In the present work, the effects of storage time and temperature on the resonance signals of unirradiated and irradiated dry pea (Pisum sativum L.) were investigated using electron spin resonance (ESR) spectroscopy. As other legumes, unirradiated pea contains Mn2+ binded proteins and very small amount natural free radicals. They give rise to an ESR spectrum consisting of an equally spaced sextet and a singlet resonance line, respectively, both appearing at g = 2.0050 +/- 0.0007. Irradiation of pea by gamma radiation did not produce any pattern changes in the ESR spectrum of pea except a significant change in the intensity of the singlet resonance line which increased exponentially with absorbed dose in the studied dose range (1.25-15.0 kGy). Cooling the irradiated samples down to room temperature caused a reversible increase in Mn2+ and a reversible decrease in free radical signal intensities. However, heating the sample above room temperature created irreversible decreases in the intensities of both species. At room temperature, the free radical signal decayed very fast, but the signal due to Mn2+ ion did not exhibit any intensity changes over the storage period of 75 days. While, annealing of irradiated samples at high temperatures (308-373 K) produced continuous decrease in the signal intensity of Mn2+, it caused a decrease at the beginning and then an increase in the free radical signal intensity. Arrhenius plot constructed using rate constants determined from the variations of the signal intensities at high temperatures was used to calculate activation energies related with oxidation of Mn2+ and free radical species responsible from singlet resonance line. (C) 2003 Elsevier Ltd. All rights reserved.