The Construction of Plastochron Ordered Sequences in Flower Ontogenesis

Flora (1990) 184: 313-323 Gustav Fischer Verlag Jena

The Construction of Plastochron Ordered Sequences in Flower Ontogenesis A. RITTERBUSCH Biologisches Institut II, Abteilung Botanik, Albert-Ludwigs-Universitiit, Freiburg, FRG Key words: Calceolaria tripartita R. et. P., Scrophulariaceae, inflorescence, flower, plastochron, developmental stability, ontogenetic sequence construction, module.

Summary Direct observation of ontogenesis taking into account for the detennination of form and especially age of a single plant organ, e.g. a leaf or a flower, from primordium to maturity is technically not feasible. However, plants organized in a modular system allow one to observe module ontogenesis indirectly. All the modules, e.g. flowers, successively being generated and being present at a certain moment in a modular system, e.g. a racemous inflorescence, remain for the different developmental stages of the single flower. A precondition for the reconstruction of module ontogenesis, especially with respect to time and age, is developmental stability, or - as an alternative criterion - developmental steady state or developmental complexity. Because the plastochron concept applies to any module of a modular system a definite metric age in plastochrons can be assigned to each module stage. In this way, equidistant, i.e. in equal steps progressing, ontogenetic sequences can be derived. The construction of complete sequences from often partial, or sparsely staged or differently dense sequences is explained with simple models and illustrated with the flowering shoot and flower of a Calceolaria tripartita R. et P. (Scrophulariaceae). Methods assigning a metric age to ontogenesis are prerequisite to any comparison and analysis of temporal patterns in terms of heterochrony and for the comprehension of form as a spatial and temporal process.

1. Aims and problems Aims and problems instigating this methodological approach arise from a comparative study of the corolla forms in Scrophulariaceae (RITTERBUSCH 1976; MEIER-WENIGER 1977; LEONHARD 1986; WUNDERLIN 1985, 1988). In spite of the striking divergence in the mature corolla form, e.g. of foxglove (Digitalis purpurea), snapdragon (Antirrhinum majus), or mullein (Verbascum thapsus) an inclusive homology, also for the corolla, can be assumed. We hope to unravel both the constant, as well as the varying properties that determine the degree of similarity by a thorough investigation of ontogenesis by which the mature corolla is brought into being (RITTERBUSCH 1980a, 1980b; RITTERBUSCH & WUNDERLIN 1989). The flowers originate from nearly uniform primordia and their mature forms must be considered a consequence of local and momentary changes in acceleration and retardation in growth, and perhaps, of changes of order in development (LEINS 1964; ERBAR 1988). For any investigation, a necessary precondition is the exact quantitative and qualitative description of the spatial and temporal aspects of ontogenesis, i.e. of morphopoiesis (RITTERBUSCH 1982). In plant morphology, the temporal aspect, in comparison to the spatial aspect, is generally too much neglected. We have, therefore, tried to establish methods allowing for a precise measurement of time and age as well as for detection, it is hoped, of the specific temporal patterns of growth and development in flowers (RITTERBUSCH 1989).

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2. Preliminaries We would like to elaborate the method for the construction of ontogenetic series and squences. This mehod is meant, first, to apply to any modular system and, second, to assign a metric age to the stages. A plant that is organized by a more orless continuous repetition of a basic unit, i.e. a mod u Ie, may be considered - in part or as a whole - a modular system (SCHRODER 1987). For instance, the flowering shoot of a Philodendron forms a monochasium and is composed of a unit - or a module - consisting of hypopodium, node with prophyll, and a sheathless leaf with reduced internode ending with an inflorescence (RITTERBUSCH 1971; RAY 1988). A monopodial shoot or a racemous inflorescence may also be seen as the repetitive addition of a module consisting of internode, leaf and axillary bud, or of internode, bract and flower, respectively. A metric age is measured by a clock relying on a periodic or continuous process defining the time unit. If no metrization is implied, one speaks of relational (or topological) age (DEPPERT 1981). The time unit may refer to a physical process defining a physical age, e. g. by a calendar in days, or to a biological process defining a biological (or physiological) age (DORMER 1965; SCHOEPP 1966), e. g. by the number of leaves or flowers formed by a shoot or a raceme in plastochrons. Age sums up the time that passes between two reference stages; e.g. the younger reference stage may be a leaf of 10 mm in length, or a flower primordium the first sepal of which becomes visible, and the older reference stage with a leaf of 50 mm in length, or the flower with the corolla fully open. A well-known measure of a metric - biological, and not a necessarily physical - age is the plastochron (ASKENASY 1880; SCHOEPP 1923) or the plastochron index ofa shoot or of a leaf (ERICKSON & MICHELINI 1957; LAMOUREAUX et a1. 1978). This concept can be generally applied - if sufficient developmental stability is guaranteed - to any modular system, to any type of growth curve of the module, and to the measurement of biological and, in special cases, of physical age. In a sequence the ontogenetic stages are ordered as to metric age, in a series according to relative age.

