Rest andactivityinvegetativebuds of treesP.ChampagnatrueLaboratoire de

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Rest and activity in vegetative buds of trees P. Champagnat rue Laboratoire de Phytomorphog6n6se, Universite Blaise-Pascal, 4, Ferrand, France Ledru, F-63000, Clermont- Introduction Rest of buds, particularly on trees or shrubs, may have 3 different origins: 1) unfavorable environmental conditions (for instance low temperatures); in this case, the bud is called ’quiescent’; 2) correlative inhibitions exerted by other organs or parts of the plant, more or less distant from the considered bud. Those may be called ’long distance correlative inhibitions (LOis)’; the apical dominance exerted by the terminal bud over axillary buds on growing shoots is the most studied example of LDI; 3) some intrinsic properties of the bud itself, such as...

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Nội dung Text: Rest andactivityinvegetativebuds of treesP.ChampagnatrueLaboratoire de

Rest andactivity invegetativebuds of treesP.ChampagnatrueL aboratoire de
buds of trees in Rest and activity vegetative P. Champagnat Ledru, F-63000, Clermont- Laboratoire de Phytomorphog6n6se, Universite Blaise-Pascal, 4, rue Ferrand, France For over 50 years, investigations on bud Introduction rest have mainly focused on plant growth regulators, and their possible involvement Rest of buds, particularly on trees or in apical dominance or in dormancy. They shrubs, may have 3 different origins: 1) did not lead to generally accepted unfavorable environmental conditions (for conclusions (Samish, 1954; Champagnat, instance low temperatures); in this case, 1965; Wareing and Saunders, 1971; the bud is called ’quiescent’; 2) correlative Phillips, 1975; Saunders, 1978; Hillman, inhibitions exerted by other organs or 1984). A few papers (Vegis, 1964; 1965a, parts of the plant, more or less distant and Usciati Guern 1975; 1972; b; from the considered bud. Those may be Mcintyre, 1977) and 1974; Miginiac, called ’long distance correlative inhibitions especially recent reviews by Mauget (LOis)’; the apical dominance exerted by (1987), Tamas (1987) and Powel (1987) the terminal bud over axillary buds on have emphasized the necessity of setting growing shoots is the most studied the problem on much wider biochemical example of LDI; 3) some intrinsic prop- and physiological bases. That will be the erties of the bud itself, such as when the aim of the present paper, with emphasis bud remains at rest even if all usual on the following aspects: 1) the structural sources of correlative inhibition have been complexity of a bud, and the tightness of removed and if environmental conditions its connections with its bearing axis are favorable; in this case, the bud is (Champagnat, 1988); 2) the state of called ’dormant’ (Champagnat, 1983). nutritional deficiency of a resting bud, In fact,taking into account the structural either during dormancy or during cor- complexity of most buds, and the strong relative inhibition of growth, and the fact interrelationship they maintain with their that, during growth recovery, the different shoot axis, we will show that the rest of deficiencies are eliminated in a precise the meristematic region and related and determined order; 3) the short tissues depends upon ’short distance distance control (SDI) of nutrient fluxes inhibitions (SDIs)’ exerted either by the towards the meristem cells in buds, by bud’s basal tissue or by the adjacent changes in membrane permeability of shoot-axis tissues. some adjacent but distinct cells; 4) the fact
a modification of the lipid composition of that this latter SDI is strongly related to the plasmalemma, which in turn causes a longer distance correlations (LDI during ) S normal inhibitions, whereas it is acting modification of membrane permeability alone and is very stable during true (Usciati et al., 1974). Moreover, change a in membrane-bound Mg ATPases is + 2 dormancy. rapidly set in motion and the cation We will examine 4 examples which have content of cells is altered (Habricot and been the subject of recent work, Sossountzov, 1984; Sossountzov and particularly in France: apical dominance, Habricot, 1985; Sossountzov et al., 1985). rhythmic growth of oak seedlings, and Near the insertion point of the axillary induction and release of dormancy in buds bud on its axis, that is near its base, an on trees and on some tubers, the latter inversion of the polarity of cells in showing physiological and biochemical conducting or proconducting tissues has features which may be compared to those been observed by many authors. The of tree buds. transverse permeability of these cells is weak or even nul in the presence of the basipetal auxin flux. After disappearance apical dominance Some facts about of the auxinic flux following decapitation, the permeability increases. The