LEAF INITIATION RATES AND VOLUME GROWTH RATES IN THE SHOOT APEX OF CHRYSANTHEMUM

The leaf initiation rate and apical volume growth rate of lateral shoots of Chrysanthemum morifolium ‘Improved Albatross III’ were determined for plants growing under controlled environment conditions. After removal of the terminal bud, the top two lateral buds produced leaves at a rate of 1.4 to 1.6 leaves/day for the first 9 or 10 days. At about the 10th day there appeared to be an abrupt shift to a lower initiation rate of 0.7 to 0.8 leaves/day. Defoliation by periodic removal of leaves larger than 1 cm caused a statistically significant increase in initiation rate, but not to as high a rate as that of the first 10 days of bud growth. Volume growth rates of the apical region were determined from transverse sections of five apices, according to the method of Richards (1951). The apical region was found to have a radial expansion rate of 0.1623/plastochron and a vertical expansion rate of 0.0494/plastochron. The volume growth rate was calculated to be 0.3740/plastochron and the volume doubling time to be 1.85 plastochrons or 56 to 63 hr.     

LEAF AND APICAL GROWTH CHARACTERISTICS IN TRITICUM

Leaf initiation rate, leaf primordium growth rates, and apical volume growth rates were determined for seedlings of Triticum aestivum cv. Ramona 50 under controlled environmental conditions. Three leaf primordia are present in the caryopsis, and three more leaves are initiated within the first two weeks after germination with a mean plastochron length of 95.5 hr. Volume growth rates of the apical region were determined on six apices which had six primordia each. The mean radial expansion rate was 0.467/plastochron, and the vertical expansion rate was 0.457/plastochron. The volume expansion rate was 1.393/plastochron. The mean volume doubling time was 0.498 plastochrons or 47.1 hr.     

A Carbon 14 Study of Metabolism in Tall Fescue Seedlings Acclimating to Growth at Low Temperatures

The youngest fully expanded leaves of young plants of tall fescue(Festuca arundinacea Schreb. cv. S 170) were allowed to assimilate¹⁴CO₂ either (a) at the fifth leaf stage immediately beforetransfer from 17/14?C to 7/4?C (non-acclimated), or (b) aftertransfer from 17/14?C to 7/4?C at the fifth leaf stage, andfurther development in 7/4?C of one or two more leaves (acclimationfor one or two plastochrons). Controls were maintained in 17/14?C and allowed to assimilate ¹⁴CO₂ at the corresponding (fifth,sixth, or seventh) leaf stages. Allocation of 14C amongst sinksand biochemical fractions was analysed during a subsequent periodof growth at 7/4 ?C (cold) or 17/14 ?C (control). Allocationof assimilate to growing parts of the shoot was less than controlsin the non-acclimated plants transferred to the cold and morethan controls in plants acclimated for one plastochron. Afterthe imbalance induced initially by transfer from 17/14?C to7/4 ?C, acclimation for one or two plastochrons brought theallocation amongst sinks and amongst biochemical fates closerto the balance existing in controls. The main shoot and sidetillers differed in the time during acclimation when they becamemore like the controls. The percentage of assimilate in thesoluble carbohydrate fraction of non-acclimated plants and ofplants acclimated for one plastochron, was higher than in controls.The percentage of assimilate in the cell wall fraction was lowerin non-acclimated plants than in controls but the differencefrom controls after two plastochrons acclimation again was less.During the first plastochron after transfer to cold, diversionof assimilate to a reserve pool in sinks could be a factor limitingcell wall synthesis and growth and this limitation may be relievedwhen the reserve pool is ‘full’. Low temperaturereduced the percentage of assimilate in the protein fractionof growing parts of the shoot and the difference was found evenafter two plastochrons acclimation. Key words: Low temperature, metabolism, tall fescue (Festuca arundinacea Schreb. cv. S. 170)     

INTERNODE ELONGATION IN XANTHIUM PLANTS TREATED WITH GIBBERELLIC ACID PRESENTED IN TERMS OF RELATIVE ELEMENTAL RATES

Xanthium plants were grown vegetatively and their developmental stages were designated by a previously described plastochron index (PI). Internodes of plants, both treated with gibberellic acid (GA₃) and untreated, were marked with India ink and photographed during 3 successive days. The relative elemental rates of elongation d(dX/dt)/dX were estimated between 15.7 and 19.0 plastochrons. The rate of growth of the GA₃-treated internodes was at least twice that of the control. The emerging pattern of acropetal internode elongation was similar in both GA₃-treated and control plants. Only rates of growth were significantly higher in the GA₃-treated plants. The acropetal pattern of internode elongation was the opposite of the basipetal pattern observed in Xanthium leaves but followed the acropetal pattern observed in Helianthus and Phaseolus internode growth.     

