Changes In Light Environment Research Articles

Light and drug induced changes of epiphysial synaptic ribbons.

In the present investigation experiments were carried out to determine whether the functionally obscure synaptic rebbons of mammalian pinealocytes can be affected by acute changes in environmental lighting and which chemical processes may be involved in their regulation. Experiments carried out in male guinea-pigs have shown that the amounts of synaptic ribbons are immediately affected by changes in the lighting pattern. Extension of the light period reduced the normally occurring increase, whereas extension of the dark period inhibited the normally occurring decrease in the amount of synaptic ribbons. Results following injections of a number of drugs known to influence pineal function (noradrenaline, L-DOPA, propranolol, reserpine and p-chlorophenylalanine, respectively) suggest that synaptic ribbons may be directly or indirectly regulated by beta-adrenergic mechanisms.

Effects of changes in environmental lighting upon levels of hydroxyindole O-methyltransferase activity in the developing-chick gland.

The level of hydroxyindole O-methyltransferase (HIOMT) activity in the pienal gland of developing chicks raised under constant illumination rose more rapidly and to higher values than in the gland of birds maintained in constant darkness. Rates of net increase in activity, and levels of activity attained, for birds raised under a diurnal cycle of illumination were intermediate between those maintained in constant light or darkness. Under each of the lighting conditions, the course of increase in enzymic activity was markedly affected by variations in an unidentified factor, the source of which appeared to be the hatching eggs. Birds transferred from constant light to the dark showed either an arrest of increase in enzyme activity or a loss of activity until the levels equalled that observed for chicks of the same age raised in constant darkness. Chicks transferred from constant darkness to constant illumination showed marked increases in levels of enzyme activity at rates comparable with the maximal values observed with birds maintained under constant illumination, regardless of age and without delay. No diurnal cycle in level of HIOMT activity was observed in the pineals of 15-day birds.

Inversion of Pineal N-Acetyltransferase Rhythm by Reversed Environmental Lighting

By reversing the regimen of environmental lighting a complete inversion of the diurnal rhythm of pineal N-acetyltransferase was brought about. The nocturnal rise of N-acetyltransferase was rapidly reversed by light, yet the diurnal fall of the enzyme was not affected by darkness. Three days’ exposure of the rats to the reversed light regimen was necessary before a 180° inversion was achieved, producing a 10- to 12-fold increase in day-time (dark period) levels of the enzyme. However, after two days a 3-fold increase in the enzyme was already evident in relation to values of control rats kept on a normal lighting regimen. It is concluded that N-acetyltransferase activity and consequently serotonin and melatonin levels of the pineal, although regulated by an endogenous mechanism in the central nervous system, can be shifted by changes in environmental light which may be synchronizing the endogenous rhythms.

Control of pineal indole biosynthesis by changes in sympathetic tone caused by factors other than environmental lighting.

The melatonin (5-methoxy-N-acetylserotonin) content and N-acetyltransferase activity of rat pineal increase rapidly in response to physical immobilization or insulin-induced hypoglycemia. Carbohydrate consumption, which causes insulin release without hypoglycemia, does not elicit these pineal responses. Prior treatment with propranolol, a beta-adrenergic blocking agent, inhibits the N-acetyltransferase responses to hypoglycemia and immobilization, indicating that these changes result from stimulation of pineal beta-receptors by a catecholamine, presumably norepinephrine released from pineal sympathetic nerve terminals. Prior destruction of those terminals with 6-hydroxydopamine does not block, but actually potentiates, the increase in melatonin content and N-acetyltransferase activity after induced hypoglycemia or immobilization. This finding probably reflects an action of circulating catecholamines, secreted from the adrenal medullae or surviving sympathetic nerve terminals, on supersensitive pineal cells. These observations indicate that factors other than changes in environmental lighting, which modify sympathetic nervous tone, can also influence pineal function.

A Critical Period for Ear Development in Maize1

The environment of hybrid corn (Zea mays L.) was changed at intervals over the life cycle by removing every other plant of plants that were equispaced in rows 97 cm apart at a population of 44,500 plants/ha. Because the low plant population environment (LPE) resulting from these thinning treatments gave great improvement in the light environment over lower shoot and relatively small changes in other environmental factors, plant response was attributed mainly to changes in light environment. The number of ears per plant was 1.76, 1.57, and 1.70 during 1964, 1965, and 1966, respectively, for the 22,250 plants/ha check, which received LPE during the entire life cycle. The 44,500 plants/ha check, which received no LPE, had 1.10, 1.05, and 1.02 ears per plant, respectively. In the three seasons, changes in number of ears per plant in response to LPE imposed at different times indicated that 56, 79, and 78% of the above reduction in ears per plant between 22,250 and 44,500 plants/ha populations took place from the time the first plants silked until soon after all plants had silked — about 10 to 12 days. The rapid abortion of ears during silking and just afterward, under the unfavorable light environment of the 44,500 plants/ha population, establishes this as an important critical period for light competition in the plant community. Individual plants within the community began aborting ears under the unfavorable light environment as early as 3 or 4 days before silks emerged and continued until about 8 days after silks emerged.

