Denitrification and N mineralization from hairy vetch (Vicia villosa Roth) and rye (Secale cereale L.) cover crop monocultures and bicultures

N mineralization, N immobilization and denitrification were determined for vetch, rye and rye-vetch cover crops using large packed soil cores. Plants were grown to maturity from seed in cores. Cores were periodically leached, allowing for quantification of NO₃ ⁻ and NH₄ ⁺ production, and denitrification incubations were conducted before and after cover crop kill. Gas permeable tubing was buried at two depths in cores allowing for quantification of N₂O in the soil profile. Cover crops assimilated most soil N prior to kill. After kill, relative rates of N mineralization were vetch > rye-vetch mixture > fallow > rye. After correcting for N mineralization from fallow cores, net N mineralization was observed in vetch and rye-vetch cores, while net N immobilization was observed in rye cores. Denitrification incubations were conducted 5, 15 and 55 days after kill, with adjustment of cores to 75% water filled pore space (WFPS). The highest denitrification was observed in vetch cores 5 days after kill, when soil NO₃ ⁻ and respiration rates were high. Substantially lower denitrification was observed on subsequent measurement dates and in other treatments probably due to either limited NO₃ ⁻ or organic carbon in the soil. On day 5, 3%, 23%, 31% and 31% of the N₂O was recovered in the headspace of fallow, vetch, rye and rye-vetch cores, respectively. The rest was stored in the soil profile. In a field study using intact soil cores, denitrification rates also peaked 1 week after cover crop kill and decreased significantly thereafter. Results suggest greater potential N losses from vetch than rye or rye-vetch cover crops due to rapid N-mineralization in conjunction with denitrification and potential leaching, prior to significant crop N-assimilation. This revised version was published online in June 2006 with corrections to the Cover Date.     

Evidence for a photoprotective function of low-temperature-induced anthocyanin accumulation in apple and pear peel

The light requirement and low-temperature stimulation of anthocyanin synthesis in peel of apple ( Malus domestica ) and pears ( Pyrus communis ) and the presence of anthocyanins in immature fruits are not congruent with a visual function in dispersal. We hypothesized that anthocyanins afford photoprotection to peel during low-temperature-induced light stress and that the protection is not a fortuitous side-effect of light absorption by anthocyanin. The extent of photoinhibition at harvest and after light stress treatment in pear cultivars differing in redness decreased with increasing red color on the sun-exposed sides of fruits. Green-shaded sides of the pears showed comparable levels of photoinhibition indicating that pears did not differ in their inherent photosensitivity. Apple and pear peel show considerable short-term fluctuation in redness in response to temperature, with red color increasing rapidly in response to low temperature and just as quickly fading in response to high temperature. Briefly, shading pears and apples during cold conditions for 2 days reduced the accumulation of anthocyanin and increased the photosensitivity of peel. Subsequent shading during warm conditions did not affect the accumulation of anthocyanin or the photosensitivity of peel indicating that the response at low temperature was not due to shade adaptation. The assessment of photosystem II (PSII) efficiency and quenching of chlorophyll fluorescence between 16 and 40°C indicated that 'Forelle' pear peel was particularly sensitive to photostress at low temperature. The photosynthetic system in mature 'Forelle' leaves was comparatively much less sensitive to light stress at low temperature. Results support the view that anthocyanins are adaptable light screens deployed to modulate light absorption in sensitive tissues such as fruit peel in response to environmental triggers such as cold front snaps.     

Host selection behavior mediated by differential landing rates of the walnut twig beetle,Pityophthorus juglandis,and associated subcortical insect species,on two western North American walnut species,Juglans californica and J. major

The walnut twig beetle, Pityophthorus juglandis Blackman (Coleoptera: Scolytidae), vectors a phytopathogenic fungus, Geosmithia morbida Kolařík et al. (Hypocreales), which causes thousand cankers disease (TCD) in walnut (Juglans sp.) and wingnut (Pterocarya sp., both Juglandaceae) trees. We investigated an early point in disease inception in two walnut species – Juglans californica S. Wats. and Juglans major (Torr. ex Sitsgr.) Heller – native to riparian forests of the western USA by comparing P. juglandis flight and landing responses to small-diameter branch sections. Twenty unbaited branch sections (10 each of J. californica and J. major) were presented in a completely randomized design to populations of P. juglandis at the USDA Agricultural Research Service, National Clonal Germplasm Repository (NCGR) Juglans collection located at Wolfskill Experimental Orchards (Winters, CA, USA) and at the California State University, Chico, Agricultural Teaching and Research Center (ATRC, Chico, CA). These assays were carried out within a 4- to 6-year period when weekly flight surveys with aggregation pheromone-baited multiple funnel traps revealed that P. juglandis flight activity–abundance was higher at the NCGR than at the ATRC. For the landing rate assays, adhesive-coated acetate sheets were wrapped around the branch sections and exchanged weekly. Three assays were completed at the NCGR (assays 1–3), whereas one assay was completed at the ATRC (assay 4). Landing rates on these traps were compared between J. californica and J. major. Two additional assays (5 and 6) were completed at the NCGR to compare responses to branch sections of J. californica and to similarly sized cardboard tubes (negative control). All six assays were completed over a 4-year span during the 4- to 6-year weekly flight survey period. Pooled landing rates of male and female P. juglandis (assays 1–4) demonstrated a preference by both sexes for J. californica over J. major. In assay 5 there was no preference by males or females for J. californica over the negative control, perhaps due to the low flight activity–abundance of P. juglandis during the assay. When repeated at a time of higher flight activity–abundance (assay 6), male and female landing rates on J. californica exceeded those on the negative control. Females of the invasive fruit-tree pinhole borer (an ambrosia beetle), Xyleborinus saxeseni (Ratzeburg), and an invasive bark beetle, Hypothenemus eruditus Westwood (both Coleoptera: Scolytidae), showed relatively higher flight responses than either sex of P. juglandis during most assays, suggesting higher population densities of these two other invasive species at the two orchards or a greater sensitivity to host volatiles. Xyleborinus saxeseni and H. eruditus preferred to land on J. major over J. californica and on J. californica over the negative control. Similarly, an invasive longhorned beetle, Nathrius brevipennis (Mulsant) (Coleoptera: Cerambycidae), showed a significant preference for J. major over J. californica, but not for J. californica over the negative control. More male N. brevipennis were trapped than females at both study sites [sex ratio ranged from 5:1 (assay 6) to 39:1 (assay 4)], and flight occurred only in the spring and early summer months. Another ambrosia beetle trapped at the NCGR and ATRC, Xyleborus affinis Eichhoff, represented the first records of this species from western North America. In summary, flight responses recorded in some of our assays for P. juglandis and several other subcortical insects on Juglans indicate that host preference by these insects may be determined by long-range olfactory cues that do not involve pheromones.