Comparison of Leaf Plastochron Index and Allometric Analyses of Tooth Development in Arabidopsis thaliana

Abstract Two methods of analyses were used to investigate tooth development in serrate (se) mutant and wild-type Columbia-1 (Col-1) Arabidopsis thaliana leaves. There were almost twice as many teeth with deeper sinuses and two orders of toothing on the margins of serrate compared with Columbia-1 leaves. The main objective of this study was to test three hypotheses relative to the source of polymorphism in tooth development: (i) Teeth share similar growth rates and initial sizes, but the deeper teeth are initiated earlier in leaf development. (ii) Teeth share similar timing of initiation and growth rates, but the deeper teeth have a larger initial size. (iii) Teeth share similar timing of initiation and initial sizes, but the deeper teeth have a faster growth rate. Leaf plastochron index (LPI) was used as the time variable for leaf development. Results showed teeth in se were initiated at −27 LPI, 15 plastochrons earlier than those of Col-1. Serrate leaf expansion was biphasic, with the early phase expanding at half the relative plastochron rate of the later phase, which equaled the constant relative expansion rate of Col-1 leaves. Allometric analyses of tooth development obscured the interactions between time of tooth and leaf initiation and the early phase of leaf expansion characteristic of serrate leaves and teeth. Timing of developmental events that allometric analysis obscured can be readily detected with the LPI as a developmental index. Received 25 January 2000; accepted 17 March 2000     

Assumptions of Plastochron Index: Evaluation With Soya Bean Under Field Drought Conditions

Erickson and Michelini (1957) derived the plastochron index(PI) and a term sometimes referred to as the plastochron ratio(PR), as quantitative expressions of the vegetative developmentof plants. With the stable plant growth in environmental chambersand glasshouses, the assumptions used to derive these termshave been validated. However, more recently these expressionsare being used to characterize growth under the unstable conditionsresulting from the imposition of stress. This study examinesthe validity of the assumptions used to derive PI and PR forfield-grown soya beans [Glycine max (L.) Merrill] subjectedto drought stress. Under stress conditions, the assumptionswere not satisfied. In fact, observing change in PR appearedto be a good method for detecting drought stress in these plants.An alternate method for calculating PI based on a single, youngleaf was developed. This alternate method appeared to be a moresensitive indicator of changes in leaf emergence rate underunstable conditions. Plastochron index, plastochron ratio, Glycine max (L.), soya bean, drought, leaf growth     

FLORAL INDUCTION DOES NOT ALTER THE GROWTH PARAMETERS OF FUCHSIA

Floral induction by night interruption of Fuchsia hybrida cv. Lord Byron, a quantitative long-day plant with decussate phyllotaxis and an indeterminate flowering habit, altered neither the rate of leaf initiation nor the rate of leaf expansion; nor did flower initiation and development change the vegetative growth of the plants. This was diagnosed using plastochron duration and plastochron ratio measurements before, during, and after a 10-day induction period. A comparison between indeterminate and determinate flowering is made using these two parameters.     

PETIOLE DEVELOPMENT AND XYLEM DIFFERENTIATION IN XANTHIUM REPRESENTED BY THE PLASTOCHRON INDEX

Petiole development and formation of xylem vessels have been investigated in Xanthium leaves from early ontogeny to maturity. Kinetics of growth was presented in terms of absolute and relative elemental rates of elongation. The process of vascularization was assessed by the number of differentiated xylem vessels. The leaf plastochron index (LPI) developed by Erickson and Michelini (1957) was used for designating the various stages of development. An exponential increase in petiole length was observed between the LPIs –3 and +4 indicating a constant relative rate of 0.20 or 20% increase per day. After cessation of lamina elongation at LPI 8, petiole elongation continued for an additional 5 day period, to LPI 9.5. Relative elemental rate analysis revealed that the basipetal pattern of elongation was maintained throughout the leaf development. At a specific plastochron age, the only growth was due to the petiole elongation. Leaves which ceased elongating had not completed their internal development, since the process of xylem formation continued for several plastochrons, or about 8 days. The highest rate of xylem formation was ten vessels per day at LPI 5. On the average, about five xylem vessels differentiated per day in the middle portion of a Xanthium petiole. Mature petioles contained an average of 218 xylem vessels. About 12 canals of schizogenous origin preceeded the development of the vascular tissue.     