Ca ions + 2 Growth inhibition of axillary buds is could play an important role in this controlled by a dominant bud, often by the mechanism (Sachs, 1986; Tamas, 1987). apical or terminal bud which has produced The presence of conducting tissues in the them. Ablation of this terminal bud lateral bud is not necessary (Ali and promotes the resumption of growth by Fletcher, 1970). axillary buds on the shoot. The auxins People who do not think that the flux of which are produced by the terminal bud, auxins may play a major role in apical and which translocate basipetally in the dominance should test if nutrient diversion stem, are thought to be a major agent of alone (sink effect) would be able to induce this LDI. Nevertheless, we should also this weak penetrability of molecules and take into consideration the fact that the ions into inhibited buds. Inversely, the dominant organ acts like a very strong supporters of a purely hormonal hypo- sink for a lot of nutrients and some plant thesis should demonstrate that this ability growth regulators (e.g., cytokinins). could not exist without auxins. Inhibited buds suffer from different kinds In short, and despite the holes in our of nutritional deficiencies: water, auxins, knowledge, we may emphasize the cytokinins, energy-rich nucleotides, but following observations (Fig. 1): 1 ) an also soluble saccharides, polyholosides, inhibited axillary bud is unable to draw the etc. It has been possible to induce their nutrients essential to its growth from the growth without cutting the dominant bud, nutrient fluxes, from storage even or through direct supply of water (Guern and compounds in surrounding tissues; 2) auxins (Sachs and Usciati, 1976), particular properties of certain cells cytokinins, the latter Thimann, 1964) or (probably on the surface of their acting through rapid modifications of membranes) located near the insertion carbohydrate metabolism (Usciati et al., point of the bu!d, are responsible for this 1972). incapacity; 3) thus, a short distance inhibition (SDI) appears in the bud; this Release from apical dominance through SDI is strongrelated to the apical decapitation is followed after 10-15 min by
command consists of a period of shoot through an LDI; it disappears flush quite rapidly after decapitation and release elongation (!12 d) followed by a period of from LDI. apparent rest (-9 d) (Fig. 2). The rest is only apparent and leaf primordia are continuously formed in the bud; further- plastochron is rather low more the in oaks Rhythmic growth between d 12 and 20 (0.4-0.5 primor- ) 1 dia-d- and higher between d 4 and 8 bud-burst (1.5-1.8 primordial) after Young oak seedlings (Quercus robur L.), (Payan, 1982). This periodic growth is grown at 25°C, under continuous illumina- independent of root growth, which is tion of 150 W!m-2, exhibit successive and continuous unlike in many other tree uniform growth flushes consisting of species (Mialoundama, 1985; Cham- morphogenetic units; however, the apical pagnat ef al., 1986b). bud alone develops. A growth flush lasts about 3 wk (21.8 ± 0.9 d in the case of the Between d 7 and 21 (end of flush), the 3rd flush after germination, which has shoot bears a scaly terminal bud enclos- been the most extensively studied). Each ing an increasing number (from 2 to 10) of
Nevertheless, it may be observed that young leaf primordia, first with aborting and afterwards with normal lamina. These during the first days of apparent rest, the leaves and their internodes cannot grow apical bud exhibits a deficient metabolism, normally; they are inhibited, like axillary very similar to that of a bud under apical buds subjected to apical dominance. dominance. It is very poor in ATP, and, overall, it is unable to synthesize enough The subject of the following discussion non-adenylic nucleotides (NTP), which are will be that period of strong growth essential for the translocation of some inability, and not the correlative inter- important molecules (UDP-glucose, for actions between leaves and the leader instance). At the end of the rest period bud, which are active during the growth and without any change in the envi- period, and which present the features of ronment, the ATP concentration increases an LDI (Champagnat et al., 1986a). 3 times and that of NTP 4 times. This The LDIs involved in this regulation spectacular modification of energyc acropetal influences and not have metabolism appears 1 or 2 d before bud- basipetal ones, like those acting during burst (Table I; E3arnola et al., 1986b). apical dominance. The mechanisms of If we measure with DMO (5,5-dimethyl- their action are unknown but, nonetheless, oxazolidine dione) the intracellular pH it is not satisfactory to consider them only (pH in buds and in the shoot axis ) i as an auxin-induced polarization of peri- immediately below them, we observe that, or provascular tissues.
