Heat waves – extended periods of abnormally hot weather – are predicted to increase in severity and frequency under climate change. The severity of heat waves should impact communities and food webs through effects on performance of individual species and through changes in the strength of interactions between them. This study tested the effects of severity of simulated heat waves, with daily maxima of either 32°C or 40°C, on a tritrophic food web consisting of plants, Capsicum anuum, aphids, Myzus persicae and two parasitoids, Aphidius matricariae and Aphelinus abdominalis. Osmolarity of plant sap (concentration of dissolved solids) was highest under 40°C heat waves, suggesting the presence of secondary plant compounds involved with stress responses. Population growth of aphids was lower under heat waves (both 32°C and 40°C daily maxima), compared to environments with periodic hot days. Development time of parasitoids was longer under heat waves. Heat waves decreased the proportion of winged aphids in the population. When both parasitoid species were present, impacts on aphid populations were greater in heat wave environments than environments with periodic hot days. When either parasitoid species was by itself, heat waves did not affect the interaction between parasitoids and aphids. Numbers of A. matricariae were reduced in heat wave environments, whereas numbers of A. abdominalis were not. In addition to direct effects on individual species, we also obtained indirect evidence for the effects of heat waves on the bottom–up effects of plant stress compounds on herbivore performance, and on the strength of inter and intra‐specific competition. Our results demonstrate that heat waves could have important effects on community structure, and on important, community‐level processes such as intra‐guild interactions and trophic cascades.

No abstract

Over 250 durum wheats, Triticum durum Desf., and close relatives were screened for resistance to the wheat midge, Sitodiplosis mosellana (Géhin). Less than 2% showed consistently reduced egg and larval densities in field and laboratory trials. Two related accessions named ’Kahla’ had stable, low larval infestation levels, which were about 30% of those in commercial durum wheat cultivars. No antibiosis was detected in durum wheats. The reduction in infestation by larvae was associated with a similar reduction in oviposition. The resistance reduced egg densities by 80% or more in choice and no-choice laboratory tests and by about 70% in the field. Egg group size was not affected, and therefore reduced oviposition must have resulted from the number of egg groups deposited by females. The resistance was not associated with any of 12 morphological traits that were quantified for spikes. ’Kahla’ is the first durum wheat identified to have resistance against the wheat midge.

. 2000. Test-weight and weathering of spring wheat. Can. J. Plant Sci. 80: 677-685. Wet weather often delays harvest and results in a grade reduction of wheat because of a decrease in test-weight, an important grading factor in Canada. The objectives of this study were to assess the effect of delayed harvest on test-weight loss of 14 Canadian wheat cultivars representing three different classes, and to develop a screening strategy for retention of test-weight for breeding programs. Non-weathered test-weight (NWTWt), weathered test-weight (WTWt), and test-weight loss (TWtLoss; i.e. NWTWt -WTWt) in the field, averaged over five field locations and 2 yr were similar within the CPS, CWAD and CWRS wheat classes, although there were genotypic differences for all three variables. Because test-weight requirements for the top grades are higher in the CWAD class than in other classes, durum cultivars would be more susceptible to downgrading during wet harvests. Historical data from the Durum Wheat Co-operative Test also suggests that, since 1950, the mean test-weight of the genetic lines has decreased by 3.7 kg hL -1 , and is now close to the minimum for grade #1 in the CWAD class. Most of the decrease in test-weight observed over several weeks in the field could be simulated by a single 5 -10 min soaking of non-weathered seed in the laboratory. Linear regression analyses of both field and laboratory samples indicated that 90% of the genotypic variation in weathered test-weight could be attributed to differences in the NWTWt. These results suggest that the screening strategy for retaining test-weight should focus on selection for increased NWTWt. This is much simpler than screening for high WTWt or low TWtLoss, which requires soaking of the seed or field weathering. Le temps pluvieux retarde souvent la moisson, occasionnant de ce fait une baisse de la valeur commerciale du blé, par suite de la diminution du poids à l'hectolitre, important caractère de classement au Canada. L'objet de nos travaux était d'évaluer l'effet d'un retard de la moisson sur la perte de poids spécifique chez 14 cultivars canadiens de blé représentant 3 classes de blé et d'élaborer une stratégie de préservation du poids spécifique dans les programmes de sélection. Sur une moyenne de 5 emplacements au champ et de 2 années, le poids spécifique sans altération sur pied (PSSA), le PS avec altération sur pied (PSAA) et la perte de poids spécifique (PPS, soit PSSA -PSAA), étaient semblables dans chacune des classes de blé : PPC, DAOC et RPOC, bien que des différences génotypiques apparaissaient pour toutes les 3 variables. Comme les exigences en matière de PS pour les grafes de supérieurs de blé sont plus élevées dans la classe DAOC que dans les autres classes, les risques de déclassement lors des moissons pluvieuses seraient plus forts pour les cultivars de blé dur. Les archives des essais coopératifs de blé dur révèlent que depuis 1950, le poids spécifique moyen des lignées généalogiques a diminué de 3,7 kg hL -1 , se rapprochant actuellement du minimum...

. Kernel colour changes associated with field weathering of spring wheat. Can. J. Plant Sci. 86: 371-377. The Canadian wheat-grading system is based on visual assessment to estimate end-use quality, both for assignment to classes and to grades within classes. Extensive training and experience are required to accurately interpret visual indicators of quality. Therefore, breeding programs are attempting to develop instrumental techniques to objectively measure visual characteristics of potential new cultivars. The objective of this research was to examine colour changes associated with field weathering of wheat cultivars from the Canada Western Red Spring, Canada Western Amber Durum, and Canada Prairie Spring Red and White classes of wheat, as measured with a portable colorimeter. The class means exhibited colour differences, although the large variability within classes suggests that, used in isolation, colour measurements would not be a satisfactory means of identifying the class of individual samples. Weathering caused an increase in L* (lightness), a decrease in a* (redness), and a slight increase in b* (yellowness), and the effects were similar for all classes. Severe weathering caused loss of vitreousness in addition to bleaching, and the colour measurements reflect the integrated colour changes associated with both effects. Within each class, although there were cultivar differences for colour, the cultivars responded similarly to weathering, suggesting that breeders should simply select lines which exhibit intense, desirable colour prior to weathering. Les obtenteurs s'efforcent donc de mettre au point des techniques instrumentales qui les aideront à quantifier objectivement les paramètres visuels des nouveaux cultivars éventuels. La présente étude avait pour but d'établir les variations de couleur associées à l'altération sur pied de variétés appartenant aux différentes classes de blé (blé roux de printemps de l'Ouest canadien, blé dur ambré de l'Ouest canadien, blé roux de printemps des Prairies et blé blanc) au moyen d'un colorimètre portatif. La couleur moyenne des différentes classes varie effectivement, mais la forte variabilité au sein de chaque classe laisse supposer qu'employés séparément, les relevés colorimétriques ne permettraient pas d'identifier de manière satisfaisante la classe des échantillons. L'altération sur pied entraîne une hausse de L* (luminosité), une baisse de a* (rouge) et une légère hausse de b* (jaune) dans toutes les classes. Une grave altération pourrait entraîner la perte de la vitrosité et le blanchiment du grain, de sorte que les valeurs reflèteraient le changement de couleur général associé à ces deux conséquences. Bien qu'ils présentent des différences de couleur, les cultivars de chaque classe réagissent de la même façon à l'altération sur pied, signe que les obtenteurs devraient se contenter de sélectionner les lignées ayant la couleur souhaitée la plus vive avant l'altération sur pied.