varieties IR22 and IR62 was shown to be due to the chemical composition of the surface wax. Reduced settling and probing of the plant surface after exploration, and a tendency to move off the stem onto the leaves as a result of chemical cues from the wax make this a potentially important resistance mechanism in rice. Bioassays have shown that the effect is caused by the hydrocarbon‐and carbonyl‐containing fractions of the wax of IR46. An investigation of N. lugens behaviour on initial contact with the surface of several other varieties of rice has indicated a similar effect for IR36, and a much stronger effect with the wild rice WR221. RÉSUMÉ Importance des structures superficielles du riz sur sa résistance à Nilaparvata lugens Stål La composition de la cire superficielle s'est révélée la cause de l'accroissement de l'activité de TV. lugens sur la variété de rix IR46, par rapport aux variétés IR22 et IR62. Un mécanisme important de la résistance potentielle du riz est dû aux conséquences des caractéristiques chimiques de la cire: établissement et sondage de la plante réduits après exploration, et tendance à quitter la tige pour gagner les feuilles. Des expériences ont montré que ces effets étaient provoqués par les fractions de la cire de IR46 contenant des hydrocarbures et des carbonyls. L'étude du comportement de N. lugens, lors du premier contact avec la surface de plusieurs autres variétés, a montré un effet semblable pour IR36 et un effet beaucoup plus fort avec le riz sauvage WR221.

ABSTRACT. The eggs of Schistocerca gregaria (Orthoptera, Acrididae) incubated under natural conditions hatch only within a few of hours on either side of dawn. This gated hatching is controlled by a circadian clock that is phase set by the diel fluctuations in the temperature of the soil surrounding the eggs. There is a circadian fluctuation in haemolymph sugar concentration which is initiated at least 4 days before hatching. However, eggs hatch arrhythmically unless given cycled temperature incubation for at least 10 days of a 12–13‐day incubation. Increase in acetylcholine esterase content of the brain during the penultimate day suggests that increased hatching rhythmicity occurring at this time is the result of increased neural organization. Embryonic activity and respiration show no circadian rhythm but do provide confirmatory evidence of a quiescent phase prior to hatching. This quiescent phase is an integral part of gated hatching behaviour.