Rates of corolla growth in tobacco determined with the plastochron index

The growth of vegetative and reproductive shoots of Nicotiana tabacum L. cv. Xanthi is analyzed with the plastochron index to estimate the relationship between corolla growth and time. The plastochron of leaves 9 through 20 declines steadily at each successive node. The flower plastochron increases steadily during the growth of an individual cyme, with the most distal flower to open having the longest plastochron. Variation in the flower plastochron is the result of variation in the rate of flower initiation, not the growth rate of individual flowers. The corolla has an extended phase of approximately constant relative growth in length (between 0.2 · d^–1 and 0.3 · d^–1) until a peak of growth (0.5 · d^–1) 2–3 d before anthesis. Corollas also have periodic peaks and troughs of growth that are low in amplitude (0.1 · d^–1), but persist throughout most of corolla development. The pattern of corolla expansion contrasts strongly with earlier reports of the pattern of tobacco leaf growth.Abbreviations PI plastochron index - PR plastochron ratio - RGR relative growth rate in length The authors thank: Drs. T. Sage and E.G. Williams for the considerable time and space they invested; the members of Dr. R. Wyatt's laboratory for allowing us to use their computer facilities; A. Tull and M. Smith for their care taken in the green-house. This research was supported by U.S. Department of Agriculture grant GAM-89-01056 and by National Science Foundation grant DCB-87-15799.     

Heterochronic Effects of Teopod 2 on the Growth and Photosensitivity of the Maize Shoot

Teopod 2 (Tp2) is a semidominant mutation of maize that prolongs the expression of juvenile vegetative traits, increases the total number of leaves produced by the shoot, and transforms reproductive structures into vegetative ones. Here, we show that Tp2 prolongs the duration of vegetative growth without prolonging the overall duration of shoot growth. Mutant shoots produce leaves at the same rate as wild-type plants and continue to produce leaves after wild-type plants have initiated a tassel. Although Tp2/+ plants initiate a tassel later than their wild-type siblings, this mutant tassel ceases differentiation at the same time as, or shortly before, the primary meristem of a wild-type tassel completes its development. To investigate the relationship between the vegetative and reproductive development of the shoot, Tp2/+ and wild-type plants were exposed to floral inductive short day (SD) treatments at various stages of shoot growth. Tassel initiation in wild-type plants (which normally produced 18 to 19 leaves) was maximally sensitive to SD between plastochrons 15 and 16, whereas tassel branching was maximally sensitive to SD between plastochrons 15 and 18. Tassel initiation and tassel morphology in Tp2/+ plants (which normally produced 21 to 26 leaves) were both maximally sensitive to SD between plastochrons 15 and 18. Thus, the constitutive expression of a juvenile vegetative program in Tp2/+ plants does not significantly delay the reproductive maturation of the shoot.     

A METHOD FOR DETERMINING PLASTOCHRON INDICES DURING HETEROBLASTIC SHOOT GROWTH

A plastochron is defined as the time interval between two successive recurring events during the growth of plant shoots, such as leaf initiation. The plastochron index (PI) formulated by Erickson and Michelini (1957, American Journal of Botany 44: 297–305) provides a method for determining 1) morphological equivalence in a developmentally variable sample of shoots and 2) rates of development in microscopic tissues and organs, by expressing shoot age as a function of plastochron number. The PI assumes that homologous organs at successive nodes grow exponentially, at equal rates, and the plastochron remains constant. These three conditions are not met in many shoots that exhibit heteroblasty in their plastochron and the growth rate of organs at successive nodes. An alternative computational method for the PI is presented that uses two measurements taken at different times from the same organ during its exponential growth phase. The method does not assume that the PI is a linear function of time. Results of an analysis of cyme internode growth in two races of Arenaria uniflora (Caryophyllaceae) demonstrate that the method proposed is in good agreement with Erickson and Michelini's (1957) method when shoot growth is not markedly heteroblastic. The current method is also used to determine the nonlinear relation between PI and time in a race of A. uniflora that has heteroblastic cyme growth. The results generalize the PI for use in studies of heteroblasty, and for shoots where the relative plastochron rate cannot be directly determined.     

