Flowering and Stem Elongation

Two long days induced some flowering and 4 or nmore long days caused 100c flowering in Silene armeria. On long days microscopically detectable flower primordia were first seen after 6 days, which is at least 1 day before the start of stem elongation. Botlh gibberellin A3 and A7 caused flowering on short days, but the results were variable and flowering was never 100%0. Th-ree different gibberellins were detected in Silene extracts. The pattern of gibberellins extracted from plants on short and long days was qualitatively the same, but on long days gibberellin contenit was up to 100% higher than on short days. Only small anmounllts of diffusible gibberellins were obtainied from Silene slhoot tips (including very young leaves) on short days. However, oII lonig days the diffusible gibberellins increased by as nuch as 10-fold after 4 to 6 long days but tlhein declined sonmewhat after 10 long days. The gibberellins extracted from the slhoot tips at the completion of the difftusion period also increased under long days, altlhouiglh the increase was not as large as for the diffusible gibberellins. All A5-like gibberellini present in extracts was not detected in diffusates. Treatment witlh AMO 1618 (2-isopropyl-4-dimetlhylamino5-methylphenyl -l -piperidine -carboxylat e inethyl chloride) completely inhibited stem elongationi oil long days but had no effect on flowering. In additionl, treatment witli AM1O 1618 caused at least an 80%' decrease ill the level of extractable gibberellin, while the diffusible gibberellini was reduced below the limits of detection in the di corn bioassay. Whein endogenous gibberellin levels were suppressed by pretreatment with AMO 1618 on short days-, gibberellin A3 caused more stem elongation in plants moved to long days than in plants left on slhort days. Thus the sensitivity of Silene plants to gibberellini with respect to stem growtlh is affected by photoperiod. It is concluded that in Silene endogenous gibberellins are a cointrolling factor for stem elongation but apparently are not requiired for flower fornmationi. Treatment on short days with gibberellic acid leads to stem elongation and flower formation in many rosette long-day 1This research was supported by a National Science Foundation Postdoctoral Fellowship to C. F. Cleland and by the United States Atomic Energy Commission under Contract AT(11-1)-1338. 2Present address: Biological Laboratories. Harvard University, Cambridge, Mass. 02138. plants (19, 21). In addition, flowering can also be induced by gibberellin treatment under SD3 in the long-short-day plant Bryophyllum daigreniontianum (46) and under long days in the short day plant Impatiens balsamina (30). In each case flower induction, whether by photoperiodic treatment or GA treatment, is accompanied by considerable stem elongation. By contrast, in most SDP and nonrosette LDP where flowering is not associated with significant stem elongation, GA treatment does not lead to flower formation (9, 21) or may even be inhibitory to flowering (9, 13, 34). In recent years additional evidence for the possible involvement of GA in the regulation of flowering has come from studies with the plant growth retardants CCC and AMO 1618, which are known to inhibit endogenous GA biosynthesis (3, 10, 31, 33). Flowering in the rosette LDP Samolus parviflorus (1), the nonrosette LDP Lemma gibba G3 (9), the SDP Pharbitis nil (44) and the LSDP Bryophv/llurn (43) could be inhibited under inductive conditions by the addition of CCC or AMO 1618, and this inhibition was reversed by simultaneous application of GA. However, in certain other plants the application of CCC or AMO 1618 apparently does not have any inhibitory effect on flowering (4, 36). Numerous attempts have been made to examine the endogenous GAs obtained by extraction from plants grown under a variety of environmental conditions and to relate them to flowering (7, 11, 12, 20, 25, 48). In general, it has been found that the level of extractable GA is higher on LD than on SD. Harada (11, 12) was able to detect increases in GA-like substances prior to the start of flower formation in several different plants. In the LSDP Bryophyllunz the transfer of plants from LD to SD resulted in an increase in the GA content prior to the formation of flower primordia (48). However, in most other studies plants kept under inductive photoperiods were not extracted until after the appearance of buds and flowers (7, 8, 23, 25). In addition, the increase in GA with LD treatment has been obtained for both LDP and SDP (7, 8). Thus, the significance of these changes in endogenous GAs for the regulation of flower formation remains doubtful. The rosette LDP Silene armeria could be induced to flower on SD by GA7, but other GAs were ineffective (26), or effective only at very high doses (40). Preliminary studies indicated that endogenous GAs could be obtained from Silene either by extraction or by diffusion from shoot tips by the agar diffusion technique (16, 17). Extraction yields higher levels of GA activity but provides information only on the GA content at any given time. In order to 'Abbreviations: SD: short days; LD: long days; LDP: long day plants; LSDP: long-short day plants; SDP: short day plants; CCC: (2-chloroethyl)trimethylammonium chloride; AMO 1618: 2-isopropyl4-dimethylamino-5-methl\iphen\i1-1 -piperidine-carbc)xylate metilhyl chloride.