IN VITRO EFFECT OF HORMONES ON THE BARK OF PLUMERIA RUBRA LINN. VAR. ACTUIFOLIA BAILEY

Young bark (with cambium) of Plumeria rubra Linn. var. acutifolia Bailey was cultured in solid media (i) without hormone, (ii) Kinetin (K), (iii) with GA₃ and (iv) with IAA at concentrations of 0.05, 0.10, 0.15 and 0.20 mg per litre. The nutrients of the media were fed laterally through the cambium zone. The amount of the phloem zone was increased considerably by GA₃, less by K and IAA. The lignified secondary wall of the pericyclic living fibres was dissolved by each of the hormones. Sieve tube member length decreased in all treatments, more in higher concentrations. K decreased the frequency of sieve tubes (most at 0.10 mg/α) and increased the frequency of parenchyma cells (optimum at 0.10 mg/α). GA₃ also favoured formation of parenchyma cells and decrease of the frequency of sieve tubes, progressively with the increase of concentration. IAA also increased the parenchyma cell frequency progressively with concentration and decreased slightly the sieve tube frequency. IAA and GA₃ increased ray frequency and decreased parenchyma cell diameter, much at high concentration. But K had less effect on ray frequency and increased parenchyma cell diameter progressively with concentrations.     

Synaptic Vesicle Dynamics: A Simple Model of Phasic Release

We present a simple model of phasic neurotransmitter release whichreproduces the salient features of chemical neurotransmission. The synapticvesicle cycle has been modelled as a set of biochemical reactionsrepresented by a system of coupled differential equations. These equationshave been solved analytically to obtain the time dependent behaviour of thesystem on perturbation from the steady state. The scheme of the synapticvesicle network has been emphasized and its role in determining some of themajor experimentally observed properties of synaptic transmission has beendiscussed, which includes the biphasic decay of the rate neurotransmitterrelease even under sustained stimulation. Another interesting outcome ofthis theoretical exercise is the saturation of total release with thecalcium dependent rate constant. The theoretically calculated values oftotal release fit very well into a sigmoidal saturating function with afourth order cooperativity exponent similar to the empiricalDodge–Rahamimoff equation. It appears that the synaptic vesiclenetwork itself is responsible for some of the major properties associatedwith chemical neurotransmission.