PLASTOCHRON2 regulates leaf initiation and maturation in rice

In higher plants, leaves initiate in constant spatial and temporal patterns. Although the pattern of leaf initiation is a key element of plant shoot architecture, little is known about how the time interval between initiation events, termed plastochron, is regulated. Here, we present a detailed analysis of plastochron2 (pla2), a rice (Oryza sativa) mutant that exhibits shortened plastochron and precocious maturation of leaves during the vegetative phase and ectopic shoot formation during the reproductive phase. The corresponding PLA2 gene is revealed to be an orthologue of terminal ear1, a maize (Zea mays) gene that encodes a MEI2-like RNA binding protein. PLA2 is expressed predominantly in young leaf primordia. We show that PLA2 normally acts to retard the rate of leaf maturation but does so independently of PLA1, which encodes a member of the P450 family. Based on these analyses, we propose a model in which plastochron is determined by signals from immature leaves that act non-cell-autonomously in the shoot apical meristem to inhibit the initiation of new leaves.     

PHOTOPERIOD INDUCED CHANGE IN PHYLLOTAXIS IN XANTHIUM

Photoperiodic floral induction in Xanthium, achieved by subjecting the plants to two long nights, is accompanied by a transient change of the phyllotaxis from the (2, 3) contact parastichy pattern of vegetative plants, to a (3, 5) pattern during the transition. To specify the phyllotaxis, two parameters were estimated from transverse sections of apical buds of control and treated plants: the divergence angle, α, and the plastochron ratio, a. The plastochron ratio decreased progressively during transition from the vegetative to the reproductive state of growth, from a = 1.48 initially to a = 1.15 six days after the beginning of induction. The divergence angle was not altered during the transition. This change in phyllotaxis is interpreted as a change in the relative positioning of leaf primordia on the transitional apex. This transient change appears to be identical with the previously described long-term change of the phyllotaxis of Xanthium brought about by treatment of plants with gibberellic acid.     

CHANGES IN EPILOBIUM PHYLLOTAXY INDUCED BY N-1-NAPHTHYLPHTHALAMIC ACID AND α-4-CHLOROPHENOXYISOBUTYRIC ACID

Preliminary studies establishing relationships between leaf plastochron index and Epilobium hirsutum L. shoot growth provide a method for rigorous selection of plants utilized in experiments designed to test the working hypothesis that endogenous auxin gradient interactions are factors of phyllotactic control in this species. Application of N-1-naphthylphthalamic acid (NPA), an auxin transport inhibitor, to one of the youngest bijugate primordia on the shoot meristem results in increased growth of the treated primordium. Fasciation between the treated primordium and one of the next primordia to be initiated alters relative vertical spacing of primordia. Angular shifts between subsequent primordia result in spiral transformation of Epilobium bijugate phyllotaxy. Application of α-4-chlorophenoxyisobutyric acid (CPIB), an auxin antagonist, to one of the youngest bijugate primordia on the shoot meristem results in decreased growth of the treated primordium that alters both radial and vertical spacing of primordia. This is followed by angular shifts between subsequent primordia resulting in spiral transformation of the bijugate phyllotaxy. Changes in the growth parameters of NPA- and CPIB-treated shoots are similar. Relative plastochron rates of radial and vertical shoot growth of induced spiral shoots are about half those of lanolin paste control shoots, as are the plastochrons and relative plastochron rates of leaf elongation. Treated shoot meristems have eccentricities of 0.5 as compared to bijugate control meristem eccentricities of 0.7. No significant difference is apparent between basal transverse areas of treated and control shoot meristems. The relative chronological rates of growth of treated shoots are not significantly different from those rates of control shoots. Spiral transformation results from changes in relative positions of leaf primordia insertion on the shoot meristem, not from changes in growth of treated shoots. These changes are accompanied by an increased rate of leaf initiation on a more circular shoot meristem. Existing theoretical models of phyllotaxy are discussed in relation to these chemically induced changes of Epilobium leaf arrangement.     

