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- Author or Editor: Wesley P. Hackett x
Abstract
There is considerable opportunity for intra-clonal variability due to nongenetic or epigenetic causes because of the well-known and striking age-related changes in developmental patterns in plants propagated from seed. It is well-known that a juvenile phase exists in the development of plants from seed, lasting up to 30-40 years in forest trees, during which flowering does not occur and cannot be induced by normal flower-initiating treatments. The ability to flower is achieved in time, however, and the plant is said to have attained the mature condition. This phenomenon has been referred to as a phase change by Brink (6). Changes in morphological and developmental characteristics, such as leaf cuticular characteristics (13), leaf shape and thickness, phyllotaxis, thominess, shoot orientation (46), and other physiological characteristics such as seasonal leaf retention, stem pigmentation (46), ability to form adventitious roots and buds (7, 46), partitioning of photosynthates into main stem vs. branches (30), disease resistance (W.J. Libby, Jr., personal communication) and cold resistance (27), are associated with the phase change. Phase-change-related characteristics are most obvious in woody, perennial plants but also have been demonstrated in herbaceous annuals and perennials (24, 54). Some of these characteristics are important horticulturally and the ability to control or manipulate them has important practical implications.
Abstract
There is very little rooting response of juvenile shoot apices to indole-acetic acid (IAA) at concentrations from 1 to 50 mg/liter when the light intensity is 400–500 ft-c. Naphthalene-acetic acid (NAA), however, promotes rooting very markedly with an optimum at 5 to 10 mg/1. There is a strong synergism between IAA at 10 mg/1 and catechol at 5 × 10−5M which results in a rooting response equal to that obtained with NAA at its optimum concentration. In low intensity light (50 ft-c) IAA is nearly as effective as IAA + catechol in high light. Rooting of adult apices in high intensity light is essentially zero using IAA, NAA or combinations of these auxins with catechol. In low intensity light rooting occurs using 10 mg/1 IAA and there is a marked synergism between IAA and catechol. The rooting response of adult tips in low intensity light is very similar to that of juvenile shoot tips in high intensity light. Three fractions of methanolic extracts of adult and juvenile shoot tissue promote rooting of juvenile shoot apices in high intensity light. Rooting of adult apices is not affected by these or other fractions.
Abstract
Low temperature is the most important environmental factor promoting flower initiation in Pelargonium domesticum Bailey, but 1 of 3 cultivars tested had little or no requirement for low temperature and flowered even when grown at greenhouse night temperatures of 21°C (70°F). High light flux can partially substitute for the low temperature requirement for flowering. At a constant temperature of 15.5°C, long days increased the total number of inflorescences and their rate of development as compared to short day control plants.
Abstract
Seeds of Citrus sinensis (L.) Osbeck cv. Valencia obtained from trees grown in north-central California would not germinate when fruit was harvested during the early part of the season (November 1978). Seeds from November-harvested fruit stored at 3 to 4°C for 21 days germinated 100%. A similar germination response to low temperature was obtained when seeds from fruit harvested April 1980 (exposed to only 103 hours below 3 to 4°) were treated for 3 weeks at 3 to 4°. Treatment with gibberellic acid (GA3) induced about 55% germination in seeds from non-cold-treated fruit. Germination rates for seeds from cold-treated fruit were found to increase when the seeds were presoaked in GA3 or germinated at high temperature.
Abstract
The floral initiation response of ‘Paul Mikkelsen’ poinsettia to low temperature under long days was saturated after exposure to constant temperatures of 60°F for 10 days. Low temperature was perceived by the shoots but not the roots. As has been reported by others, high temperatures (80°F) during and after short days inhibited floral initiation. Gibberellin A3 was an effective inhibitor of low temperature stimulated floral initiation under long days and Cycocel promoted flowering under long days at 70°F but not at 80°. Neither light source nor intensity greatly influenced low temperature stimulated floral initiation. These findings are discussed in relation to a possible mechanism by which low temperatures stimulate floral initiation in this short day plant and in relation to cultural practices.
