Gibberellins, Cell Divisioyi, cuid Plant Flowering b21 



other words, why does GA activate cell divisions of the whole meristem 

 in the first case and not in the second? 



Many hypotheses are possible. We can suppose, as Tschailachian 

 does, that in SD plants an activator other than GA is necessary and 

 that this activator is lacking in SD plants mider long days. We can 

 also suppose that the action of GA on metabolism is not exactly the 

 same for LD and for SD plants, or that the necessary level of activa- 

 tion must be higher in SD plants than in LD plants and cannot be 

 achieved through GA application. But there is another possibility 

 which cannot be neglected. It is known that chloroplast structure is 

 very delicate and that it is very rich in many enzyme systems. Within 

 the plastid, chlorophyll is not distributed at random but is in close 

 association with protein and lipide, the spatial organization of which 

 is now under study in some laboratories (44). To some extent the or- 

 ganization protects the chlorophyll from photodestruction. The de- 

 gree of protection varies from one species to another, or in the same 

 species in accordance with the conditions of its culture. This appears 

 evident when one studies photooxidative effects. We have found that 

 Chlorella pyrenoidosa (Kandler's strain K) was relatively resistant to 

 photooxidation, while Chlorella vulgaris (Pirson's strain P) was much 

 more sensitive (39). In some mutants, photooxidation is very easy 

 (17), but chlorophyll destruction appears to be only the final conse- 

 quence of photooxidation. Long before it occurs, photosynthesis has 

 completely ceased in high-intensity light (18,20), phosphorylations 

 are inhibited (18), and oxygen consumption rises probably with at- 

 tendant peroxide formation (18, 20, 31). A general poisoning of metab- 

 olism occurs. Crawford (inipublished) has studied the sensitivity of the 

 LD plant Salvia splendens to photooxidation by intense light. He 

 found that photooxidation (as measured by the inhibition of photo- 

 synthesis in white light) is much more marked in the leaves of Salvia 

 splendens grown in short days; the plants grown in long days are evi- 

 dently more resistant to photooxidation. The high photosensitivity of 

 Salvia grown in short days may be due to an insufficient protection of 

 chlorophyll inside the chloroplasts, possibly resulting from an ab- 

 normal structure of the jDlastids themselves. Indeed, under short days 

 the chloroplasts of Salvia do not accumulate their pigments in a 

 normal fashion. 



In practice this means that, during a given short day with light 

 of sufficient intensity, the metabolism of an LD plant grown in short 

 days can be partially inhibited through photooxidative processes. It 

 can therefore be concluded that short days do not permit flowering 

 of LD plants for two interrelated reasons: (1) suitable metabolic con- 

 ditions (of the kind described above) for increased cell divisions in 

 the meristem are lacking, and (2) photooxidation products poison 

 metabolism during the light period. 



