Light-dark transitions

Dunlap But it is happening at the light—dark transition, not the other way round. This is why I have a hard time understanding why this is called an acute effect . Does the induction happen when the light is turned off ... 

Dunlap What about the 10 min before the lights went off Why didn t it acutely induce then So it s not acute induction by the light—dark transition that you... 

The molecular assays in Clk"mAic2As bom fide rhythms with a predominant effect on circadian rhythm amplitude and no more than a modest effect on phase or period. With circadian per and tim enhancers, we observed reduced enhancer activity and a reduced cycling amplitude in a Clk" background, consistent with the role of Clk in regulating these enhancers. Nonetheless, the phase of oscillating bioluminescence is similar to that of wild-type flies. The presence of molecular rhythms contrasts with the absence of detectable behavioural rhythms. We favour the notion that this reflects a level or amplitude reduction below a critical threshold for behavioural rhythmicity. The absence of anticipation of light—dark transitions makes it very unlikely that an effect restricted to the lateral neurons — the absence of the neuropeptide PDF, for example — is primarily responsible for the behavioural phenotypes. This is also because LD behavioural rhythms are largely normal in flies devoid of PDF or the pacemaker lateral neurons (Renn et al 1999). However, we cannot exclude the possibility of selective effects of Clk" on other behaviourally relevant neurons. 

From E. suberosa were isolated two alkaloids, erysodine (102) and erysothrine (106). These compotmds were investigated for anxiolytic properties. Both alkaloids produced anxiolytic-like effects in mice exposed to a two widely used anxiety tests - the elevated plus-maze and the light -dark transition model. In the elevated plus-maze, only erysodine increased the percentage of open arm entries and tended to increase the percentage of time spent in the open arms, the two conventional measures of anxiety in the elevated plus-maze [71]. 

Two observations indicate that the increase in absorbance at 520 nm is due to a reversible process 1) at light/dark transitions, the absorbance decreases 2) the light-induced absorbance increase can be repeated several times with the same leaf at 1 min dark intervals (fig. 3). 

Figure 20-1. An experimental arrangement for determining the average lifetime t of chains by the rotating sector method. Focusing is such that the light/darkness transition is sharp. The ratio of light/darkness is r = 3/1 in this example. LS, light source RS, rotating sector D = thermostated dilatometer.

The emissions of isoprene, acetaldehyde, methanol, and monoterpenes from Sitka spruce correlating with photosynthetic photon flux and temperature have been investigated, and an anticorrelation phenomenon between isoprene and acetaldehyde was reported during the sudden light-dark transitions [44]. Biogenic VOCs... 

Acetaldehyde is mostly formed by the enzymatic oxidation of ethanol, the latter is formed in roots under anoxic conditions or in anoxic stems of woody plants [13]. Large fluxes of acetaldehyde have been observed from the leaves when roots are flooded (limited access to oxygen) or exposed to anoxia. It has also recently been discovered that acetaldehyde can arise in leaves directly from metabolism that occurs during light-dark transitions [14]. 

Figure 5.4 Changes in acetaldehyde and isoprene levels following light-dark transitions subjected to sycamore leaves. Note that acetaldehyde shows a rapid burst and decline during the dark period. The ability to measure such rapid changes in volatiles emitted from leaves illustrates one of the key benefits of PTR-MS. These results were replicated in several leaf varieties. Light intensity is given in terms of irradiance (E has units of W m ). Reproduced with permission from [223]. Copyright 2002 John Wiley Sons, Ltd.

Graus, M., Schnitzler, J. P., Hansel, A. et al. (2004) Transient release of oxygenated volatile organic compounds during light-dark transitions in grey poplar leaves. Plant... 

THE EFFECTS OF LIGHT-DARK. TRANSITION AND OF SPECIFIC INHIBITORS ON THE ATP LEVEL IN SOME CYANOBACTERIA... 

Further study of the effects of light-dark transitions on the levels of RuDP suggested to Bassham that the affinity of RuDP carboxylase for RuDP was lower in the dark than in the light. This may be one aspect of control mechanisms which co-ordinate the catabolic and anabolic reactions of the cell. The regulation of the flow of carbon between the two by way of the several compounds which are intermediate in both chloroplastic and non-chloroplastic reaction sequences and which can traverse the chloroplast membrane may also be involved. Doubtless many subtle interactions between the photosynthetic and non-photosynthetic reactions of the green cell await discovery. 

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