Shoot‐forming tobacco (Nicotiana tabacum L. cv. Wisconsin 38) callus produces less endogenous ethylene than non‐shoot‐forming tissue cultured in the light (16 h photoperiod) or the dark. In shoot‐forming tissue more ethylene is produced early in culture (days 0–5) than later. Also dark‐grown tissue produces much more ethylene than light‐grown. On the basis of experiments in which (1) gaseous ethylene was added to or (2) CO2 removed from the flasks, (3) Ethrel (an ethylene releasing agent) and (4) 1‐aminocyclopropane 1‐carboxylic acid (an ethylene precursor) were added to the medium, it was determined that this gaseous phytohormone had two contrary effects on shoot initiation (shoot primordium formation). Early in culture (days 0–5) endogenous or exogenous ethylene inhibited organogenesis, but later (days 5–10) exogenous ethylene or increased endogenous ethylene production speeded up primordium formation.

We examined ethylene effects on root regeneration in tomato leaf discs cultured in vitro. Applied ethylene or Ethephon did not stimulate rooting in the leaf discs. In the presence of indoleacetic acid. 5 × 10‐6M, these substances significantly inhibited root formation. Ethylene production (nl C2H4· (24 h)‐1. flask‐1) was positively correlated with increased IAA concentrations at various times during the culture period and, as a consequence, with the rooting response after 168 h. However, separate testing of equimolar concentrations of seven different auxins and auxin‐like compounds showed no positive correlation between the rate of ethylene production and subsequent rooting response. Aeration of gas‐tight flasks containing leaf discs and absorption of ethylene evolved from the discs by mercuric perchlorate in gas‐tight flasks or pre‐treatment of leaf discs with AgNO3 significantly enhanced IAA induced root regeneration. Thus, these studies indicate that ethylene is not a rooting hormone per se. Furthermore, ethylene (whether applied externally or synthesized by the tissue) does not appear to account for the ability of auxin to stimulate rooting.

Tobacco callus cultures grown on defined agar-solidified media produced ethylene in differing amounts, which were related to cultural treatment and age of the callus. There was a close correlation between the rate of etbylene production and growth. In darkness, maximal rates occurred in the third week of growth with ethylene production in the range of 750 ni (calius piece)^^ d^' (fr, wt, = 1,5 g),, and in the light, maximal rates occurred in the first week of growth, 200 nl (callus piece)"' d"' (fr, wt, = 200 mg). Growth was also correlated with ethylene production when the latter was altered by exposure of the callus to inhibitors of ethylene synthesis, L-canaline, benzyl isothiocyanate, and 3,5-diiodo-4hydroxy-beazoic acid. No correlation was found following treatment with AgNOj, a presumptive inhibitor of ethylene action. The inhibition of growth and ethylene production by L-canaline was partially reversed by gassing the cultures with etiiyiene (1 ^1/1), A mercuric perchtorate sink had no significant effect on growth, A possible relationship between ethylene evolution and growth is discussed.Abbreviations: BITC, benzyl isothiocyanate; DIHB, 3,5-diiodc-4-hydroxybenzoic acid; IAA, iiidole-3-acetic acid; GC, gas chromatcgraphy. Materials and MethodsCultural procedures. Calltis of tobacco stem pith wa: obtained from Nicotiana tabacum L, cv, Wisconsin 3S 0031-9317/79/080374-07 $03,00/0 © 1979 Physiologia Plantarum Physiol, Plant, 46, 1979