No abstract

The branch bag method was used to monitor photosynthesis and transpiration of trembling aspen (Populus tremuloides Michx.) and hazelnut (Corylus cornuta Marsh.) over a 42-day midsummer period in 1996, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). During the same period, daytime measurements of stomatal conductance (g(s)) and leaf water potential (Psi(leaf)) were made on these species, and sap flow was monitored in aspen stems by the heat pulse method. Weather conditions during the study period were similar to the long-term average. Despite moist soils, both species showed an inverse relationship between daytime g(s) and vapor pressure deficit (D) when D was > 0.5 kPa. Daytime Psi(leaf) was below -2 MPa in aspen and near -1.5 MPa in hazelnut, except on rainy days. These results are consistent with the hypothesis that stomatal responses are constrained by hydraulic resistance from root to leaf, and by the need to maintain Psi(leaf) above a minimum threshold value. Reductions in g(s) on sunny afternoons with elevated ambient D (maximum 2.3 kPa) were associated with a significant decrease in photosynthetic rates. However, day-to-day variation in mean carbon assimilation rate was small in both species, and appeared to be governed more by solar radiation than D. These results may be generally applicable to healthy aspen stands under normal midsummer conditions in the southern boreal forest. However, strong reductions in carbon uptake may be expected at the more extreme values of D (> 4 kPa) that occur during periods of regional drought, even if soil water is not locally limiting.

Antifeedant and growth inhibitory effects of toosendanin, a limonoid allelochemical from the bark of the trees Melia toosendan and M . azedurach, were determined for the variegated cutworm, Peridroma saucia, using different bioassays. Toosendanin significantly deterred feeding of 2nd and 4th instar larvae in diet choice and leaf disc choice bioassays, respectively. Refined bark extracts containing 60-75% toosendanin were less deterrent than the pure compound in both bioassays. However, the 60% extract significantly inhibited growth of neonate larvae in a dose-dependent manner when incorporated into an artificial diet, with an ECso (concentration reducing growth by 50% relative to controls) of 42.3 ppm at 7 days. Extracts containing 60% and 75% toosendanin were more inhibitory than equivalent amounts of pure toosendanin in this chronic growth bioassay, suggesting the presence of other minor constituents which must be significantly more inhibitory, though less deterrent to feeding, than toosendanin itself. Nutritional analyses of 4th instar larvae following both oral and topical administration of toosendanin or the 75% extract confirm that both materials possess a combination of antifeedant and growth inhibitory properties.

No abstract