it would be interesting at the beginning of the rest period, the pH apical dominance, i in bud cells is lower (more acidic) than in NTP-synthesizing ability and, to measure oak resting apical buds axial cells; the difference may reach 2-3 in the of on case pH gradients in pH units. One or two days before bud- monitor i seedlings, to lipid composition of mem- burst, this gradient is inverted. It is now addition to the branes and Mg ATPases, -associated + 2 well known that the pH whose regulation , i could be hormone-linked, is an essential and thus identify in both examples the whole spectrum of nutritional and hor- element in the regulation of short distance nutrient transport between cells. Transport monal deficiencies. always moves from tissues or cells with We would like to underline the fact that, the lowest pH to those with the highest i in both cases (rhythmic growth and apical (cotransport protons-K protons-sucrose, , + dominance), there is an association be- etc.) (Delrot and Bonnemain, 1981; tween LDIs and SDI Growth regulators . S Spanswick, 1981; Guern et al., 1982; could step in for the latter, which act as Maslowski et al., 1984; Matsumoto and relays of the former. Yamaha, 1984; Sze, 1984; Marre and LDis evolve spontaneously with size Ballarin-Denti, 1985). These deficiencies and age of leaf lamina (Champagnat et lead to true dormancy, under our never al., 1986a); this characteristic explains the and remain indefinitely of conditions, endogenous nature of the rhythm. It is correlative origin (Champagnat et aL, likely that this evolution initiates the rapid 1986a). modification of the SDI as does deca- , S In case, at the beginning of rest our pitation during apical dominance. SDis (and probably several the days before), always exhibit poor stability. bud is unable to draw from the axis the nutrients essential for its growth. Afterwards, it acquires this ability and may burst and grow. Causes of this transforma- Dormancy induction and release in tion may lie in the modification of leaf buds current year shoots of trees on , S LDI which may parallel lamina evolution (Fig. 3) (Champagnat et al., 1986a; Preliminary remarks Gendraud and Lafleuriel, 1983; 1985; Barnola et al., 1986b). A parallelism with a bud subjected to apical dominance and to the definition of bud dor- According gave in the introduction, this afterwards released from it, may be we mancy stage of bud development follows period reasonably envisaged. In the case of a
of active correlative inhibitions, which may plants under apical dominance. We will be necessary for the induction of true not consider here the case of sylleptic axillary shoots (often called ’anticipated’ dormancy (Libbert, 1961; Champagnat 1955). The scaly and dormant buds do not shoots), which originate from spontaneous show any appreciable elongation of their releases of apical dominance, and leaves or their internodes. Yet they are not consequently the disappearance of the in a complete state of rest, as they still SDI associated with trophic deficiencies. exhibit active organogenesis and even The complete developmental cycle slight but significant growth for a few described below is illustrated in Figs. 4 weeks. This rest is of the same nature as and 5. indeed, many authors accept that that seen in the apical bud of an oak different kinds of correlative inhibitions seedling, as described previously, and that appear successively in trees between the in axillary buds of many herbaceous
stage of bud-burst and the beginning of short internode, and if this ’node-cutting’ is dormancy, in contrast to annual placed in a favorable environment, the true herbaceous plants, which have been used bud will burst and resume growth after a delay of about 8 d at 20-25°C. If we most often for studies on apical domi- accept that the wound due to cutting plays nance. Typical apical dominance may only be recorded during the first weeks of no major role in this process, which is spring: decapitation alone initiates the difficult to demonstrate, we may postulate resumption of growth of axillary buds. that the bud is under the influence of a Later, decapitation has to be accompanied correlative inhibition originating from the by complete defoliation to enable a few bearing axis (Champagnat, 1965; 1983); quiescent axillary buds to resume growth. in fact, during this period, the axis exhibits From July-August on, even this drastic active secondary growth, intense lignifica- treatment becomes totally ineffective. tion and production of storage compounds Despite this increasing inability of axillary (hardening process). The axis therefore buds to react to decapitation and acts as a sink. we consider that real dor- defoliation, Later, between mid-September and mid- has not yet begun: in fact, if an October, depending upon climate condi- mancy axillary bud at this stage is isolated on a tions and tree species, the node-cuttings
and resuming growth, Some specifics and hypotheses reacting stop of the environmental conditions regardless and time of exposure used. We may Despite the lac:k of extensive studies on therefore conclude that dormancy has interrelations between a bud and the been induced at this moment. This adjacent axis tissues, we may hypothesize induction is progressive but rapid: only 2 the following succession of events (Fig. 6). wk are necessary to pass from 10 d of 1) Between March and September, long burst delay at 25°C to complete inertia. distance correlations (LDI are respon- ) S This delay in bud-bursting on node- sible for the relative quiescence of the cuttings is used as a measure of the buds. The SDI which act as relays for , S intensity of dormancy and its evolution the LDI must be permanently active. It is , S during winter (Fig. 5) or during expe- not obvious if the influence of auxins, rimental chilling. In temperate climates, via tissue polarization, remains deter- this delay diminishes from December on minant during the whole period, even if, in and becomes quite weak in January. Later this regard, the axial cambium may take on and until the spring bud-burst (March- the succession of young leaves from the April), we may qualify the end of the rest apex. The fact that no significant crisis period as post-dormancy, the absence of may be recorded during the replacement significant growth being due of one inhibition source by the next one any exclusively to low temperatures (Nigond, (apical bud, then leaves and finally axis), strongly supports the hypothesis of the 1967; Mauget, 1976; 1987).
