Plant Cell, Tissue and Organ Culture 24: 187-191, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands. Effect of potassium humate on apple cv. 'Golden Delicious' cultured in vitro Rita Baraldi 1, Francesca Fasolo Fabbri Malavasi 1, Stefano Predieri I & Marco Castagneto 2 lCentro di Studio per la Teeniea Frutticola, CNR Via Filippo Re, 6, 40126 Bologna, Italy; 2Veneta Mineraria, Via Palmanova, 24, 20132 Milano, Italy Received 3 April 1990; accepted in revised form 25 October 1990 Key words: apple, benzyladenine, indole-3-butyric acid, in vitro culture, potassium humate Abstract The effects of humic substances on in vitro culture of 'Golden Delicious' apple are reported. Potassium humate (KH) when used in proliferation showed a negative interaction with BA while it enhanced rooting when IBA was not present in the culture medium. In the presence of IBA, KH increased root number and reduced root growth. The highest concentration tested, 500 mg 1- i, caused a drastic reduction in root system development. 50 mg 1-1 KH hastened rooting and plants grew more rapidly when transferred to soil. Introduction Humic substances (HS) can affect plant physiology and stimulate growth but their hormone-like ac- tivity has often been questioned. A recent study, where bioassays were used, reported that HS have cytokinin and auxin-like activity, 100 and 10 times lower than that of benzyladenine (BA) and indole- 3-acetic acid (IAA), respectively [1]. The auxin-like activity of HS has also been reported by several authors in some herbaceous species [8,10-13,16]. The present paper deals with the influence of humic substances on the in vitro culture and acclimation of 'Golden Delicious' apple. Materials and methods 'Golden Delicious' (Malus pumila Mill.) apple shoots were initially grown on a proliferation medium containing MS [9] salts, LS [4] vitamins, sucrose 2% (w/v), agar (B&V, Italy) 0.65% (w/v) and BA 1 mg1-1. Media pH was adjusted to 5.7 with KOH 0.1N solution before autoclaving for 20 min at 120°C. Cultures were incubated in growth chambers at 22 +_ 2°C with 16h/daylight from cool white lamps at a photon flux of about 60/~Mm :s '. Shoots were subcultured every 20 days. Shoots, 1.5 cm long, were used for trials. Exclud- ing hormones, the proliferation medium used in trials was the same, as was the rooting medium except that the mineral salts were reduced to one- half. Humic substances Commercial potassium humate (KH) (Veneta Mineraria S.p.A, Italy) was extracted from peat with KOH solution. It consisted of a humic frac- tion and the elements K, Na, Ca, Mg, Fe (41.4, 20.92, 0.1, 0.49, 0.04, 0.26%, respectively), traces of fulvic acid, an insoluble fraction (12.1%) and water (24.69%). Experimental design The effect of KH, which was added before adjust- ing the pH and autoclaving the medium, was tested at 0, 50, 250 and 500mg1-1 of active ingredient (humic fraction) and combined with BA (0, 0.2 or 1 mg1-1) or indole-3-butyric acid (IBA) (0, 0.06