Showing papers by "Kenneth V. Thimann published in 1967"

The role of auxins and cytokinins in the release of buds from dominance

Abstract: The paper deals with the general problem of the physiological basis of branching, and the roles of known and unexplored factors in sensitivity to apical domirnance. It is shown that when pea seedling shoots are completely or partially inhibited by other shoots on the same plant auxin can promote their elongation, even though it does not have this effect on inhibited buds. This influence of auxin is only exerted on internodal elongation and not on apical growth. When kinetin in a solution of alcohol and carbowax is applied directly to the lateral buds of pea seedlings, it releases them from inhibition by the growing apex. It is shown that the role of alcohol in this solution is to act as a surfactant, permitting good contact with the buds, while that of carbowax, being hygroscopic, is to maintain a thin film of solution over the buds. Buds thus released from apical dominance by kinetin do not elongate as much as do uninhibited control buds. Such kinetintreated buds can, however, be made to elongate normally by the application of auxin locally to their apices. It is concluded that growing shoots are relatively insensitive to correlative inhibition because they synthesize two types of growth substances, namely, auxin, which antagonizes the inhibitory effect on internodal elongation, and cytokinins, which permit the apex itself to develop. In the discussion it is brought out that many cases of branching, which appear at first to bear little relation to one another, can be understood on the basis of two principles, namely: (1) Any reduction in the growth rate of a dominant apex reduces its inhibitory effect on other apices, and (2) once an apex starts growing it becomes less sensitive to inhibition by other apices These generalizations and the experimental results are tentatively interpreted in terms of an interaction between the syntheses of auxin and of cytokinin.

Observations on the fine structure of the oat coleoptile

Abstract: I,n pre~i,ou.s ~pape.r,s ( O ' B r i e n an, d T , h i , m a n n 1965; T h i m a n n and O ' B r i e n 1965) ~the h,i:s.to.l, ogy of the eoleop,tiIe a p e x has been desc r ibed a n d i l lus t ra ted in s.ome ,detail. I't was ,shown t.ha,t the a p e x e,onsist,s of th ree ti~ssue:s: eipi, derm.is (.inner a n d outer) , va,scular sys,te~m, a n d p a r e n c h y m a . T h e e.el}s of the ~arenehyLma range b e t w e e n ,tw.o ,extremes. O n tfhe one hand , the cells nea res t 4he o,uter epi,derm:its ,o.n ,the p M o e m side .of the va's.eular bund les conta:in plast~ds which t u r n q.ud~te g reen .in t.he ~ig,ht. O n the o the r hand , .the pa renehy ,ma o.f the a v a s c u l a r r (w,hieh ,clo,s.e,s ,the eoleo.ptd~e) and the cells nea r e r ,the ~i.n~ne.r .epidermis f,a:il t.o Jd.ev.el,o~p a n y ap~preeiabl.e gr.ee~ p i g m e n t in the ~i~ht. The re,st .of the ~parenehy~m eel~Ls lie b.etween ~he,se two extrem,es, and it i,s ~el,e.ar t,hat ,in ~he s.ho~f,t o,f the eol,eoptile the c a p a c i t y to ' turn g reen show, s a ,marked r ad i a l g r a d i e n t t.ow,ar, ds the outside. A l though ,par,enehyma .eell,s .e,o~nst~,tute the b u l k ,of ,the l iv ing tis,s,ues of hi,gher .plant,s, s.urpr.i.sdrrg.ly l!i,ttle d a t a has b,een p u b l i s h e d on ' their fine s t ruc ture . S,u.eh infor,mat,i,an o~s .~her.e ,is co,m,es larg, e ly fr,o~m eell,s .of the eoleo,ptile. T,hu,s, A r r i g o n i and R o ~s .s ~i (,1962; 1964,) ,make b r i e f Inent ion of {he parenehy ,ma cel.Ls .but thei'r w o r k i,s concerned p r i m a r i l y w i t h the va scu l a r .spstem. W a r ,d r o O and F o s :t ,e r (1964) h a v e ,stud!i,ed 'the p a renehy.ma and ep:idermds .o,f y,o,ung e,o.leoptiles ,in .mater ia l f ixed ~n per~mangana te , whi le T ,h .o r n ,t o n arid T h i m a n n (19,64,) and C r o n s h a w (1964) h a v e b o t h r e p o r t e d on the crystal-e,onta~ining bo,dd.es ,o,f 4.hese e,ell~s in the o.at eo.leoptile. The wal'l~s of t:his ,tissue h a v e been s tud ied more e x t e n s i v e l y

Transient Effects of Light on Auxin Transport in the Avena Coleoptile

Abstract: The transport of indole-3-acetic acid-1- 14 C (IAA) through 4 mm segments of etiolated Avena coleoptiles was studied as a function of time by applying IAA in apical agar blocks and measuring the basal IAA export rate at 5-minute intervals. The transport velocity found in this way is at least 15 mm/hour at 26°. Following a 30-minute equilibration period, the export rate is nearly constant for at least 50 minutes at physiological donor concentrations. Exposure to about 3 × 10 5 ergs/cm 2 blue light for 15 minutes leads to a transient reduction in the export. The export rate reaches a minimum about 25 minutes after the onset of illumination, then rises to reach a maximum by 35 minutes, and subsequently declines again. The result is a net export depression during the first 80 minutes, amounting to some 12 to 17%. Its timing closely matches the timing of the light growth response elicited by the same light dosage. At higher IAA concentrations (0.5 and 1.8 mg/l), the export rate reaches a peak about 60 minutes after the initial application of auxin, and thereafter declines rapidly. Light increases this decline in export rate, without causing peaks and troughs, and even at 0.25 mg/l IAA the transient changes in export appear to be superimposed on a gradual decline in export rate after illumination. Blue light is effective in all these phenomena; the red far-red system appears to exert no effect. The results are discussed in relation to the mechanism of action of light both in the light growth response and in phototropism.