permanency of the SDI. The stability of sympodic growth has no influence on or the membrane structures associated with the fact that the bud has to evolve this SDI could progressively increase from to the the according pattern one same as spring onwards. 2) The correlative com- the apex of a young oak tree grown at on mands seem to progressively approach 25°C. However, rest is a temporary stage the inhibited bud meristem and to become for the young oak seedling, but will diffuse (global action of leaves or shoot become permanent and will require a axis). The probability that, on one-node chilling period to be removed in case of cuttings, these commands are identical true dormancy. It is interesting to note that with SDIs is weak; in fact, the evolution dormancy generally appears later and is from an extensive ability of bud-burst more profound for terminal buds than for toward a very weak one may as well be axillary ones. due to an increase in the intensity of the inhibitory action of the stem as to a Some experimental results stabilization of the structures responsible for the SDI. This stabilization is currently the most characteristic feature of true The ability to synthesize non-adenylic dormancy, as will be emphasized below. nucleotides (NTP) has been measured in 3) Whether the species show monopodic ash buds and in adjacent tissues of the
the stem in respect to axis from September to March (Table 11; independent of Lavarenne et al., 1982; Barbola et al., NTPs and may grow without importing 1986a). During September, when 100% of them from the stem. At the end of October, the in.ability to synthesize NTPs node-cuttings may resume growth at 25°C, this ability is considerable in both reaches the bud; bud-burst is now almost tissues (bud and stem). A few weeks later, under favorable condi- impossible even During December and January, NTP it becomes very weak in stem tissues, tions. while it remains quite extensive in the bud, synthesis resumes in the stem, due to the which may therefore be considered to be influence of chilling conditions, while it still - w: on n
only slightly water and carbo- remains impossible in the bud. In addition, which is hydrate consuming. It is difficult to reduce will show that the bud is still unable to we this situation to a polarization of basipetal import NTPs because of an opposing pH i gradient. In February, a long time before translocations imposed by auxins and it would be hazardous to regard it as the spring bud-burst (which happens at the unique cause for SDI 3) The most . S end of April on our ash trees), the important fact remains that SDIs are September situation returns. essentially under the regulation of LDI . S The has been measured i pH gradient They disappear once the latter have been the period, in distal and during same released but with an increasing time lag. proximal parts of the bud and near its This fact is important, because this insertion on the stem (Fig. 7; Lavarenne increasing stability leads to a permanency and Barnola, unpublished observations). of SDIs even when LDIs are no longer Throughout autumn and winter and until 5 present; thereafter, a specific treatment or 6 d before spring bud-burst, the (chilling for instance) is necessary to gradient opposes a translocation flux remove them. This stability and this re- which would be essential for good nutri- quirement for an external release tional supply of the bud: the basal as well treatment are, to date, the most useful as adjacent stem tissues have a higher characteristics of a dormant state: what pH than the distal tissues in the bud. The i initially a simple relay strongly was difference may be as high as 0.5-1.0 pH by LDis has evolved into an controlled units. The inversion may happen very independent barrier. It may be associated rapidly (2-3 d max.), and is immediately with membrane properties of certain cells followed by a dramatic increase in water the bud. near content, which coincides with bud-burst (Cottignies, 1983). Two to three weeks after bud-burst, in the axillary buds of rapidly flushing shoots of ash trees, the in Dormancy induction and release gradient between apex and bearing stem tubers again becomes unfavorable to the apex. The bud differentiates, nevertheless, and as on oak trees, organogenesis continues Here will the work refer only to we until the final number of leaf primordia is conducted with Helianthus tuberosum at reached. the University of Clermont-Ferrand (Cour- duroux, 1967; Gendraud, 1981; Tort etaL, 1985). Apparently not known outside Discussion France (see Ewing, 1987), the following results provide some answers to the The results remain fragmentary and the questions we mentioned earlier for tree conclusions open to discussion. buds. are Nevertheless, a few facts may be outlined. 1) It appears quite obvious that nucleotide Tuberization in Helianthus tuberosum metabolism is a good marker for the dor- mant state of buds; further results ob- tained with tubers will support this point of We usually mention the concept of tuber view. 2) We may once more note that an dormancy and not tuber-bud dormancy. unfavorable pH gradient does not exclude i This detail is important: the subject of the slow growth, for instance, organogenesis, following report is the parenchymatous