3. Approaches 3.1. Direct vs indirekt observation One way of gathering data on ontogenesis is the direct observation of one and the same living specimen, e.g. of a certain flower. This is possible only in special forms or for larger stages. A serious handicap arises from the organs covering each other, as is the case for calyx and corolla, or corolla and stamina and pistil etc. At the moment, we do not see any feasible techniques for overcoming these difficulties and keeping the flower alive. Consequently, one has to rely on an indirect observation of different flowers of different developmental status. Indirect observation is based on the modular organization of a plant: the more or less numerous and successively produced modules represent developmental stages of the individual module (SCHOEPP 1966; RITTERBUSCH 1976). For instance, in some cases up to 250 floral stages of the inflorescense of the foxglove represent as many discrete steps of the continuous ontogenesis of a single flower from emergence to blossom. The method of indirect reconstruction of ontogenesis requires high developmental stability of the module, which means that neither external nor internal fluctuations should influence the stages differentially. So far little is known about developmental stability in plants on the level of organs. In lieu one may consider complexity as a criterion of developmental stability. E.g. the specialized zoophilous flower of most Scrophulariaceae should exhibit rather high developmental stability (BERG 1959; RITTERBUSCH 1976), because it must fit the pollinator rather precisely at maturity.

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3.2. Series and sequences On the basis of the above-mentioned preconditions it will be possible to establish ontogenetic series comprising the different stages in view of their relative ages, i. e. in a younger - older order only. Our most prominent aim, however, is the construction of ontogenetic sequences from stages to which a metric age (either biological or physical) is assigned, i.e. which are arranged due to a howmuch-younger-or-older measure. Consider the foregoing example of the foxglove: following the concept of the plastochron in its generalized form an age in plastochrons is assigned to each floral stage, with two consecutive stages differing in age by one plastochron. The question arises how one may be sure that the time and age measuring clocks run constantly or, in other words, are linearily related. This is not the place to discuss such an epistemologically difficult question (DEPPERT 1981); without any criterion on this point, however, statements on temporal patterns are easily declassified as mere artefacts of irregular clocks. Again, a necessary precondition is the developmental stability, in this case of the modular system, e.g. the inflorescence, or, more precisely, of the module generating center, e.g. the apex. The following criteria ought to be considered: (1) Only morphologically equivalent modules are to be taken into account, (2) the development of the modular system on which modules are produced should be open, and (3) the modules should be formed in a stationary or steady state phase of development of the modular system. Ad (1): the plastochronal age, e.g. ofa flower, depends on the pattern of flower formation in an inflorescence; therefore, the morphological equivalence, or homology, of the flowers ought to be stated. The knowledge of the "typus" of an inflorescence (TROLL 1964; WEBERLING 1983; SCHRODER 1987) is often of great help but it is not a prerequisite. For example, homology and typus of the inflorescence of a Calceolaria are not easy to prove (ANDERSSON & MOLAU 1980; MOLAU 1988) whereas the regular mode of flower arrangement and succession can be quite easily detected (RITTERBUSCH 1976) Ad (2): The development of many inflorescences, e.g. of a raceme, is open for at least some time. The direct determination of biological age in inflorescences with closed development, e.g. of a capitulum, is problematic (ERICKSON & MICHELINI 1957); they are treated in analogy to single flowers. Ad (3): With regard to the plastochron, an "oscillating steady state", i.e. a steady state of repeated and periodic organ formation, must be ascertained and flowers developing during non-steady state should not be taken into account. One should avoid, e.g., flowers developing along with the change from vegetative to reproductive state or at the end of the flowering period, i.e. flowers at the lower end or at the abnormal looking - sometimes elongated, sometimes truncated - upper end of an inflorescence. Steady state is usually guaranteed in an artificially controlled optimal environment and any externally measurable parameter, e.g. flower bud length, may be used for the calculation of the duration of one plastochron in physical time. Many species, however, are difficult to raise in a phytotron and one has to make use of field-grown plants. Then one may consider constant rei at i v e developmental densi ty of a flower (or a module) as a criterion ofthe developmental stability of the inflorescence (or the modular system). This means that the number of stages occuring between two distinct reference stages is constant in a developmentally stable modular system. Thus, the number of flowers of the raceme of the foxglowe between a flower bud of 6 mm length and the youngest flower with the corolla fully opened should be constant for some time. This can usually be assumed to be the case if the environment does not change beyond the limits to which the plant is adapted, and if the time of development of the flower is short in relation to that of the inflorescence. Unlike an artificially controlled environment, one measures age in biological time, i.e. in plastochrons. The plastochrons as compared to physical time may be different from the fluctuations of the environment.