Abstract
Translocation of 14C-labelled assimilates between partners of juvenile–adult grafts of English ivy (Hedera helix L.) was influenced by 14CO2 application technique, stem girdling, selective defoliation, and cytokinin treatment. Applications of the cytokinin, 6 benzylamimo purine (BA), to the shoot tip of the juvenile scion increased the amount of label translocated to the juvenile shoot. The results indicate the importance of precise manipulation of assimilate sources and sinks in order to insure translocation from presumptive donor to receptor in the grafted ivy system.
Abstract
Chemical control of plant height has been achieved for many herbaceous and woody species. Horticultural practices in the greenhouse, orchard, and landscape have been altered to include the use of numerous compounds, the main function of which is to eliminate overgrowth. The problems encountered in selecting and using even the registered materials cannot be readily generalized since each compound presents special difficulties. Nevertheless, for the purpose of this review, 7 challenges to effective use usually presented by all compounds will be discussed, namely: 1) identifying the primary cause of inhibition of stem elongation; 2) timing the application of compounds to the appropriate stage of plant development; 3) determining the best method of application; 4) determining the optimum dosage, formulation, and frequency of application; 5) testing for cumulative phytotoxicity; 6) noting species specificity; and 7) taking note of potential environmental effects. Many chemicals have been made available for testing, but relatively few of them are registered expressly for control of overgrowth (Fig. 1).
Abstract
The members of the genus Aesculus, known as buckeyes and horsechestnuts, includes trees commonly planted in parks, arboreta, college campuses, and home landscapes (2). Aesculus ‘Autumn Splendor’ is a selection that combines leaf scorch resistance, cold hardiness in USDA zone 4a, handsome flowers, and brilliant fall color to make it a particularly desirable medium-sized landscape tree.
Exogenous ethylene could not substitute for NAA to induce adventitious root initiation in juvenile petiole explants of English ivy (Hedera helix L.), indicating that the action of auxin-stimulated root initiation was not directly mediated through ethylene production. Mature petioles did not initiate roots under any auxin or ethylene treatment combination. Ethephon or ACC supplied at 50 or 100 μm was inhibitory to NAA-induced root initiation in juvenile petioles. The pattern of ethylene production stimulated by NAA application was significantly different in juvenile and mature petioles. Ethylene evolution by juvenile petioles declined to near control levels during from 6 to 12 days after NAA application. Reduction in ethylene production was due to reduced availability of ACC in juvenile petioles. Mature petioles continued to produce ethylene at elevated levels throughout the course of the experiment. Ethylene does not appear to play a significant role in the differential root initiation response of juvenile and mature petioles treated with NAA. However, ethylene appeared to have an inhibitory effect during root elongation stages of adventitious root development in juvenile petioles. Chemical names used: 1-aminocyclopropane-1-carboxylic acid (ACC); 1-napthaleneacetic acid (NAA); 2-chloroethylphosphonic acid (ethephon).
Several inhibitors of ethylene biosynthesis and action, as well as an atmospheric ethylene scrubber, were used to investigate the role of ethylene in adventitious root initiation in de-bladed petioles from the juvenile and mature phase of English ivy (Hedera helix L.). Induction of root primordia required NAA regardless of the inhibitor treatment. Difficult-to-root mature petioles have been shown to produce higher amounts of ethylene than easy-to-root juvenile petioles. However, mature petioles failed to root under any combination of NAA and inhibitor treatment, indicating that the continued evolution of ethylene in NAA-treated mature petioles was not responsible for the absence of a rooting response. Root initiation in juvenile petioles was not affected by treatment with the ethylene action inhibitors STS and NDE, nor by removal of atmospheric ethylene with KMnO. Inhibition of ethylene biosynthesis using AVG or AOA reduced root initiation in juvenile petioles, but this response was not well-correlated to the observed reduction in ethylene evolution. The inhibitory action of AVG could not be reversed by the addition of ethylene gas or ACC, which indicated that AVG could be acting through a mechanism other than the inhibition of ethylene biosynthesis. Chemical names used: 1-naphthalene acetic acid (NAA); l-aminocyclopropane-l-carboxylic acid (ACC); silver thiosulfate (STS); 2,5-norbornadiene (NDE); aminoethyoxyvinyl-glycine (AVG); aminooxyacetic acid (AOA).