The effects of simulated spring goose grazing on the growth rate and protein content of Phleum pratense leaves.

The effects of simulated goose grazing on Phleum pratense plants were tested in an Iceland hayfield during the spring goose staging period (19 April-11 May 1997). Plants in an area exclosed from the influence of grazing and the nutrient effects of goose faeces were subject to the removal of the youngest lamina once, three and four times during this period. Clipping three and four times resulted in 25-41% increases in cumulative elongation of youngest laminae compared with unclipped plants. Total cumulative lamina growth of entire plants showed no significant difference between unclipped plants and those clipped three and four times, hence no overcompensation occurred. Sequential clipping elevated the protein content of the youngest laminae from 20% to 27-33%, whereas there was no change amongst shoots clipped only once. Because geese only consume the youngest lamina of each Phleum plant, measurements from this experiment showed that regular physical removal of growing biomass doubled the biomass of preferred tissue available to geese and increased the potential protein intake 3.5 times at experimental clipping frequencies similar to levels of sequential harvesting observed amongst staging geese compared to less frequent harvesting. These increases were achieved without any fertilising effects of goose faeces implicated in such effects in previous studies.

Simulations to derive membrane resistivity in three phenotypes of guinea pig sympathetic postganglionic neuron.

The electrotonic behavior of three phenotypes of sympathetic postganglionic neuron has been analyzed to assess whether their distinct cell input capacitances simply reflect differences in morphology. Because the distribution of membrane properties over the soma and dendrites is unknown, compartmental models incorporating cell morphology were used to simulate hyperpolarizing responses to small current steps. Neurons were classified as phasic (Ph), tonic (T), or long-afterhyperpolarizing (LAH) by their discharge pattern to threshold depolarizing current steps and filled with biocytin to determine their morphology. Responses were simulated in models with the average morphology of each cell class using the program NEURON. Specific membrane resistivity, R(m), was derived in each model. Fits were acceptable when specific membrane capacitance, C(m), and specific resistivity of the axoplasm, R(i,) were varied within realistic limits and when underestimation of membrane area due to surface irregularities was accounted for. In all models with uniform R(m), solutions for R(m) that were the same for all classes could not be found unless C(m) or R(i) were different for each class, which seems unrealistic. Incorporation of a small somatic shunt conductance yielded values for R(m) for each class close to those derived assuming isopotentiality (R(m) approximately 40, 27, and 15 k omega cm(2) for T, Ph, and LAH neurons, respectively). It is concluded that R(m) is distinct between neuron classes. Because Ph and LAH neurons relay selected preganglionic inputs directly, R(m) generally affects function only in T neurons that integrate multiple subthreshold inputs and are modulated by peptidergic transmitters.