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Robert L. Geneve, Wesley P. Hackett, and Bert T. Swanson

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).

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Robert L. Geneve, Wesley P. Hackett, and Bert T. Swanson

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).

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Michael R. Evans, Harold F. Wilkins, and Wesley P. Hackett

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).

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Michael R. Evans, Harold F. Wilkins, and Wesley P. Hackett

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.

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Gregory T. Browne, Charles A. Leslie, Joseph A. Grant, Ravindra G. Bhat, Leigh S. Schmidt, Wesley P. Hackett, Daniel A. Kluepfel, Reid Robinson, and Gale H. McGranahan

Species of Phytophthora are serious soilborne pathogens of persian (english) walnut, causing crown and root rot and associated production losses worldwide. To facilitate the development of improved walnut rootstocks, we examined resistance of 48 diverse clones and seedlings of Juglans species to P. cinnamomi and P. citricola. Plants were micropropagated, acclimatized to a greenhouse environment, and then exposed to the pathogens in artificially infested potting soil mix. Inoculated plants, as well as noninoculated controls, were subjected to soil flooding for 48 hours every 2 weeks to facilitate infection by the pathogens. Two to 3 months after inoculation, resistance to the pathogens was assessed according to the severity of crown and root rot. Clonal hybrids of J. californica × J. regia were highly susceptible to the pathogens (means 52% to 76% root crown length rotted), while several clones of J. microcarpa × J. regia were significantly less susceptible (means 8% to 79% crown length rotted). Among clones of other parentages tested, including: J. microcarpa, (J. californica × J. nigra) × J. regia, J. hindsii × J. regia, (J. hindsii × J. regia) × J. regia, [(J. major × J. hindsii) × J. nigra] × J. regia, and J. nigra × J. regia, responses varied, but tended to be intermediate. When ‘Serr’ scions were budded or grafted on J. microcarpa × J. regia clone ‘RX1’ or Paradox (J. hindsii × J. regia) seedling rootstocks in a commercial orchard infested with P. cinnamomi, all trees on ‘RX1’ remained healthy, whereas only 49% of those on Paradox survived. Thus, useful resistance to Phytophthora is available among J. microcarpa × J. regia hybrids and is evident in ‘RX1’ rootstock.

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Kourosh Vahdati, James R. McKenna, Abhaya M. Dandekar, Charles A. Leslie, Sandie L. Uratsu, Wesley P. Hackett, Paola Negri, and Gale H. McGranahan

Walnuts (Juglans spp.) are difficult-to-root woody plants. The rolABC genes (rolA + rolB + rolC), derived from the bacteria Agrobacterium rhizogenes, have been shown to increase the rooting potential of other difficult-to-root woody plants. We inserted the rolABC genes into somatic embryos of a `Paradox' hybrid (J. hindsii × J. regia) clone PX1 using the A. tumefaciens gene transfer system. A transgenic sub-clone, designated PX1 rolABC 2-2 was selected and compared to the untransformed clone for a variety of phenotypic characteristics, including rooting potential. Transformed and untransformed shoots were budded onto seedling J. regia rootstock in the greenhouse and established in the field. Transformed trees displayed reduced internode length, an increase in lateral branching, and wrinkled leaves. In another test, a commercial persian walnut cultivar J. regia `Chandler' was grafted onto rooted cuttings of both the untransformed and transformed plants. The presence of the rolABC genes in the rootstock had no visible effects on the grafted scion. Several of these trees were excavated from the field and the root systems of each genotype were examined for root number, diameter, and biomass. Trees with the rolABC rootstock had significantly more small diameter roots compared to the controls and less recovered biomass. Tests of the rooting potential of leafy semi-hardwood cuttings for two years resulted in 14% to 59% rooting of the transformed cuttings compared to 51% to 81% rooting of the control. Both transformed hardwood cuttings and microshoots in tissue culture also rooted significantly less (52% and 29% respectively) than untransformed hardwood cuttings and tissue cultured shoots (82% and 54% respectively). Thus, although the rolABC genes induced a shorter internode length and a more fibrous root system (typical of rol-tranformed plants), they were not useful for increasing the rooting potential, and as rootstock they did not affect the phenotype of the scion.