The sulfonylurea herbicides are characterized by broad-spectrum weed control at very low use rates (c. 2-75 g ha-'), good crop selectivity, and very low acute and chronic animal toxicity. This class of herbicides acts through inhibition of acetolactate synthase (EC 4.1.3.18; also known as acetohydroxyacid synthase), thereby blocking the biosynthesis of the branched-chain amino acids valine, leucine and isoleucine. This inhibition leads to the rapid cessation of plant cell division and growth. Crop-selective sulfonylurea herbicides have been commercialized for use in wheat, barley, rice, corn, soybeans and oilseed rape, with additional crop-selective compounds in cotton, potatoes, and sugarbeet having been noted. Crop selectivity results @om rapid metabolic inactivation of the herbicide in the tolerant crop. Under growth-room conditions, metabolic half-lives in tolerant crop plants range fiom 1-5 h, while sensitive plant species metabolize these herbicides much more slowly, with half-lives >20 h. Pathways by which sulfonylurea herbicides are inactivated among these plants include aryl and aliphatic hydroxylation followed by glucose conjugation, sulfonylurea bridge hydrolysis and sulfonamide bond cleavage, oxidative 0-demethylation and direct conjugation with (homo)glutathione. Sulfonylurea herbicides degrade in soil through a combinatiori of bridge hydrolysis and microbial degradation. Hydrolysis is signijicantly faster under acidic (PH 5 ) than alkaline (PH 8 ) conditions, allowing the use of soil pH as a predictor of soil residual activity. Chemical and microbial processes combine to give typical field dissipation half-lives of I d weeks, depending on the soil type, location and compound. Very short residual sulfonylurea herbicides with enhanced susceptibility to hydrolysis (DPX-L5300) and microbial degradation (thifensuljiuron-methyl) have been developed.

SUMMARY1. Electrical properties of the membrane of photoreceptor cells in the lateral ocelli of barnacles, Balanus amphitrite and B. eburneus were investigated by intracellular recording, polarization and voltage-clamp techniques.2. The resting potential of a dark adapted cell was 36.3 + 6-6 mV (S.D.) and depended-mainly on the external K+ concentration.3. Current-voltage relations obtained from voltage-clamp experiments in the absence of light were non-linear and varied with time after the onset of a step change in membrane potential; the steady state was reached after about 0 5 sec.4. Illumination resulted in a membrane potential change under current clamp and in a change of membrane current (light-initiated membrane current (L.I.C.): total membrane current with illumination minus current without illumination) under voltage-clamp conditions. Amplitudes and time course of L.I.C. depended on the light intensity as well as membrane potential.5. The L.I.C.-voltage relation was non-linear and corresponded with a slope conductance increase with increasing positive membrane potential.6. The reversal potential of L.I.C. was independent of the light intensity and the time after onset of illumination; the average value obtained in normal saline was + 26-9 + 5 0 mV.7. The membrane conductance estimated from instantaneous L

Degradation of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-yl)amino] carbonyl] benzenesulfonamide} in acidic and alkaline soils was evaluated using plant bioassay and high-performance liquid chromatography (HPLC) radiotracer techniques. Soil sterilization with either ethylene oxide (Et0) or gamma irradiation significantly reduced breakdown of chlorsulfuron; the ability for degradation was restored by reinoculation with indigenous soil microorganisms. Streptomyces griseolus (a soil actinomycete), Aspergillus niger, and Penicillium sp. (soil fungi) were demonstrated to degrade 14C-chlorsulfuron in pure culture. In addition to microbial breakdown, chemical hydrolysis was an important factor in the disappearance of chlorsulfuron from soil. The contribution of chemical hydrolysis to total degradation was a function of soil pH, with hydrolysis occurring most rapidly in acidic soils. Both dissipation processes slowed markedly at low temperatures.

The need to estimate mineralization has long been recognized in making N fertilizer recommendations, but little progress has thus far been made in identifying a specific fraction of soil organic N that affects crop responsiveness to N fertilization. After eliminating major defects in the methodology employed to fractionate the N in soil hydrolysates, a study was conducted to compare N‐distribution analyses for soils differing in N‐fertilizer responsiveness by corn (Zea mays L.). Hydrolyses with 6 M HCl were performed on composite soil samples (0–30 cm) that had been collected in late March or early April of 1990, 1991, or 1992, from 18 sites in a N‐response study involving 75 site–years throughout Illinois with different soil types, crop rotations, and N management practices. Concentrations of amino sugar N were 33 to 1000% greater (P < 0.001) for 11 nonresponsive than for seven responsive soils, whereas no consistent difference was observed in their content of total hydrolyzable N, hydrolyzable NH4–N, or amino acid N. Upon aerobic incubation for 3 mo with biweekly leaching, production of (NH4 + NO3 + NO2)‐N averaged 260% greater for three nonresponsive soils than for two responsive soils, and was accompanied by a net decrease in amino sugar N but not in amino acid N. Soil concentrations of amino sugar N were very highly correlated with check‐plot yield (r = 0.79***) and fertilizer‐N response (r = −0.82***). On the basis of amino sugar N, all 18 soils were classified correctly as responsive (<200 mg kg−1) or nonresponsive (>250 mg kg−1) to N fertilization.

Illumination of an Aplysia giant neuron evokes a membrane hyperpolarization which is associated with a membrane conductance increase of 15%. The light response is best elicited at 490 nM: the neuron also has an absorption peak at this wavelength. At the resting potential (-50 to -60 mV) illumination evokes an outward current in a voltage-clamped cell. This current reverses sign very close to Es calculated from direct measurements of internal and external K+ activity. Increases in external K+ concentration shift the reversal potential of the light-evoked response by the same amount as the change in Es. Decreases in external Na+ or C-do not affect the response. Therefore, the response is attributed to an increase in K+ conductance. Pressure injection of Ca2+ into this neuron also hyperpolarizes the cell membrane. This effect is also due largely to an increase in K+ conductance. The light response after Ca 2+ injection does not appear to be altered. Pressure injection of EGTA abolished or greatly reduced the light response. The effect was reversible. We suggest that light acts upon a single pigment in this neuron, releasing Ca 2+ which in turn increases K+ conductance, thereby hyperpolarizing the neuronal membrane.