tissue in the tuber and not the bud itself, different processes: the latter appears to as for trees. As a matter of fact, before the be controlled by reversible correlations. recent studies summarized above, stem A dormant tuber, even during the period tissues were only rarely regarded as of maximum rest intensity, may resume regulatory elements of tree dormancy. growth without releasing dormancy when placed under appropriate temperature The tuberization of a stolon, a long conditions. Its buds do not evolve as long underground stem with scaly leaves, shoots, but as new young tubers, directly occurs under the influence of correlations inserted on the ’mother’ tuber. This tuber exerted by the aerial shoots of the plant growth is very useful for assessing dor- (LDIs). As a matter of fact, when we mant or non-dormant stages of growing suppress these LDI we stop the tuber- , S tubers. A chilling treatment (or any other ization in progress and we initiate both the treatment of the same kind) may trigger inflexion of the terminal bud, due to dormancy release, and brings back the gravitropism, and its growth and transfor- ability to develop long leafy shoots. mation into a long shoot bearing as- The in vitro culture of buds attached to a similating leaves (Fig. 8). The tuber is considered to be dormant only later, when little pyramid of parenchyma produces either little dormant tubers from a dormant it reaches its final length; at this stage, even when isolated, it does not develop a explant or long shoots from a non-dormant explant, but als.o non-dormant tubers from growing shoot, but only a new tuber (Fig. 8). Dormancy and tuberization are quite non-dormant explants if the culture
Detailed studies of energetics, different medium exerts a correlative inhibition (due enzymatic systems and carbohydrate the presence of cytokinins, for in- to metabolism in dormant and non-dormant stance). These latter tubers may rapidly tubers have been made (Poole, 1978; develop long shoots when transplanted Spanswick, 1981; Maslowski ef al., 1984; into a non-inhibiting medium (Courduroux, 1984; Matsumoto and Yamaha, Sze, 1967). 1984; Marre and Ballarin-Denti, 1985; Petel, 1986; Petel and Gendraud, 1986). Some results Similar studies have never been con- ducted on trees. These investigations The parenchyma of a dormant tuber is provided very interesting new information. unable to synthesize NTPs. That from a We will focus on experiments run with tuber able to produce long shoots is cellular extracts, enriched with plasma- always able to synthesize them, regard- lemma, containing an ATPase which could less of whether it is a cutting from in vitro be isolated, purified and analyzed for plantlets or from a plant growing in the Michaelis characteristics (Table III). This field. This observation may be related to membrane-bound ATPase is different in the fact that the rate of biomass increase dormant tubers and in non-dormant, non- is much higher in a growing shoot than inhibited ones able to produce leafy during the production of a daughter tuber. shoots. In the first case, due to modifica- The pH in the parenchyma of a dormant i tions of the electron transporting system tuber is higher than that in a non-dormant and of the resulting particular energy tuber. It has been demonstrated that the metabolism, the cells of the parenchyma former exports stored nutrients with much export with great difficulty ions and more difficulty and that its buds are nutrient molecules towards the potential therefore poorly supplied (Gendraud and center of use constituted by the buds. Lafleuriel, 1983; 1985; Tort and Gendraud, They do it with more ease in the second 1984). It is a pity that the pH of the related i case (Fig. 9). The whole dormant paren- buds could not be measured. In fact, the chyma is a poor exporter, despite its theory supposes that it varies only slightly, richness in available storage compounds; and that it is lower than the pH of dormant i all cells are organized as a differentiated tuber parenchyma, and higher than the barrier between stem and bud, whereas in pH of non-dormant parenchyma, as in i trees only a few cell layers play this role. tree buds. These arguments are indirect and were easier to demonstrate with The most significant result seems to be trees, but the results are, nevertheless, the following: a young tuber, growing very similar in both cases. under correlative inhibitions originating