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4. Models and cases 4.1. Combining complete sequences As an example of the construction of an ontogenetic sequence, one should reconsider the raceme of the foxglove. One starts - for practical reasons - with the preparation of an oldest flower that shall be included in the sequence; it is assigned the preparation number 1, its younger neighbour, the number 2, and so forth; the youngest flower to be included in the sequence gets the number n. The successively prepared flowers are fixed and stored. Each flower, or only every second, third or i-th, is processed, e.g., for whole mounts, microtome sections or SEM. From these preparations, diverse parameters are measured for the construction of growth curves in size vs preparation number offlower, i.e. in size vs plastochron plots. The construction of sequences in selected cases is shown with the linearized growth curve of one parameter for reasons of simplification. However, for a more precise and reliable evaluation of data two, three or even more parameters and growth curves, respevticely, should be used. Eventually the different procedures, as explained for the special cases, will have to be applied in combinations. Case 1: An inflorescence covering the full range of ontogenesis with only a few flowers and, therefore, wide gaps between the successive stages of a sequence may be combined with another inflorescence or interpolated into another sequence, whereby the flowers differ in their developmental status. In the case of equal relative developmental density, i.e. of equal slope of the growth curves (in our simplistic example), the stages of both the inflorescences differ in age by some constant fraction of one plastochron (Fig. 1).

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Case 2: Inflorescences may produce different numbers of flowers between two reference stages and exhibit different relative developmental density. As a consequence, stages of identical developmental status are given different ages, and growth curves are given different slopes. Flowers and inflorescences can be compared to each other or be combined into one sequence with reference to age only if age is transformed linearly by multiplication with the quotient of the ages of two developmentally identical stages of each inflorescence (Fig. 2).

4.2. Interlinking partial sequences Case 3: If two or even more inflorescences do not cover the required range of ontogenesis, one may interlink the partial sequences into one complete sequence. This is possible ifthe growth curves overlap at their ends in at least two stages (Fig. 3, curve 1). The overlapping end must display the same slope, otherwise the curves must be transformed in advance (see case 2).

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Case 4: If growth curves of two partial sequences concerning a certain parameter show zero growth, i.e. the slope in our example is zero in the overlapping ends, then one has to resort to the growth curve of an additional parameter which - during this phase - continues to grow monotonously. This precaution should also be considered if the slope is very flat (although not zero) (Fig. 3, curves 1 and 3).

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Fig. 3. Interlinking two partial sequences a and b of equal relative developmental density. The slopes of the partial growth curves, Fig. 3.1 and Fig. 3.2, of the parameters 1,2 and 3, respectively, are equal; the growth of parameter 3 stops temporarily. Sequences a and b are interlinked with respect only to curve 1 by their overlapping ends, with respect only to curve 2 by extrapolation bridging the gap between the last and the fIrst stage of a and b, with respect only to curve 3 a and b cannot be interlinked because the slope of the overlapping ends is zero, and another growth curve I additionally must be taken into consideration.