On the photoreactivity of some hardwood species studied by their leaf characteristics

A certain number of measurable characteristics of tree leaves (morphological characteristics, absorption of light radiation, intensity of respiration and photosynthesis) are clearly linked with the presence of physiologically active pigments in the leaves. Leaf characteristics are highly and inequally influenced by changing conditions of light environment, especially those related to light intensity, light quality and duration of the daily illumination period. These modifications do not only apply to light radiation as created under laboratory conditions, but also to light conditions ensuing from the place in the crown of a single tree, the social position of the tree in a forest stand and the site factors in general. There are also changes taking place due to the progression of the vegetation period, at the end of which all species are less tolerant or more light demanding. The reaction of the leaves towards light radiation out of different regions of the spectrum is also different. The so-called blue light radiation (λmax = 440 nm) seems to be of the greatest importance in this relation, as species react quite different to its action. The biggest variation in leaf characteristics due to changing light environment was measured for oak and beech, which both react quickly and are qualified as 'photolabile species'. No important variations occur in leaves of ash and maple, which therefore are qualified as 'photostable species'. As a consequence of variable reactions to changing light conditions, the relationships between the species are continually modified, even in such a way that their potential for dominance is not constant. The classical division into tolerant and intolerant species or classification of the species based upon the degree of light demand, is highly inaccurate and it seems preferable to speak of relative light demands and relative tolerance. All these observations and conclusions bring about a clear confirmation of the necessity to recognize the individuality of the single tree, the special character of each growth condition, the own structure of each forest stand, the specific reaction to one sided modifications of environmental factors. This is especially important for an intensive sylvicultural practice. They also prove the necessity for more physiological and biochemical research to arrive at a better understanding of growth and its mechanism. Sylviculture in fact must try to regulate, on an expanded scale, the phenomens of growth, which is the exchange, absorption and transformation of energy. A practical interpretation and regulation of fundamental laws of physiology and growth will be possible as soon as a clinical form of sylviculture is created and the adequate instrumentarium developed.

Evidence for a cholinergic influence on pineal hydroxyindole O-methyltransferase activity with changes in environmental lighting

Hydroxyindole O-methyltransferase (HIOMT) activity of pineals of rats kept in constant light for one week was significantly lower than HIOMT in pineals of rats kept in constant darkness for one week. These inhibitory effects of light on pineal HIOMT have been shown by others to be mediated by the sympathetic nervous system. What factors mediate the facilitatory effects of the absence of light? In order to answer this question we studied the role of the parasympathetic system by giving female rats on constant light, constant dark or normal lighting schedules daily injections of atropine sulfate, atropine methyl bromide or the cholinomimetic, oxotremorine oxalate, with atropine methyl bromide. The peripherally acting parasympathetic effector blocker, atropine methyl bromide, inhibited the high levels of HIOMT in constant darkness, while the cholinomimetic restored HIOMT activity in darkness. Rats on a normal lighting schedule also had significantly higher levels of HIOMT after oxotremorine than after atropine sulfate. These results indicate that cholinergic mechanisms are involved peripherally, perhaps also centrally, in mediating the response of pineal HIOMT in darkness.

Central control of the pineal gland: visual pathways.

UNDER 1 normal lighting conditions the rat pineal gland exhibits a diurnal variation in its content of the amines serotonin 1 and norepinephrine 2 and the enzyme hydroxyindole-O-methyl transferase (HIOMT). 3 Each of these rhythms is affected by changes in environmental lighting. The norepinephrine and HIOMT rhythms appear to be predominantly under photic control and are suppressed in animals kept in continuous light or darkness and in blinded animals. 3,4 The serotonin rhythm, in contrast, is eliminated when animals are placed in constant light but persists in animals deprived of visual stimulation. 1 Each of the rhythms is maintained by neural information transmitted to the pineal from the central nervous system by the cervical sympathetics. Denervation of the pineal by superior cervical ganglionectomy abolishes all of the rhythms. 2-4 In addition, destroying these nerves in the female rat suppresses the vaginal estrus response to continuous environmental illumination, a response

The Influence of Light Intensity upon the Growth of an S. 37 Cocksfoot (Dactylis glomerata) Sward

A sward of S. 37 cocksfoot (Dactylis glomerata) was grown in three different light intensities varied by means of muslin screens. Decreasing the light intensity to 60 and 40 per cent of full daylight decreased the growth-rate and the rate of tiller production, and so decreased competition between tillers and the death-rate of tillers. Light intensity affected the area of leaves but had no effect upon the number of leaves per tiller. The rate of increase in the total dry weight fluctuated because of loss through decay, and the rate of increase in the weight of green tissue was taken as a measure of the rate of assimilation by the swards. Since the growth-rate of the green tissue at each light intensity varied little over a wide range of leaf area index, it seems that grass swards may be able to increase the efficiency with which they use light by adaptation to a changing light environment as growth proceeds. The difficulties of performing a growth analysis in the presence of the high and variable rates of senescence and decay of plant parts found on an established sward are discussed.