Roger D. Meicenheimer

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

A bstractTwo 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.

Consideration of the geometry of the phyllotaxic triangular unit and discontinuous phyllotactic transitions

The Phyllotaxic Triangular Unit (PTU) has been proposed as the common element of all patterns of phyllotaxis, analogous to the primitive unit cell of crystallography. The geometry of the PTU is examined within the geometric framework of classical contact parastichy phyllotaxis models. Three natural categories of phyllotactic patterns are defined in terms of the geometric characteristics of the PTU. Discontinuous transitions, analogous to edge dislocations in crystals, provide a theoretical mechanism by which multiple (more than one leaf per node) and simple (one leaf per node) phyllotactic patterns can be transformed one to the other.

Growth and Developmental Stages of Alaska Peas

Growth and developmental stages of Alaska peas, from planting to dry seed maturity, were examined and described. Node counts were determined by designating the cotyledonary node as 0 and counting successive nodes up the plant to the highest visible one with unexpanded stipules. Five additional nodes are already determined in the shoot apex beyond the highest one with unexpanded stipules, indicating that damage to the shoot apex would result in losses of flowers and fruits that would have developed later if damage had not occurred.

Short‐Day‐Grown Arabidopsis thaliana Satisfies the Assumptions of the Plastochron Index as a Time Variable in Development

A plastochron index (PI) based on a 3‐mm leaf reference length was established for the Columbia‐1 (Col‐1) ecotype and the se (serrate) mutant of Arabidopsis thaliana for quantitative analysis of leaf development. The three underlying assumptions for PI establishment—early exponential leaf growth rate, similar relative growth rates (RGRs), and plastochrons between successive leaves—were tested. Under a 10‐h day length, the three assumptions held for leaves 11–37 of Col‐1 and for leaves 7–37 of se. The mean plastochron of se was about 1.5 times longer than that of Col‐1, and the mean RGR of Col‐1 was about 1.25 times greater than that of se.

Anatomical basis for biophysical differences between Pinus nigra and P. resinosa (Pinaceae) leaves

Differences in the flexibility of Pinus nigra and P. resinosa leaves can be used to discriminate these two similarly looking pine species from one another. When bent along the longitudinal axis, P. resinosa leaves snap, while P. nigra leaves appear flexible. This useful field test has had no known biophysical or anatomical explanation until now. Analysis of the first order mechanics of bending and buckling of the pine needles was used to elucidate any important anatomical differences between these two species that can account for their different biophysical behaviors when bent. Neither the cross section of the total leaf area nor the inner core area between the two species differed significantly. Differences in the pattern of cell wall thickening and lignification of the endodermal layer of the inner core of the leaves best explain the differences in bending behavior. Thus, subtle variation in anatomy can influence the biophysical properties of naturally occurring structures, which in turn could have important implications for the engineering of manufactured objects.

Gravitropic Response of Kalanchoë Stems

Gravitropic response of Kalanchoë stems was analyzed in terms of instantaneous curvature along the stem, relative elemental growth rates, and epidermal cell dimensions. A bimodal pattern of instantaneous curvature along the stem axis was detected at the end of the gravitropic response. Results of the study revealed hitherto unreported complex spatial and temporal patterns of growth rate oscillations in both gravistimulated and unstimulated control stems. Spatial and temporal growth oscillations, thought to be associated with circumnutation, appear to be overridden by local changes in growth rates in gravistimulated stems. Overall, rate of growth is initially depressed and then stimulated in response to reorientation in the gravitational field. There is a basipetal migration in amplified growth rate on the lower side and a return to oscillatory growth rate on the upper side of gravistimulated stems. Analysis of epidermal cell dimensions reveals that, in general, there is a redistribution of cell elongation potential to the lower side of the gravistimulated stem but no evidence for overall enhancement of cell expansion in response to gravity.

The plastochron index: Still useful after nearly six decades

The plastochron index (PI) introduced by Erickson and Michelini in 1957 provides a solution to a long-standing problem, of how to measure time in growing plant populations, such that the occurrence of critical developmental events can be more readily detected, compared, and analyzed, than if chronologic time is used. The PI reduces the rather large variation associated with chronologic time in measuring such events by taking advantage of the growth characteristics of stem organs that repeat at regular intervals (the plastochron) and has found widespread application in botanical research. The original formulation and derivation of the PI and associated leaf plastochron index (LPI) is reviewed. Additional formulations that have been developed to overcome some of the limitations of the original PI formulation are examined. Major advancements that have been achieved in understanding the physiology, growth, and development of agriculturally important and current model plant species are reviewed to illustrate how various researchers have used the PI in such studies. Potential uses to which the PI and LPI might be applied in emerging frontiers of plant science are suggested. A searchable bibliography of most all the primary research studies that cite the original PI article is provided.

