Effects of rust and post-infection drought on photosynthesis, growth and water relations in groundsel

Exchanges of CO2 and water vapour were examined in groundsel, Senecio vulgaris, grown and infected with rust, Puccinia lagenophorae, under laboratory conditions. The effects of drought were examined by withholding water from plants from 2 days until approximately 10 days after infection, after which further daily water losses were restored. Net photosynthesis was inhibited and dark respiration was stimulated in rust-infected leaves but, in young healthy leaves on the same plant, gross and net photosynthesis were stimulated and photorespiration was inhibited. The growth of plants was stunted 15 days after infection because the growth of leaf area was inhibited and, thereby, the photosynthetic capacity of the plant was reduced. Drought inhibited the growth of leaf area in both healthy and infected plants and, when combined with rust, had additive deleterious effects on net photosynthesis per plant and on growth. After sporulation, rusted plants transpired more rapidly than did healthy controls and leaf water potential fell to reach - 0.95 and-1.92 MPa in well-watered and drought-stressed plants respectively, compared with -0.18 and - 1.2 MPa in healthy controls, 20 days after infection. Rust impaired the normal increase in Water-Use Efficiency in response to drought

Microbial Populations and Enzyme Activities in Soils Fumigated with Methyl Bromide Alternatives

Methyl bromide (MeBr; CH3Br) use for soil fumigation will be banned in 2005 due to its ozone depleting properties. Potential alternative chemicals to replace MeBr include chloropicrin (CP; CCl3NO2), 1,3-dichloropropene (1,3-D; C3H4Cl2), iodomethane (IM; CH3I), and propargyl bromide (PrBr; C3H3Br). The goal of this research was to assess changes in soil fungal populations, microbial biomass C (MBc) and respiration, nitrification potential, and enzyme activities after fumigation with MeBr and alternative fumigants. Four formulations of alternative fumigants (CP, InLine [61% 1,3-D plus 33% CP], Midas [50% IM plus 50% CPJ, and PrBr) were applied at commercial rates through drip irrigation systems to two field plots located in main strawberry production areas in California, USA. Soil samples (0-15 cm) were taken at 1, 4, and 30 or 37 wk after fumigant application. Fumigation with MeBr plus CP and the alternative chemicals eliminated soil-borne fungal pathogens in soil and reduced culturable fungal populations up to 4 wk post fumigation. Soil microbial respiration decreased with fumigant application and was the least (>40% reduction relative to the control) in the PrBr treatment I wk after fumigation, while soil MBc was not affected by fumigation. The activities of acid phosphatase and arylsulfatase were generally lower in fumigated soils over the 30-or 37-wk study, and those of P-glucosidase and dehydrogenase were lower up to 4 wk past fumigation. Potential nitrification rates were substantially reduced (>55% reduction relative to the control) by the fumigants, but rates recovered toward the end of this study. Results of this study suggested that fungal populations and the activities of acid phosphatase and arylsulfatase were more sensitive to fumigation with the tested MeBr and the alternative fumigants than total microbial biomass, microbial respiration, nitrification, and the activities of dehydrogenases and P-glucosidase. Short-term impacts of MeBr and its alternative fumigants on microbial activities and enzymatic processes suggest that all the tested fumigants have the potential to alter important microbial and enzymatic functions such as nutrient cycling