A procedure for adventitious shoot regeneration from leaf explants of quince (Cydonia oblonga Mill.) using thidiazuron (TDZ) was developed. Excised leaves of cultures grown on Murashige and Skoog (MS) medium containing 5 p~M benzyladenine (BA) and 0.9% Gibco Phytagar were used. Several experiments were conducted to determine optimum concentrations of thidiazuron, a-naphthaleneacetic acid (NAA) and sucrose. When the medium contained 1.5 txM TDZ and 2.5 ~M NAA, 85% of the discs regenerated shoots with an average of eight shoots per leaf disc. An incubation period of three weeks in the dark was necessary for optimum shoot regeneration. Leaves excised from four to six-week-old cultures gave a higher percent shoot regeneration than leaves from cultures older than six weeks. Regeneration percentages were significantly reduced when sucrose concentration in the medium was less than 3%. A significantly higher percentage of shoots regenerated when leaf discs were placed on the regeneration medium abaxial side down as compared to the adaxial side.Regenerated shoots were cultured on MS medium containing 5 txM BA and rooted on half-strength MS medium containing 10 txM NAA. Rooted plantlets were acclimatized to greenhouse conditions for evaluation of any somaclonal variation. The importance of these findings are discussed in relation to in vitro improvement of plants.

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Arachidonic acid-stressed potato tuber discs synthesized the phytoalexin rishitin. This synthesis was inhibited by salicylhydroxamic acid (SHAM), and to a lesser extent by tetraethylthiuram disulfide (disulfiram pg/ml. This mixture was shaken on a Vortex mixer and centrifuged at 10,000g for 30 min. The droplets of fatty acid floating on the surface were removed by aspiration and the actual arachidonic acid concentration was determined by titration of a 5-ml aliquot. Ethanol was added to completely dissolve the fatty acid prior to the titration. The arachidonic acid suspension was then diluted to 100 ,ug/ml with Mes. When used, SHAM and disulfiram in methanol were added to the arachidonic acid suspension before it was diluted. The total methanol concentration was 2% (v/v)

Potato (solnum tuberosum L. cv Katahdin) tuber discs treated with arachidonic acid become necrotic and accumulate sesquiterpenoid phytoalexins. The arachidonic acid also causes increases in both phenylalanine ammonia lyase and lignin, but no change in total alcohol-soluble phenols. Linoleic acid does not alter any of these parameters. A high concentration of nonanoic acid promotes both necrosis and accumulation of low levels of phytoalexins, but decreased levels of phenols, phenylalanine ammonia lyase, and lignin. The respiration of the control discs and those treated with linoleic acid declines by 24 hours after treatment, but the respiration of arachidonic acid-treated discs remains constant for at least 48 hours.Plants often respond to attempted fungal invasion by synthesizing toxic metabolites called phytoalexins (12,22 (3,24).In this paper, we have extended the studies ofarachidonic acid by investigating the effect of this fatty acid on both phenol metabolism and respiration in potato tuber tissue. In addition, we include data on the effect on nonanoic acid treatments because this fatty acid has been reported to cause necrosis of plant tissues (15, 20), although it is not known to be an elicitor of phytoalexins. MATERIALS AND METHODSChemicals. Arachidonic and linoleic acids (both 99%) and nonanoic acid (97-99%) were purchased from Sigma Chemical Co. Both stock and working solutions were prepared as previously described (21), except that the buffer was pH 7.0 rather than pH 5.5.Plant Materials. Tubers of white potato (Solanum tuberosum L. cv Katahdin) were obtained from field plantings at a West Virginia University Agricultural and Forestry Experiment Station farm. They were stored at 4°C for 8 to 10 months and removed from cold storage 1 d before use. Discs (2 mm x 1 cm) were cut from the pith area of these tubers, rinsed in distilled H20, and incubated in Petri dishes at 20°C for 24 h.The aged potato discs were immersed for 7 min in buffer as control, or buffer containing one of the fatty acids and then incubated at 20C in Petri dishes or, when tissue respiration was going to be measured, in respirometer flasks. At the end of the incubation period, the extent of necrosis was estimated visually, and the discs were used immediately for respiration or PAL assays, or stored at -80°C for later analyses.Phytoalexins. The phytoalexins rishitin and lubimin were determined after homogenizing the entire potato discs by the method of Henfling and Kuc (10).Phenylalanine Ammonia Lyase. A crude preparation of PAL was made (11) and the enzyme was then assayed by the method of Zucker (30) except that the buffer was Tris-HCl rather than borate.Phenols. The potato discs were extracted with ethanol in absence of reducing agent by the method of Khanna et al. (13) and the extracted phenols were then assayed by the colorimetric procedure of Swain and Hillis (23).Lignin. Potato tuber discs were thinly sectioned with a razor blade and 1 g of tissue was then shaken with 20 ml of methanol for 1 h. The sample was centrifuge...

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