In three rubber-producing species, in vitro, the rates of initiation and polymerization and the biopolymer molecular weight produced were affected by the concentration of farnesyl diphosphate (FPP) initiator and isopentenyl diphosphate (IPP) elongation substrate (monomer). Ficus elastica, a low molecular weight-producer in vivo, synthesized rubber polymers approximately twice the molecular weight of those made by Hevea brasiliensis or Parthenium argentatum (which produce high molecular weights in vivo), possibly due to its lower IPP Km. In all species, increasing FPP concentrations increased rubber biosynthetic rate and new molecules initiated but decreased molecular weight by competition with the allylic diphosphate (APP) end of elongating rubber molecules for the APP binding site. Increasing IPP concentrations increased rubber biosynthetic rate and rubber molecular weight, but only when FPP concentrations were below the FPP Km's or where negative cooperativity operated. In conclusion, rubber transferase is not the prime regulator of rubber molecular weight in vivo.

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Plant regeneration from embryogenic cells of two Rosa hybrida cultivars, Kardinal and Classy, was increased by dispersing embryogenic callus in liquid medium for 3 h followed by size-fractionation to isolate proembryogenic masses that were smaller than 530 microm. Dispersed callus of three cultivars, Kardinal, Classy, and Tineke, produced 61-135 cotyledonary-stage embryos/100 mg fresh weight (FW) as compared to intact callus that had not been dispersed, which produced only zero to three cotyledonary-stage embryos/100 mg FW. Over 500 cotyledonary-stage embryos/100 mg FW callus developed from proembryogenic masses of Kardinal, Classy, and Tineke following 2 months of culture on solidified Murashige and Skoog's basal salts medium supplemented with 0.25% activated charcoal. Cotyledonary-stage embryos of Classy that developed from both dispersed callus and fractionated cells of various sizes showed a significantly higher conversion frequency to plants (28%) than cotyledonary-stage embryos isolated from intact callus (9%). The highest conversion frequencies for Kardinal (50-58%) occurred from cotyledonary-stage embryos that developed from dispersed callus and from the fraction of cells smaller than 850 microm.

No abstract

Embryogenic callus cultures of three genetically diverse cultivars of rose (Rosa hybrida L.), the floribunda `Trumpeter', the multiflora `Dr. Huey', and the hybrid tea `Tineké', were used to study the effect of various carbohydrates and osmotically active compounds on somatic embryo maturation and conversion. Cotyledonary-stage embryos were produced by dispersing callus in liquid medium followed by filtration to isolate globular-stage embryos. Quantitative experiments were conducted to determine maturation and conversion of the three rose cultivars in response to medium with sucrose, glucose, fructose, or maltose as the primary carbon source and also in response to various concentrations of either myo-inositol, polyethylene glycol, or mannitol in combination with 3% sucrose. Conversion of 27% was achieved for `Trumpeter' embryos following their maturation on 3% fructose. `Dr. Huey' embryos required maturation on medium containing 3% sucrose supplemented with either 2.5% or 5% mannitol for 36% and 61% conversion, respectively. Maturation of `Tineké' embryos on either 3% sucrose, 3% glucose, or 3% fructose resulted in a maximum 12% conversion.