Solar UV-B radiation limits seedborne anthracnose infection and induces physiological and biochemical responses in Lupinus mutabilis

The aim of this study was to test whether solar UV-B radiation and temperature have an effect on infection of lupin seeds by Colletotrichum acutatum. Samples of infected seed were placed in a solar oven and exposed on sunny days for 15, 30, 45, 60, 75, 90 and 120 min. The degree of reduction in disease incidence and seed germination was dependent on the exposure time. Exposure times of 75 min (UV-B 4.41 kJ m(-2), approximate to 76 degrees C) and higher reduced incidence from 5% to undetectable levels, but also reduced seed germination by around 10% compared with untreated seed. Therefore, in a second experiment, infected seeds were exposed for 45 or 60 min in the solar oven (UV-B 2.83 or 3.75 kJ m(-2), respectively, approximate to 76 degrees C), for 60 min at ambient temperature (UV-B 3.75 kJ m(-2), approximate to 21 degrees C) or to dry heat for 60 min at 75 degrees C. Exposure for 60 min in the solar oven reduced seed infection by 99%, while UV-B-radiated seed at ambient temperature or dry-heat reduced infection by 60% or 32%, respectively. To evaluate the effect of UV-B plus high temperature on seedlings, lupin seed exposed for 45 or 60 min (UV-B 2.83 or 3.75 kJ m(-2), respectively, at approximate to 76 degrees C) were grown and physiological and biochemical responses of the seedlings were assessed. Seedlings from exposed seed had higher total concentrations of chlorophyll, protein and peroxidase activity than those grown from unexposed infected seed.