A new method to measure leaf age: Leaf measuring-interval index

We propose a new method, the leaf measuring-interval index (LMI), to estimate leaf age in morphological and physiological studies of leaves. When the plastochron, the interval between the initiation of successive leaves, is constant, the well-known leaf plastochron index (LPI) provides a robust measure of leaf age. When the duration of the plastochron is not uniform, however, we show that the LPI can (in simulations) and does (with actual data) turn variation in duration of the plastochron into variance about the regression estimates of leaf growth curves. The method we present in this paper, the LMI, is plastochron independent. This new method is particularly suited, therefore, for studies of plants growing in natural environments rather than in controlled growth facilities where the assumptions of the LPI method can be met.     

Microarray analysis of vegetative phase change in maize

Vegetative phase change is the developmental transition from the juvenile phase to the adult phase in which a plant becomes competent for sexual reproduction. The gain of ability to flower is often accompanied by changes in patterns of differentiation in newly forming vegetative organs. In maize, juvenile leaves differ from adult leaves in morphology, anatomy and cell wall composition. Whereas the normal sequence of juvenile followed by adult is repeated with every sexual generation, this sequence can be altered in maize by the isolation and culture of the shoot apex from an adult phase plant: an 'adult' meristem so treated reverts to forming juvenile vegetative organs. To begin to unravel the as-yet poorly understood molecular mechanisms underlying phase change in maize, we compared gene expression in two juvenile sample types, leaf 4 and culture-derived leaves 3 or 4, with an adult sample type (leaf 9) using cDNA microarrays. All samples were leaf primordia at plastochron 6. A gene was scored as 'phase induced' if it was up- or downregulated in both juvenile sample types, compared with the adult sample type, with at least a twofold change in gene expression at a P-value of < or =0.005. Some 221 expressed sequence tags (ESTs) were upregulated in juveniles, and 28 ESTs were upregulated in adults. The largest class of juvenile-induced genes was comprised of those involved in photosynthesis, suggesting that maize plants are primed for energy production early in vegetative growth by the developmental induction of photosynthetic genes.     

Transgenic tobacco over-expressing a homeobox gene shows a developmental interaction between leaf morphogenesis and phyllotaxy.

The tobacco gene, NTH1, encodes a polypeptide of 326 amino acids and is a member of the class1 KN1-type family of homeobox genes. Expression of NTH1 has mainly been observed in vegetative and reproductive shoot apices, not observed in roots or expanded leaves. Over-expression of NTH1 in transgenic plants caused abnormal leaf morphology, consisting of wrinkling and curvature. Interestingly, the direction of leaf curvature tended to be conserved among almost all of the leaves in any given transformant. In transgenic plants exhibiting clockwise or anticlockwise phyllotaxy, leaves curved to the right or left, respectively, when looking from the shoot apex toward the base. Micro-surgical experiments demonstrated that the presence of the shoot apex is necessary for the development of leaf curvature, indicating that the order of formation of leaves on the stem (the generative spiral) affects leaf development. We found a correlation between the severity of leaf curvature and the value of the plastochron ratio, a parameter of phyllotaxy. Transformants with more severe phenotypes had larger plastochron ratios. From these findings, we discuss the possibility that an increase in the plastochron ratio, caused by over-expression of NTH1 in the shoot apex, may be involved in leaf curvature.     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

Responses of ivy (Hedera helix L.) to combinations of gibberellic acid,paclobutrazol and abscisic acid

A reduction in concentration of gibberellins has been implicated in the phase change from juvenile to mature forms of ivy (Hedera helix L.). Attempts were made to increase the effective internal concentration of gibberellins by exogenous application of GA₃, and to decrease them by various applications of abscisic acid (ABA) and paclobutrazol (PP333), alone or in combination with GA₃. ABA and GA₃ were fed directly into the xylem of ivy plants by a wick system (a less drastic procedure than the defoliation or decapitation used by earlier workers) whereas PP333 was applied as a soil drench.Mature ivy responded to the application of GA₃ by reversion to the juvenile form, although this reversion was incomplete with respect to leaf lobing and red (anthocyanin) pigmentation and could occur spontaneously without the application of GA₃. Contrary to expectation, applications of ABA and PP333 caused the stimulation of growth in juvenile ivy. No adult characteristics were induced. As similar concentrations of ABA and PP333 produced severe retardation of growth (which could be alleviated by the application of GA₃) in other species, it is suggested that ivy may be an unsuitable model system for the investigation of phase change in woody plants.