Meristem characteristics of normal, afila (af), acacia (tl), reduced stipule (st), and combinations of these leaf phenotypes were investigated in pea (Pisum sativum L.). The multiple tendrils of the afila leaf are formed from numerous secondary branches on the leaflet primordia. Adaxial and marginal meristems are absent in afila leaflets. The tendril-like morphology of the terminal and secondary branches of the afila leaflets is derived from a radial marginal meristem, which is characteristic of normal tendril development. The small terminal leaflet lamina on tendrils of the acacia leaf is produced by adaxial and marginal meristems which become apparent in the distal portion of the tendril late in leaf ontogeny. The reduced stipules of the reduced stipule leaf result from early loss of abaxial and adaxial stipule marginal meristems. Combinations of the af, tl, and st genes apparently have no modifying influences on their mutual expression with one exception; the aftlst mature reduced stipule is significantly wider than stipules in st, afst, and tlst phenotypes. The greater final width of triple recessive stipules is attributed to the persistence of the adaxial stipular marginal meristem in this phenotype.
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 Nsl-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 Epilabium bijugate phyllotaxy. Application of a-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 ofleafprimordia 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.
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.
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.
Growth of Ranunculus shoots through ontogeny is quantified by techniques utilizing scanning electron microscopy and studies on living plant material. The order of the contact parastichy phyllotaxy in the apical system is related to the relative plastochron rates of growth of the shoot. There is a change in the (2, 3) contact parastichy pattern of vegetative phyllotaxy to a transitional (3, 5) contact pattern which is maintained through sepal production. Formation of a 5(1, 1) whorl of petal primordia establishes a (5, 8) contact pattern with the sepal primordia. Subsequent initiation of stamen primordia, in spiral sequence, results in (5, 8, 13) triple contacts between petal and stamen primordia. The stamen primordia and carpel primordia arrangement is characterized by a (8, 13) contact parastichy pattern of phyllotaxy. Through ontogeny the volume of the shoot apex progressively increases but the shape of the apex, described by a second degree polynomial, remains constant. The plastochron and the relative plastochron rates of radial and vertical displacement of primordia progressively decrease during transition but there is no alteration of the chronological rate of apical expansion. The change in contact parastichy phyllotaxy through ontogeny is interpreted as a change in the relative positions of primordia insertion on the apex resulting from an increase in apical volume and an increased rate of primordia initiation.
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