No abstract

We studied the effects of a 2-day exposure to aluminum (Al), copper (Cu) and zinc (Zn) stress on root exudates and root apices of aseptically grown poplar (Populus tremula L.) cuttings. Aluminum induced root exudation of oxalate and citrate, Cu induced root exudation of oxalate, malate and formate, and Zn induced root exudation of formate. The threshold treatment concentrations were 100 microM for Cu and 500 microM for Al and Zn, corresponding to about 30 microM Cu2+, 140 microM Al3+ and 290 microM Zn2+. Simultaneous with the increase in organic acid anion exudation, sulfate and the nutrient cations K+, Ca2+ and Mg2+ were released into the solutions. Significant positive correlations between the organic acid anions and the cations indicate possible co-release. Toxicity symptoms of the poplar roots included browning of the root apices, which occurred at Cu concentrations of 50 microM and above, at Zn concentrations of 500 microM and above, and at an Al concentration of 1000 microM, and callose formation, which was observed solely in response to Al concentrations of 500 microM or higher. The results indicate that the composition of the exuded organic acid anions from poplar roots and the toxicity symptoms are specific to each of the applied heavy metals.

inducing a higher RLD in the topmost layer under NT (Gregory, 1994;Cannell and Hawes, 1994). There is relatively little information about root growth responseInformation about the impact of tillage on the root under different tillage systems in cool temperate regions. In a 5-yr growth of small-grain cereals is scarce. The root growth field trial at two sites [loamy silt (1995)(1996)(1997)(1998)(1999) and sandy loam soils of winter wheat was similar at later growth stages under (1996-2000)] in the Swiss midlands, the effect of tillage intensity [notillage (NT) and conventional tillage (CT)] on the morphology and NT and CT in temperate regions (Ellis and Barnes, 1980; distribution of winter wheat (Triticum aestivum L.) roots at harvest Dzienia and Wereszczaka, 1999), but the impact of tillwas studied for 3 yr (1997-1999). The root length density (RLD), age on root growth may depend on the length of time mean root diameter (MD), and relative length per diameter-class since the implementation of the NT system (Pearson et distribution of the roots were determined using washed roots from al., 1991). soil cores taken from the row and the midrow. Averaged across allThe spatial distribution of the roots reflects the crop's the other factors, NT resulted in a slightly lower RLD and a slightly potential to take up nutrients and water. The RLD is larger MD compared with CT. However, compared with CT, the RLD often used to describe root distribution. In general, the was higher in the upper soil layer (0 to 5 cm), similar from 5 to 10 cm, RLD is higher in the row than in the midrow (Rubino and lower from 10 to 30 cm in NT. The tillage effect disappeared and Franchi, 1990). Roots are more abundant in the below 30 cm. This tillage-induced difference in root distribution was upper soil layer (Wilhelm et al., 1982; Barraclough et more and more marked from 1997 to 1999. In the row, the MD was greater from 0 to 15 cm, was similar down to 25 cm, and was smaller Dzienia and Wereszczaka, 1999). The impact from 25 to 50 cm in NT compared with CT while below 50 cm, the of tillage on root distribution was evident in the layer MD was hardly affected by tillage intensity. However, MD in the affected by plowing (Gerik et al., 1987; Rasmussen, midrow was usually significantly higher from 0 to 10 cm under NT 1991). The roots in the NT system accumulated to a than under CT.

Preplant soil fumigation is an important pest management practice in coastal California strawberry production regions. Potential atmospheric emissions of fumigants from field treatment, however, have drawn intensive environmental and human health concerns; increasingly stringent regulations on fumigant use have spurred research on low-emission application techniques. The objectives of this research were to determine the effects of a new low-permeability film, commonly known as totally impermeable film (TIF), on fumigant emissions and on fumigant distribution in soil. A 50/50 mixture of 1,3-dichloropropene (1,3-D) and chloropicrin (CP) was shank-applied at 314 kg ha in two location-separate field plots (0.4 ha each) in Ventura County, California, in fall 2009. One plot was surface-covered with standard polyethylene (PE) film, and the other was covered with TIF immediately after fumigant application. Data collection included emissions, soil-gas phase concentration profile, air concentration under the film, and soil residuals of the applied fumigants. Peak emission flux of 1,3-D and CP from the TIF field was substantially lower than from the PE field. Total through-film emission loss was 2% for 1,3-D and <1% for CP from the TIF field during a 6-d film covering period, compared with 43% for 1,3-D and 12% for CP from the PE field. However, on film-cutting, greater retention of 1,3-D in the TIF field resulted in a much higher emission surge compared with the PE field, while CP emissions were fairly low in both fields. Higher concentrations and a more uniform distribution in the soil profile for 1,3-D and CP were observed under the TIF compared with the PE film, suggesting that the TIF may allow growers to achieve satisfactory pest control with lower fumigant rates. The surging 1,3-D emissions after film-cutting could result in high exposure risks to workers and bystanders and must be addressed with additional mitigation measures.