Phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), and peroxidase (POD) are considered important biochemical markers in host plant resistance against pest insects. Constitutive activity of these enzymes was analyzed in resistant and susceptible wheat cultivars against cereal aphid Sitobion avenae (F.) at various developmental stages, i.e., tillering, stem elongation, flag leaf, and ear. Following aphid infestation, the activity of these enzymes was determined at the flag leaf and ear stages. Resistant cultivars exhibited greater constitutive PAL activity than susceptible ones at the tillering, stem elongation, and flag leaf stages. Aphid infestation enhanced levels of PAL activity in the flag leaf and ear stages in both resistant and susceptible cultivars. Constitutive PPO activity was higher in the resistant cultivars at all developmental stages. Aphid infestation induced increases in PPO activity in the flag leaf and ear stages of one susceptible cultivar, whereas induction in resistant cultivars was weaker. Resistant cultivars showed greater constitutive POD activity in the tillering, stem elongation, and flag leaf stages, while aphid infestation induced POD activity in all cultivars, especially in susceptible ones. The potential role of PAL, PPO, and POD in wheat defense against aphid infestation is discussed.

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. 2009. Use of sonication to determine the size distributions of soil particles and organic matter. Can. J. Soil Sci. 89: 413Á419. Applying ultrasound energy to soil-water suspensions (sonication) is an established method of determining the size distributions of soil primary mineral particles and associated organic matter. The size distributions may vary, however, with sonication input energy and soil type. The objective of this study was to determine the effects of sonication input energy on the size distributions of soil mineral particles and organic matter for a range of soil textures and carbon contents typical of agricultural soils in southwestern Ontario. The soils included a Brookston clay loam, a Brookston clay, a Huron silt loam, a Perth silt loam and a Harrow sandy loam. All soils were under no-tillage management. Nine sonication energies ranging from 50 to 1500 J mL(1 were applied to soil-water suspensions (1:4 mass ratio), and the soil particle size distribution results were compared with those obtained using the standard chemical dispersion (pipette) method. The three medium-and coarse-textured soils (Huron, Perth, Harrow) required about 250 J mL(1 for complete dissociation of soil aggregates, while the two fine-textured soils (Brookston) required sonication energies of 600Á750 J mL (1 . Increasing sonication energy increased the amount of soil organic carbon (SOC) measured in the clay-size fraction and decreased the amounts in the sand and silt fractions. Therefore, accurate determinations of particle size distribution and SOC contents require an initial assessment of the amount of sonication energy required for the complete dispersion of the particle size fractions. For the Brookston clay loam and Brookston clay soils, 40Á52% less particulate SOC was found in the sand fraction at 750 J mL(1 sonication energy than that obtained using the standard pipette method, indicating particle size reduction by sonication of particle organic matter. It should be noted that the sand-size SOC typically represents a small fraction. Furthermore, sonication had a minor effect on the SOC content of the clay fraction. It was concluded that sonication is a viable technique for determining the size distribution of soil primary mineral particles, as well as the amount of SOC associated with the silt and clay fractions.Key words: Sonication, ultrasound energy, particle size distribution, organic carbon fractionation, clay soil Yang, X. M., Drury, C. F., Reynolds, W. D. et MacTavish, D. C. 2009. Recours a`la sonication pour de´terminer la granulome´trie des particules du sol et la quantite´de matie`re organique. Can. J. Soil Sci. 89: 413Á419. L'e´tude des suspensions sol-eau aux ultrasons (sonication) est une me´thode bien e´tablie pour de´terminer la re´partition granulome´trique des principales particules mine´rales du sol et de la matie`re organique qui y est associe´e. Cette re´partition peut ne´anmoins varier avec la puissance des ultrasons et la nature du sol. L'e´tude devait pre´ciser quels effets la puissance des ...

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