2.1. Plant material R. hybrida cv. ‘Mercedes’ plants were planted in containers filled with perlite and grown in a plastic greenhouse at Ben-Gurion University of the Negev (Israel). Temperatures were maintained year round at 14 8C minimum to 30 8C maximum. Plants were irrigated until drainage with a nutrient solution consisting of 1.5 mL L1 of a commercial fertilizer (N:P:K, 7:3:7), 100 mg L1 of (NH4)2SO4, 80 mL L1 of microelement solution (Koratin, Fertilizers and Chemicals Ltd., Israel), containing 0.55% Fe; 0.27% Mn; 0.2% B; 0.13% Zn; 0.02% Cu and 0.015% Mo, and 80 mg L1 of chelated Fe (Sequestrene, Ciba-Geigy, Switzerland). The pH of the nutrient solution was adjusted to 5.5 by addition of H2SO4.