Kinetics and relative significance of remobilized and current C and N incorporation in leaf and root growth zones of Lolium perenne after defoliation: assessment by 13C and 15N steady-state labelling

The contribution of pre-defoliation reserves and current assimilates to leaf and root growth was examined in Lolium perenne L. during regrowth after defoliation. Differential steady-state labelling with ¹³C (CO₂ with δ¹³C = -0.0281 and -0.0088) and ¹⁵N (NO₃^? with 1.0 and 0.368 atom percentage, i.e. δ¹⁵N = 1.742 and 0.0052, respectively) was applied for 2 weeks after defoliation. Rapidly growing tissues were isolated, i.e. the basal elongation and maturation zones of the most rapidly expanding leaves and young root tips, with a biomass turnover rate > 1 d^?1. C and N weights of the elongation zone showed a transient decline. The dry matter and C concentration in fresh biomass of leaf growth zones transiently decreased by up to 25% 2 d after defoliation, while the N concentration remained constant. This ‘dilution’ of growth zone C indicates a decreased net influx of carbohydrates relative to growth-related influx of water and N in expanding cells, immediately after defoliation. Recovery of the total C and N weights of the leaf elongation zone coincided with net incorporation of currently absorbed C and N, as shown by the kinetics of δ¹³C and atom percentage ¹⁵N in the growth zones after defoliation. C isotope discrimination (Δ¹³C) in leaf growth zones was about 23‰, 1–2‰ higher than the Δ in root tips. Δ¹⁵N in the leaf and root growth zones was 10±3‰. The leaf elongation zones (at 0–0.03 m from the tiller base) and the distant root tips (about 0.2 m from the base) exhibited similar kinetics of current C and N incorporation. The amount of pre-defoliation C and N in the growth zones, expressed as a fraction of total C and N, decreased from 1.0 to 0.5 at 3 (C) and 5 (N) d after defoliation, and to 0.1 at 5 (C) and 14 (N) d after defoliation. Thus, the dependence of growth zones on current assimilate supply was significant, and stronger for C than for N. The important roles of current assimilates (as compared to pre-defoliation reserves) and ‘dilution’ of dry matter in regrowth after defoliation are discussed in relation to the method of labelling and the functional and morphological heterogeneity of shoot tissues.     

Comparative Study of Six Strains of Naegleria with Special Reference to Nonpathogenic Variants of Naegleria fowleri*

SYNOPSIS. Naegleria fowleri strains HB-1 and KUL, pathogenic for humans, Naegleria gruberi strain 1518/1e, and 3 strains (Vm1, LvH₁, and LvH₂) of Naegleria isolated from a body of water polluted with thermal effluents were compared in an attempt at specific identifications of the latter strains. The 3 environmental isolates were morphologically almost identical with N. fowleri and had almost the same temperature tolerance, although at 37 and 42 C the growth rates of LvH₁ and LvH₂ were higher than those of the human pathogen, N. fowleri, and of isolate Vm1, which was pathogenic for mice. Serologic examinations by indirect fluorescent antibody method revealed a very close relationship of the new isolates with the human pathogens. While Vm1 was indistinguishable from N. fowleri, LvH₁ and LvH₂ were not, when cross-absorbed antisera were used. Of all the strains examined, only the 2 LvH isolates were not inhibited by amphotericin B, while only N. gruberi was not inhibited by fumagillin. The cytopathic effect in Vero cell cultures suggested that the LvH strains could have a certain degree of virulence, although this was not confirmed by intranasal and intracerebral inoculations of mice. The cytopathic effects of the human pathogens and of the isolate pathogenic for mice were related to their virulence for mice. It is concluded that there exists an intermediate form between N. gruberi and N. fowleri, with a strong relationship to the latter species. We refer to such strains as nonpathogenic variants of N. fowleri. Further research is needed to reveal their place in the taxonomy.     

Modeling a Neural Oscillator that Paces Heartbeat in the Medicinal Leech

SYNOPSIS. Heartbeat in the medicinal leech is paced by a neuraloscillator comprising two elemental oscillators whose activityis coordinated intersegmental coordinating fibers. The elementaloscillators each consist of a bilateral pair of heart interneuronslinked by reciprocal inhibitory synapses. The activity cycleof each elemental oscillator consists of alternating burstsof action potentials (plateau/burst phase) and periods inhibition(inactive phase). Oscillation ensues in the reciprocally inhibitorypairs because each neuron is able to escape from the inhibitionits contralateral partner and thus move on to the plateau/burstphase. We have identified and described membrane currents thatcontribute to oscillation and studied graded synaptic transmissionbetween the neurons, using discontinuous current clamp and switchingsingle electrode voltage clamp techniques. A hyperpolarization-activatedinward current, Ih, plays a major role in escape from inhibition,and Ca²⁺ currents produce plateau potentials that support burstformation and mediate graded synaptic transmission. To consolidate our knowledge and guide future research, we haveconstructed a first generation computer model of a neural oscillatorbased on reciprocal inhibition, using Hodgkin-Huxley equationsand a synaptic transfer model, derived from our biophysicalstudies, with Nodus software (De Schutter, 1989). This modelhas confirmed an important role for I_h in sustaining oscillationand has implicated a similarly important role for outward currents(particularly I_A), which remain to be studied. Neural oscillatorsbased on reciprocal inhibition appear to be ubiquitous, andour studies, biophysical and computational, provide insightsinto how they may operate.