Short Light Exposure Research Articles

Interactions between photosynthesis and respiration in Chlorella I. Types of transients of oxygen exchange after short light exposures

1. Synchronous Chlorella fusca was exposed to light for periods of 0.1 sec-15 min. Subsequent to the illumination, the rate of O 2 exchange passes through a series of transient phenomena. These transients vary greatly with cellular age and depend also upon conditions prior to and during the experiment. However, the individual maxima and minima of O 2 exchange specified as T 1, T 2, T 3..., occur at reproducible times provided that algae of the same developmental stage and pretreatment are used. 2. At least 5 individual components are involved in the complex changes: (a) A sudden photosynthetic O 2 evolution ( T 1) which, after darkening, declines rapidly and without oscillations. (b) A brief and distinct O 2 uptake ( T 2) immediately after switching off the light, which is only observed under special conditions. (c) The next maxima and minima T 3, T 4, T 5, and partly also T 6 obviously are damped oscillations of the respiratory O 2 uptake, started by an inhibition of some respiratory step by photosynthesis. (d) A slow declining stimulation of O 2 uptake superimposed on the foregoing transients occurs after longer light exposures. (e) A very strong stimulation of respiratory O 2 consumption ( T 8), culminating not until 6 to 8 min after the start of illumination, is induced only by irradiation with short-wave light (< 540 nm) after longer dark periods. 3. The individual components differ with respect to their dependence on intensity, wavelength and duration of the light exposure and their sensitivity to several inhibitors. According to their characteristics all transients except T 8 depend on, but in different ways, the function of the photosynthetic apparatus. The components b and c seem to be closely connected with System I.

THE BLUEBERRY RHIZOME: IN VITRO CULTURE

The successful in vitro culturing of blueberry (Vaccinium angustifolium Ait). rhizome apices is reported. A variation in sensitivity to short light exposures is noted that may be seasonal. In spring the organ appears less light sensitive. Later no exposure is tolerated without response. In dark culture of spring-gathered rhizomes, the diageotropic habit continues with the evolution of scale leaves separated by long internodes. Exposure to a 16-hour light/dark cycle, however, causes the apex to elevate and induces changes in habit that are manifested by the shortening of the internode and the expansion of the scales into green leaves. Elongating growth essentially ceases. Histological studies suggest that a disruption of the apex follows light exposure. Necrotic cells appear behind the dome, and the meristem decreases in size and changes shape even as it rapidly evolves foliar initials. Apparently the meristem rapidly loses function.The method is offered as a technique suitable for the controlled study of rhizome physiology and development.

BIOELECTRIC POTENTIALS IN HALICYSTIS

The effects of light upon the potential difference across the protoplasm of impaled Halicystis cells are described. These effects are very slight upon the normal P.D., increasing it 3 or 4 per cent, or at most 10 per cent, with a characteristic cusped time course, and a corresponding decrease on darkening. Light effects become much greater when the P.D. has been decreased by low O(2) content of the sea water; light restores the P.D. in much the same time course as aeration, and doubtless acts by the photosynthetic production of O(2). There are in both cases anomalous cusps which decrease the P.D. before it rises. Short light exposures may give only this anomaly. Over part of the potential range the light effects are dependent upon intensity. Increased CO(2) content of the sea water likewise depresses the P.D. in the dark, and light overcomes this depression if it is not carried too far. Recovery is probably due to photosynthetic consumption of CO(2), unless there is too much present. Again there are anomalous cusps during the first moments of illumination, and these may be the only effect if the P.D. is too low. The presence of ammonium salts in the sea water markedly sensitizes the cells to light. Subthreshold NH(4) concentrations in the dark become effective in the light, and the P.D. reverses to a negative sign on illumination, recovering again in the dark. This is due to increase of pH outside the cell as CO(2) is photosynthetically reduced, with increase of undissociated NH(3) which penetrates the cell. Anomalous cusps which first carry the P.D. in the opposite direction to the later drift are very marked in the presence of ammonia, and may represent an increased acidity which precedes the alkaline drift of photosynthesis. This acid gush seems to be primarily within the protoplasm, persisting when the sea water is buffered. Glass electrode measurements also indicate anomalies in the pH drift. There are contrary cusps on darkening which suggest temporarily increased alkalinity. Even more complex time courses are given by combining low O(2) and NH(4) exposures with light; these may have three or more cusps, with reversal, recovery, and new reversal. The ultimate cause of the light effects is to be found in an alteration of the surface properties by the treatments, which is overcome (low O(2), high CO(2)), or aided (NH(4)) by light. This alteration causes the surface to lose much of its ionic discrimination, and increases its electrical resistance. Tests with various anion substitutions indicate this, with recovery of normal response in the light. A theory of the P.D. in Halicystis is proposed, based on low mobility of the organic anions of the protoplasm, with differences in the two surfaces with respect to these, and the more mobile Na and K. ions.