Case 5: Circumstances may allow only for non-overlapping growth curves. In cases of equal relative developmental density, i.e. of equal slopes at the neighbouring ends of the curves, one may bridge the gap by extrapolation (Fig. 3, curve 2). Eventually one has to refer to the foregoing cases as well.

4.3. Smoothing the bends due density dijJerences The more or less sudden change of slope of a single growth curve - and even more the correlated change of many curves - may bring up the question whether the change is the expression of intrinsicly phased development or rather the reaction of extrinsicly induced growth. This problem can be solved if one compares the growth curves of the sequences of inflorescences grown either in

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variant or - contrarily - in constant environmental regimens. If in variant conditions, the change of slope is always correlated with the same developmental status and is also observed in constant conditions, it is intrinsic; otherwisde it is extrinsic, and one may equalize the contingent different slopes according to the procedure discussed in the course of the foregoing cases.

4.4 Interpolating the single stage Having established a complete sequence - the stages of which are assigned definite (plastochronal) ages - it is now possible to insert a single flower and to determine its age. The same size parameters already used to construct the growth curves are measured within the single flower and interpolated "by eye" or by some computational procedure minimizing the deviation with respect to size. The measurement of one parameter may be sufficient, however, - although usually small - with regard to deviations from the curves, two, three or more size parameters should be taken into account. In comparative studies one often needs one or even two reference stages, as is the case with the application of the normalized-plastochron concept (RITTERBuscH 1976, 1989) for the detection of temporal patterns of ontogenesis in different taxa. In practical work, however, one often has to deal with sequences the youngest (and/or the oldest) stage of which does not coincide exactly with the independently defined reference stage. If the sequence starts with a younger stage, one interpolates the reference stage (as a single flower) and then assigns to that the age of 0 plastochrons (see Fig. 5), consequently the younger neighbouring stage receives a negative, the older neighbouring stage, a positive value in plastochrons. If the sequence starts with an older stage one has to extrapolate the growth curves (in the direction from older to younger stages) and than interpolates the reference stage, as before, as a single stage.

4.5. Remarks Instead of proceeding via growth curves based on one or a very few selected size parameters, one may refer to the Bildscharen method (RITTERBuscH 1980a) ., making direct use of the whole pictures of the stages. Bildscharen would also allow for the graphic (computer aided) construction of stages of a certain age, for example, of a youngest or oldest reference stage and consequently of sequences the stages of which represent exactly equal steps of ontogenesis (starting from the reference stage of zero age). The discussed procedures also apply to series or to correlation curves i.e. to size vs size plots (DoRMER 1965) with one size being selected as a reference size. Results from correlative studies are of eminent importance, however, they reflect relative, but not metric time and age. It is unnecessary to discuss in detail that the proposed method includes correlation studies and refers to both the relative and metric time and age. Correlation curves can be transformed into growth curves if the time course of the reference, or any other correlated size parameter is known. Ontogenesis is a spatial and temporal process. If one argues that developmental stability is restricted to the developmental constancy ofform of the module, one may consider the variability of the modular system with respect to time and age, e.g. if the order of two interpolated sequences is not consistent with the developmental status of the stages, i.e. an "older" stage precedes a "younger" stage, then one may take into consideration a varying plastochron and a non-steady state phase of development (consult for details LINDENMAYER 1984). If however, on the other hand, it is restricted to the constancy of age of the modular system, one may consider the variability of size and form of the module. The methods will allow for the analysis of developmental stability by a stress experimental approach. If the whole inflorescence is treated, a subsequent analysis of the derived ontogenetic sequence will show, if at all , when and where and how the stages are affected. A possible result could be that the highly specialized animal pollinated flowers may prove more stable as compared to wind pollinated flowers, and that the pollen-producing stamens and the seed-producing pistil will come out as the most stable parts of the whole flower.

