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  • Author or Editor: Wesley P. Hackett x
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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.

Open Access

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.

Open Access

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.

Open Access

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.

Open Access

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).

Open Access

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.

Open Access

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.

Open Access

The poinsettia [Euphorbia pulcherrima (Willd. ex. Klotzsch)] is a short-day plant (SDP) for floral initiation that will also initiate floral structures (cyathia) under long days (LD) after the apical meristem produces a cultivar-dependent number of nodes (long-day node number). Leaf removal, root restriction, and air layering failed to affect the long-day node number (LDNN) of the apical meristem. Repeated rooting of shoots, which resulted in the removal of nodes, did not affect the total number of nodes initiated by the apical meristem before floral initiation, although the number of nodes intact on the plant at the time of floral initiation was reduced. Reciprocal grafting of axillary buds of `Eckespoint Lilo' and `Gutbier V-14 Glory' plants did not affect the LDNN of the grafted meristem since the LDNN was the same as for nongrafted buds of the same cultivar. Further, grafting axillary buds from different positions along the main axis that differed in LDNN did not affect the LDNN of the grafted meristems. On the basis of these results, it was concluded that LD floral initiation in poinsettia is a function of the ontogenetic age of the meristem and that the LDNN represents a critical ontogenetic age for floral initiation to occur under LD.

Free access

Exogenous foliar spray applications of gibberellic acid (GA3) applied at 7- or 14-day intervals providing 50 or 125 μg per plant inhibited long-day (LD) floral initiation in poinsettia [Euphorbia pulcherrima (Willd. ex. Klotzsch)]. Periodic application of GA3 resulted in an additional number of nodes being produced by the plant before floral initiation equivalent to the number of nodes over which GA3 was applied. Further, GA, application eliminated the nodal position dependence of the long-day node number (LDNN) of axillary meristems observed in control plants. It was concluded that GA3 application inhibited the inclusion of nodes into the LDNN count and thus inhibited ontogenetic aging of the meristem. Exogenous application of GA, also inhibited LD floral initiation, while application of GA4 had no effect. Application of GA7 delayed LD floral initiation, but plants did initiate cyathia by the termination of the experiment. All gibberellins increased the average internode lengths similarly. The gibberllin-biosynthesis inhibitors chlormequat and paclobutrazol had no effect on LD floral initiation when applied as single or multiple foliar sprays or as soil drenches, although heights and internode lengths were reduced by application of the inhibitors. The LDNN of plants grown at 31C was significantly higher than of plants grown at 16, 21, or 26C. All plants eventually initiated cyathia regardless of temperature. When plants were grown under a range of day/night temperatures, an increase in the LDNN occurred only when plants were grown at 31C during the day. Chemical names used: 2-chloroethyl-trimethyl-ammonium chloride (chlormequat); (+/-)-(R*,R*)-β -(4-chlorophenyl)methyl-α -(1,1-dimethylethyl)-1-H-1,2,4-triazole-1-ethanol (paclobutrazol).

Free access

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).

Free access