Combined effect of hormonal priming and salt treatments on germination percentage and antioxidant activities in lettuce seedlings

Hormonal priming is a pre-sowing treatment that improves seed germination performance and stress tolerance. To understand the physiology of hormonal priming and its association with post priming stress tolerance, we investigated the effect of hormonal priming with increasing gibberellic acid (GA3) concentrations (0, 3, 4.5 and 6 mM) on seedling growth and antioxidant system in lettuce. Germination percentage was higher in lettuce seedlings derived from primed seeds. Radicle and hypocotyl length and dry weight were reduced by salt treatment to a greater extent in non-primed than in primed seeds. Hormonal priming with 4.5 mM GA3 induced the most dramatic decreases in electrolyte leakage (EL) and malondialdehyde (MDA) levels. NaCl increased catalase (CAT) activity in primed and non-primed seeds. The total ascorbate level remained constant in both primed and non-primed seeds under NaCl constraint. These results suggest that hormonal priming might have increased the salt tolerance of lettuce seeds through enhancing the activities of antioxidant enzymes and reducing the membrane damage as estimated using EL and MDA biomarkers.Key words: Ascorbate, germination, hormonal priming, lettuce, salinity

Scission, Cross-Linking, and Physical Relaxation during Thermal Degradation of Elastomers

Elastomers are susceptible to chemical ageing, i.e., scission and cross-linking, at high temperatures. This thermally driven ageing process affects their mechanical properties and leads to limited operating time. Continuous and intermittent stress relaxation measurements were conducted on ethylene propylene diene rubber (EPDM) and hydrogenated nitrile butadiene rubber (HNBR) samples for different ageing times and an ageing temperature of 125 °C. The contributions of chain scission and cross-linking were analysed for both materials at different ageing states, elucidating the respective ageing mechanisms. Furthermore, compression set experiments were performed under various test conditions. Adopting the two-network model, compression set values were calculated and compared to the measured data. The additional effect of physical processes to scission and cross-linking during a long-term thermal exposure is quantified through the compression set analysis. The characteristic times relative to the degradation processes are also determined

Erroneous or Arrhenius: A Degradation Rate-Based Model for EPDM during Homogeneous Ageing

To improve the predictive capability of long-term stress relaxation of elastomers during thermo-oxidative ageing, a method to separate reversible and irreversible processes was adopted. The separation is performed through the analysis of compression set after tempering. On the basis of this separation, a numerical model for long-term stress relaxation during homogeneous ageing is proposed. The model consists of an additive contribution of physical and chemical relaxation. Computer simulations of compression stress relaxation were performed for long ageing times and the results were validated with the Arrhenius treatment, the kinetic study and the time-temperature superposition technique based on experimental data. For chemical relaxation, two decay functions are introduced each with an activation energy and a degradative process. The first process with the lower activation energy dominates at lower ageing times, while the second one with the higher activation energy at longer ageing times. A degradation-rate based model for the evolution of each process and its contribution to the total system during homogeneous ageing is proposed. The main advantage of the model is the possibility to quickly validate the interpolation at lower temperatures within the range of slower chemical processes without forcing a straight-line extrapolation