Anisotomous dichotomy results from an unequal bifurcation of the original shoot apical meristem in Diphasiastrum digitatum (Lycopodiaceae)

PREMISE OF THE STUDY Two types of dichotomy are recognized in Lycopodiaceae: isotomous (equal) and anisotomous (unequal). Anisotomous dichotomy (anisotomy) has been hypothesized to result from unequal growth of an equal bifurcation of the original shoot apical meristem (SAM). Diphasiastrum digitatum (Lycopodiaceae) exhibits anisotomy at various locations. We thus used D. digitatum to test this classic hypothesis about anisotomy. METHODS Transverse areas of original and derived SAMs of anisotomy exhibited by the rhizome and the vertical aerial vegetative stem were measured using scanning electron microscopy. The difference between half of the original SAM and one derived SAM in terms of transverse area were compared using paired t-tests. KEY RESULTS During the anisotomy exhibited by the rhizome SAM, 77.4% of the transverse area of the original rhizome SAM contributed to the derived rhizome SAM. During the first anisotomy exhibited by the vertical aerial vegetative stem SAM, 66.2% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. During the second anisotomy exhibited by the vertical aerial vegetative stem SAM, 49.4% of the transverse area of the original vertical aerial vegetative stem SAM contributed to the derived vertical aerial vegetative stem SAM. Nonetheless, the shape of the two derived SAMs differed though they did not differ in size. CONCLUSIONS In D. digitatum, anisotomy results from an unequal bifurcation of the original SAM. This finding sheds light on plant body architecture evolution as well as plant organ (megaphyllous leaf) evolution.

The ontogeny, phyllotactic diversity, and discontinuous transitions of Diphasiastrum digitatum (Lycopodiaceae)

PREMISE OF THE STUDY Fibonacci phyllotactic patterns in seed plants are well documented, but whether such predominance holds true for lower vascular plants is relatively unknown. We investigated Diphasiastrum digitatum (Lycopodiaceae) phyllotaxis throughout its ontogeny to extend our knowledge of pattern frequency of lower vascular plants and to measure quantitative variables associated with discontinuous phyllotactic transitions. These investigations allowed us to test whether the same mechanisms inherent in shoot apical meristem (SAM) development of seed plants are applicable to early-diverged lower vascular plants SAM development. METHODS Divergence angle, plastochron ratio, leaf insertion angle, circumferential ratio, radial ratio, half conic angle, area, circumference, and circularity of the shoot apical meristem were compared among different phyllotactic patterns and different meristem types observed throughout D. digitatum ontogeny, using scanning electron microscopy. KEY RESULTS Fibonacci patterns were not predominant during six stages of D. digitatum ontogeny. In all five cases of discontinuous transition associated with strobili formation, divergence angle was the only variable that has changed consistently. CONCLUSIONS The predominance of non-Fibonacci patterns/series in D. digitatum is inconsistent with the prediction of interpretive model of phyllotaxis. We hypothesize this is because its SAM, due to its frequent dichotomy, is not circular and primordia initiation is restricted spatially and temporally at the beginning of pattern formation. Change in divergence angle associated with discontinuous transitions is most likely due to the change of the location of new auxin maxima, due to the change of SAM shape and size.

Empirical models of stem growth and vasculature differentiation processes

Abstract A new notation for leaf trace patterns was developed which is consistent with contemporary contact paratichy phyllotaxis notation. New computer-aided methods for generating accurate stem tissue maps were developed. Study of the tissue maps for the various leaf trace patterns exhibited by Linum stems through ontogeny generated a set of observations which permits more rigorous definition of the developmental rules for vascular pattern formation. Changes in Linum stem growth through ontogeny was defined by empirical models generated by computer-aided analysis of scanning electron micrographs. Linum stems undergo progressive changes from low order to higher order patterns of leaf trace interconnections and phyllotaxis through ontogeny. Positional relationships between interconnected leaf traces is found predictable using long-known geometric principles of phyllotaxis.