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The interpolation of a set of curves of a sequence b into another set of curves of a sequence a by optimal fit, e.g. the two curves I and 2 of the sequences A and B of Fig. 5, may be performed with the help of a computer program or graphically "by eye", by drawing the (if necessary slope-adapted) set of curves B asterisk, on semitransparent paper that is moved along the abszissa until the curves of B asterisk conic ide optimally with the curves of A. All the graphic and computational procedures enumerated in this chapter on models and cases are speeded up enormously by computer assistance.

5. Example We illustrate the method with the flowering shoot of Calceolaria tripartita R. et P. of which the ontogenesis of the flower is shown elsewhere (RITTERBUSCH 1976). It is much more complicate than a raceme, e.g. of a foxglove, thereby allowing one to evaluate the limits of theoretical applicability and practical feasihility of the method.

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Fig. 4. Partial flowering shoot of Calceolaria tripartill1 R. ell'. Fig. 4.1 shows the scheme with the obliterated apex of the main axis E (at asterisk), two branches C and D developing from axillary buds of the last pair of leaves of E, and further more or less developed branches and resting axillary buds may be found (arrows). C becomes reproductive producing always a pair of flowers, the older terminal (T·) and the younger front (F·) flower. Two branches, A and B, develop from axillary buds from the subtending paired leaves (bractoles). B forms a monobracteolate monochasium (mB), A changes from a dibracteolate (dA) to a monobracteolate (rnA) monochasium. Fig. 4.2 Simplified diagram of Fig. 4.1 showing obliteration of apex (asterisk) of the main axis (E) and last pair of leaves (bbO, bO) with developing axillary buds corresponding to branches C and D (aO). The bracteole (bbl) with a resting axillary bud (a I) is always smaller than the bracteole (b I ) of which the axillary bud continues and extends the monochasium; with the change from two to one bracteole the smaller is totally reduced.

Fig. 4.1 is the scheme of the flowering shoot of a certain plant. (A detailed description is found with the legend.) It is not the point here to analyze the flowering shoot in terms of a certain classificatory or typological system of inflorescences (see WEBERLING 1983) but to detect - with respect to the construction of a ontogenetic sequence of the flower - the repetitive unit of the module making the regular organization of the modular system.

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The axis of the early monopodial shoot stops developing by inactivation and obliteration of the apex (Fig. 4 asterisk). The later sympodial shoot develops from one to several axillary buds. A lateral branch may form a pair or a single of leaves and either continue vegetative development forming further leaves or turn to reproductive development terminating in always a pair of flowers. Usually, soon after the change from vegetative to reproductive development quite regularly organized flowering branches appear. These are - in the case of Calceolaria tripartita, but not generally in Calceolaria (MoLAU 1988) - monochasia with the pecularity of the older terminal (T-) flower always being accompanied by a younger front (F-) flower, both flowers being oriented in the same direction, perpendicular to the pair or single of leaves, i.e. the bracteoles.

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The monochasial flowering portion of a branch is considered the modular system made of the module consisiting ofintemode, node with two or one bracteoles, the terminal flower and the front flower; the larger of the two bracteoles bears a resting bud of another module or a monochasium respectively, while the bud of the smaller bracteole produces the next module, and so forth, thereby establishing the monochasium (see Figs. 4.1 and 4.2). With the switch to monochasial organisation a steady state of development can be assumed - at least for some time and with tolerable deviation. Strictly speaking one only should take into consideration the monochasia forming one bracteole per node with the opposite smaller bracteole being totally suppressed. With regard to the paired flowers only the terminal at one hand and the front flower at the other are morphologically equivalent, and the two may serve for the construction of two independent sequences of flower ontogenesis. The

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two sequences can also be combined into one. The developmental distance of terminal and front flower is in our example half a plastochron (in the sequences A and B, Fig. 5) and is calculated by the interpolation of the front flower sequence into the terminal flower sequence (or vice versa, see Figs. 5.1 and 5.2). In this case T- and F-flowers differ by half a plastochron, thereby allowing for the construction of one sequence progressing in steps half as large and still equal. The more interesting aspect is that the bifloral monochasium behaves like a raceme in organization as well as with respect to the successive effloration in equal steps. Further examples of ontogenetic sequences of flowers of other species illustrated with about 20 stages may be found elsewhere (see: Aims and problems). From such sequences chronological tables or graphs may be prepared, listing the developmental events in correlation to age (RITTERBUSCH 1976). Tables and graphs of different species in which the (plastochronal) age is normalized (RITTERBUSCH 1989) help to detect the relative temporal patterns of heterochrony, of acceleration and retardation, etc. (MA YERS& LORD 1984; CANNE-HILLIKER 1987; TUCKER 1988) and allow for a better analysis of form as a spatial and temporal process.