from shoots from the culture associated ATPase is replaced or rapidly leafy or medium, and therefore unable to produce properties. However, in the acquires new a leafy shoot, has the same biochemical of a dormant tuber, a chilling treat- case characteristics as a dormant tuber. In ment is necessary to slowly and progres- particular, the ATPase is of the ’dormant’ sively lead to the same result. type. The tuber exhibits a poor ability to What may therefore best distinguish a grow and not a specific property of a correlative inhibition from dormancy is the dormant stage per se. stability of membrane structures to which But there is a fundamental difference ATPases and other enzymes, for instance between both situations, and we have dehydrogenases, are bound. This stability stressed it several times: for a non- is weak during correlative inhibitions, that dormant tuber, the release from correlative is when SDis act as relays for LDis, but inhibitions (LDI immediately suppresses becomes very strong during dormancy; so ) S the inability to translocate nutrients to the strong, in fact, that only a specific induc- bud, in the same way as when a pea tion treatment, such as chilling, may make plantlet is decapitated or when the termi- the membranes return to their initial stage. nal bud of an oak seedling attains the end This hypothesis remains to be confirmed of apparent rest. The plasmalemma- by experimental evidence.
a tissue emits a signal, which is General discussion and conclusion or received and translated by another tissue or another organ whose behavior is thus Assuming that a resting bud is a bud modified. What is the vector of this suffering from nutritional deficiencies information? A hormone? The evidence during apical dominance, may be common supporting the latter is still weak. Indeed, How else could we describe the sense. auxins may be directly responsible for the physiological status of resting buds, while polarization of some neighboring cells at we no longer believe in the existence of the base of an inhibited axillary bud. But is specific inhibitors or real correlation such a mechanism present during all hormones, whose presence would para- correlative inhibitions? What happens lyze the cells they inhabit? The fact that when the ’source’ organs do not diffuse such nutritional deficiencies have been any auxin, as in adult leaves? How does discovered in oak seedlings with rhythmic abscisic acid act, if its accumulation is a growth and in some different cases of consequence and not the cause of the dormancy is not a surprise. But there is deficient stage, as in many stress still a problem to be solved: what are the situations? Concepts of this kind, that origins of these deficiencies and the were once used in plant growth regulator physiological mechanisms underlying studies are now being strongly criticized them? Are they identical in all cases? (Trewavas, 1981).But growth regulators To assume that short distance cor- surely act at the cellular level or at relations (SDI are present in all these ) S membrane interfaces, in order to control cases in only a first element of an answer. permeabilities and/or translocations. But It means that a meristem and the distal the auxins (for example) acting at this part of the bud, where active organo- level certainly do not originate directly genesis and elongation are taking place, from the apical bud! not independent ’self organizing are That SDIs associated with correlative centers’ as we had imagined, but that they inhibitions disappear immediately after the respond to external commands, produced disappearance of the corresponding LDIs by their surrounding cells, which, even seems normal after the above mentioned when they are active, only slightly trans- considerations. The signal for LDIs has to locate metabolites, and, therefore, isolate circulate permanently in order to remain other cells, which would be able to grow effective. In the case of dormancies, the actively. Some enzymes such as mem- SD[s remain active even while the brane-bound ATPases, could play a corresponding LDIs have completely determining role. Unfortunately, the loca- vanished. This stabilization of SDIs seems tion of these cells remains imprecise and to be the most significant characteristic of could vary for different physiological stage. Understanding the a dormant situations. Assuming the existence of well- mechanisms will be a fascinating chal- determined barriers does not fit in all lenge. cases; the whole tuber parenchyma or the stem tissue of a hardening tree may be like to recall that some Finally, we would involved. Also, the concept of short dis- kinds of LDI emitted by an organ or a , S tance is more a model than a concrete tissue, may be very short-lived (some 10 reality. min in Phyllanthus, Nozeran ef al., 1971 ), The primary interest of SDI is to but nevertheless induce long-term or somehow clarify the idea of a relay acting permanent modifications. These self- between correlative influences: an organ have, with maintaining states reason,
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