SECRETARF'S REPORT 79 



ulatory reactions to see if they are mediated by the same basic mech- 

 anisms. 



A National Science Foundation grant for 3 years is supporting the 

 major portion of the work at present. 



Chloroplast development. — It has been found by Dr. J. B. Wolff 

 and L. Price that radiant energy is necessary for the maturation of 

 the chloroplast, the photosynthetic organ of the leaf of higher plants. 

 The progress of this photomorphogenetic development has been fol- 

 lowed by measuring the gradually increasing rate at which the leaf 

 is able to synthesize chlorophyll when placed in high red or blue 

 irradiances. In the leaf of a dark-grown seedling the rate of chloro- 

 phyll formation is at first very slow, but after two or three hours of 

 high-intensity irradiation, the rate begins to increase. Therefore, 

 the time lag before the leaf begins to form chlorophyll rapidly is 

 taken as the time required for certain developmental changes in the 

 proplastid as it is transformed into a functioning chloroplast. 



Irradiation of dark-grown bean or corn leaves with a small amount 

 of red energy (prior to incubation in the dark) has been found to be 

 more effective than blue energy for stimulation of the ability to form 

 protochlorophyll. Oxygen is required for the developmental proc- 

 esses, since it was noted that little or no chlorophyll is formed in an 

 atmosphere of nitrogen. When the temperature at which the leaves 

 are kept was lowered from 25° C. to 15° C, the metabolic processes 

 necessary for synthesis of the chloroplast components are almost com- 

 pletely stopped; too high a temperature has a similar effect. These 

 metabolic processes are being studied in greater detail. 



Photoperiodic chlorosis. — Chlorophyll content of a number of 

 plants is markedly influenced by the relative lengths of the light and 

 dark periods. The leaves of young plants often show a marked 

 chlorosis, with a definite pattern of interveinal yellowing when given 

 long light periods in a 24-hour cycle. Often associated with the 

 mottling are nastic responses, very similar to those occurring with 

 certain types of virus infection. Temperature is closely correlated 

 with light in influencing this process. At some temperatures, the 

 plants are yellow and at others green, regardless of the photoperiod. 

 Particularly does a cyclic alternation in temperature promote or in- 

 hibit the chlorosis under light conditions where the converse is true 

 at constant temperatures. This type of chlorophyll deficiency has 

 not been observed in plants growing under outdoor or greenhouse 

 conditions with daylight, but seems to be a response unique to irradi- 

 ation with artificial sources, particularly the incandescent lamp. Dr. 

 Alice Withrow and Walter Shropshire this year have found that the 

 far-red is the region of the spectrum which most effectively promotes 

 the chlorosis. Far-red also promotes a very marked lengthening of 

 the stem. 