References ANDERSSON, L., & MOLAU, U. (1980): The inflorescence of Calceolaria. Bot. Notiser 133: 21-32. ASKENASY, E. (1880): Ober eine neue Methode, urn die Verteilung der Wachstumsintensitat in wachsenden Teilen zu bestimmen. Verh. naturk.-med. Ver. Heidelberg N. F. 2: 70-153. BERG, R. L. (1959): A general evolutionary principle underlying the origin of developmental homeostasis. Amer. Naturalist 93: 103-105. C-\NNE-HILLIKER, J. (1987): Patterns of floral development in Agalini' and allies (Scrophulariaceae). II. Floral development of Agalini, densiflvra. Amer. J. Bot. 74: 1419- 1430. DEPPERT, W. (\ 981): Grundlagen einer Theorie der Systemzeiten. Allg. Zeitschrift Philosophie 6: 1-25. DORMER, K. J. (1965): Correlations in plant development: general and basic aspects. Encyc/. Plant. Physiol. 15, I. Springer. Berlin. 452-478. ERHAR. C. (1988): Early developmental patterns in tlowers and their value for systematics. In: LEINS, P., TUKER. S. C. & ENDRESS, P. K. (eds.): Aspects of Floral Development. Berlin, Stuttgart, 7-23. ERICKSON, R. 0., & MICHELINI, 1'. 1. (1957): The plastochron index. Amer. 1. Bot. 44: 297-305. LAMOREAUX, R. 1., CHANEY, W. R., & BROWN, K. M. (1978): The plastochron index: A review after two decades of use. Amer. J. Bot. 65: 586-593. LEINS, P. (1964): Das zentripetale und zentrifugale Androeceum. Ber. Deutsch. Bot. Ges. 77: 22-26. LEONHARD, 1. (1986): Untersuchungen zur Entwicklungsgeschichte der Bltiten von Scrophulariaceen - Pseu"O/y.lilllll,.hi,,/l SpiClIIUIll OPIZ. Diplomarbeit, Fakultat Biologie, Universitat Freiburg. LiNDENMAYER, A. (1984): Positional and temporal control mechanisms in inflorescence development. In: BARLOW, P. W. & D. 1. CARR (eds.): Positional controls in plant development. Cambridge. 461-486. MAYERS, A. M. & LORD, E. M. (1984): Comparative flower development in the cieistogamous species Viola odorata. III. A histological study. Bot. Gazette 145: 83-91. MEIER-WENIGER, E. (1977): Untersuchungen zur Entwicklungsgeschichte der Bliiten von Pedicularis foliosa L. und P. recuritll L. (Scrophulariaceae). Dissertation, Universitat Base!. MOLAU, U. (1988): Scrophulariaceae, part!. Calceo/arielle. Flora Neotropica 47: 1-326. The New York Botanical Garden; New York. RAY, T. S. (1988): Survey of shoot organization in the Aracelle. Amer. 1. Bot. 75: 56-84. RIIIERBlJSl'H, A. (1971): Morphologische Untersuchungen zur Wuchsform von Philodendron. Bot. lahrb. Systematik 90: 527-549. - (1976): Die Organopoiese der Bliite von Calceolarill trip"rtita R. et P. (Scrvphuillriaceae). Bot. lahrb. Systematik 95: 267-320. (J980a): The modelling of growth and development, a transformational approach to l10ral ontogenesis. Hora 169: 498-509. (I 980b): The spatio-temporal patterns of growth and development in noral ontogenesis as visualized by bildscharen and trajectories. Flora 169: 405-423. (1982): Wachstum und Entwicklung als Begriffe der Morphologie. Ber. Dtsch. Bot. Ges. 95: 127-131. (1989): Comparison of temporal patterns in flower ontogenesis using a normalized age. Ann. Bot. 64: 179-183.

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