526 C. Sironval 



grown in long days undergoes a rapid mollification following several 

 short days. Moreover, modifications in the nucleic acid fraction of the 

 meristems occur during this SD induction. 



(b) It is well known that photoperiodic induction rapidly changes 

 the type of the gas exchange between the plant (in particular its leaves) 

 and the environment (13, 35). Respiration measurements of very young 

 isolated leaves (including the meristem) of an LD strain of Salvia 

 splendens showed that during the induction phase the respiration is 

 significantly higher under long-day conditions than under short-day 

 conditions (10). 



(c) Studying the total hematin content of leaves of LD and SD 

 plants of Perilla nankinensis (SD), Caymahis saliva (SD), Sinapis alba 

 (LD), and Salvia splendens (LD), we found (unpublished) that induc- 

 tion always causes a decrease of the molar ratio chlorophyll.hematin 

 of the leaves. This decrease is most evident in young leaves. The modi- 

 fication is very rapid and is measurable a few days after the beginning 

 of induction. We always observed that in the very young leaves the 

 chlorophyll accumulation becomes slower upon induction, ^\•hile 

 hematin accumulates more rapidly. 



(d) In flowering Fragaria vcsca the vitamin E content of the young 

 leaves is approximately proportional to the day length. In field ex- 

 periments a maximum is found in June to July, coinciding \vith the 

 increase of flower initiation (38). As shown by Nason and Lehman 

 (32), vitamin E acts in vitro as an activator of cytochrome c reductase. 



Points c and d directly relate to chlorophyll metabolism which is 

 controlled by day length, although the exact site of the photoperiodic 

 control is not yet known (9, 11, 29, 36, 37). Points b, c, and d suggest 

 some inductive change in enzyme systems of the young tissues, a pos- 

 sibility which is very consistent with point a. Taken together, the 

 four classes of facts support the following hypothesis: 



In affecting chlorophyll metabolism, photoperiodic induction acts 

 on several important metabolic processes; it enhances the respiration 

 of the young tissues of the stem and it provides them with an im- 

 proved system of hydrogen carriers passing through tlie series of cyto- 

 chromes [the cytochrome carriers are known to be regularly associated 

 in higher plants with meristematic activity (15)]. It therefore in- 

 creases the ATP supply which is necessary for the changes in the pro- 

 tein fraction (27, 28) as well as for increased cell division and flower- 

 ing. It would be very interesting to sec il the activation of cell divi- 

 sions by GA follows a scheme of this type. 



ON A POSSIBLE DIFFERENCE BETWEEN SD 

 AND LD PLANTS 



Finally, we may ask why GA induces flowering of LD plants grown 

 in short days but is ineffective in SD plants grown in long days. In 



