THE INFLUENCE OF LIGHT INTENSITY ON THE SUSCEPTIBILITY OF PLANTS TO CERTAIN VIRUSES

Reducing the light intensity under which plants were grown in summer to one-third increased their susceptibility to infection with tobacco necrosis, tomato bushy stunt, tobacco mosaic and tomato aucuba mosaic viruses. With the first two viruses shading increased the average number of local lesions per leaf by more than ten times and by more than five times with the second two.
Reducing the light intensity increased the virus content of sap from leaves inoculated with Rothamsted tobacco necrosis virus by as much as twenty times. As it also reduced the total solid content of sap by about one-half, purification was greatly facilitated; crystalline preparations of the virus were readily made from shaded plants but not from unshaded controls.
Reducing the light intensity also increased the virus content of systemically infected leaves; the greatest effect was with tomato bushy stunt virus with which increases of up to ten times were obtained, but with tobacco mosaic and aucuba mosaic viruses there were also significant increases.
The importance of controlled illumination in raising plants for virus work and the possible mechanisms responsible for the variations in susceptibility are discussed.     

RESULTS OF IRRADIATING SACCHAROMYCES WITH MONOCHROMATIC ULTRA-VIOLET LIGHT : II. THE INFLUENCE OF MODIFYING FACTORS

Possible variation in the probability that absorbed quanta of ultra-violet energy will produce observable inhibitory and lethal effects in the yeast cell, due to non-uniformity in sensitivity of the different regions of the cell, may be further modified by the reproductive stage of the cell at the time of irradiation. Tests of the survival of yeast cells of 15 day and 24 hour cultures indicate that the older resting cells are more resistant to ultra-violet irradiation effects than cells undergoing rapid cell division. The effects of temperature changes within the range of normal growth are evidently small, as judged from the temperature coefficient (1.10). Possible inhibitory effects due to the action of ultra-violet radiation on the malt agar medium and to toxic substances diffused from cells killed by irradiation were not found under the conditions of the experiments. Tests of the validity of the Bunsen-Roscoe reciprocity law for variation in the intensity of the incident ultra-violet radiation up to 30 per cent indicate that for this range the rate of absorption of quanta by the cell does not produce any marked change in the lethal effects observed.     

CONCERNING THE INFLUENCE OF POLARIZED LIGHT ON THE GROWTH OF SEEDLINGS

While these experiments are not exhaustive, a sufficient number have been made to warrant the statement that the effect of polarized light of the visible spectrum on the growth of various seedlings and See PDF for Structure more particularly on the growth of Lupinus albus is somewhat different from that of non-polarized light. This is especially convincing in view of the results obtained with double sets of plants which were alternately exposed to polarized and non-polarized lights of the same intensities and at the same temperature. In every experiment thus performed the set which was placed in a polarizing chamber grew better. It is, furthermore, interesting to note that the phenomenon above observed did not take place when the seed portion of the plants was protected from light by wrapping with tinfoil. This agrees well with previous findings concerning the action of diastase on starch in polarized light. The above researches will be continued on a more elaborate scale but the results so far obtained are deemed worthy of publication in the form of a preliminary communication at the present time.