Effect of short duration exposure to high temperatures on weed seed germination

Physical soil management is considered an alternative to chemical disinfestation to lower the pressure of many soil-borne pests ad diseases, particularly on high-value crops. Thermal methods, and steaming in particular, have proved to be effective also in reducing weed seed germination. However, limited information exists on requirements in terms of temperature and time of exposure for seed thermal death in different weed species. Some studies pointed out that in the case of steaming, the degrees of weed seed devitalisation is strongly related to the maximum temperature achieved into the soil, while treatment duration seems to play a secondary role. At this purpose, we started a series of experiments aimed at defining, for some weed species, the effects on germination of several combinations of temperature and time under controlled conditions. The objective of this work, in particular, was to estimate the effect of temperature only, by exposing the seeds for a very short time to a wide range of temperatures. Seeds of Echinochloa crus‑galli (barnyardgrass), Solanum nigrum (black nightshade), Galinsoga quadriradiata (sin. G. ciliata, hairy galinsoga), Setaria viridis (green bristlegrass), Portulaca oleracea (little hogweed) and Amaranthus retroflexus (redroot amaranth) were treated at twenty thermal levels, ranging from 48°C to 86°C with a 2°C‑interval between temperatures. For each species and temperature three 10ml Pyrex test-tubes were filled with 3g of sandy-loam soil mixed with 60 seeds. Soil moisture was previously adjusted to 80% of field capacity. The tubes were maintained at 4°C during the 24h before the thermal treatment (2h in the case of P. oleracea) to prevent seed germination, yet allowing the seeds to imbibe water from the soil. Thirty minutes before thermal treatment, the tubes were dipped in a 23°C water bath. Soil temperatures were monitored using T‑type thermocouples inserted into the tubes by‑passing the cap and in contact with the soil. Temperatures were acquired by a datalogger every 2s and average of two probes were used to monitor the temperature into the tubes. The tubes were dipped in a water bath in which the temperature was set 3°C higher than treatment temperature, in order to quickly heat the soil. As soon as the temperature in the tubes reached the treatment temperature, the tubes were immediately immersed in a 1°C water bath until the attainment of the initial temperature of 23°C. Immediately after, the seeds of each tube were distributed in three Petri dishes (9cm diameter; 20 seeds/dish), wetted and incubated at 25°C for 19 days. For each species three untreated tubes were maintained at room temperature as control. The thermal resistance to very short exposure of each species was described using a 3‑parameters log-logistic model, plotting the percentage of mortality respect to control (dependent variable) against the temperature (independent variable). Results showed that the species have a different thermal susceptibility. Yet, in the majority of species seed germination was severely affected by short exposure to temperatures falling in the range 54-74°C. The only exception was E. crus-galli, which was much more resistant to heat than other species and a temperature higher than 82°C was required to totally avoid seed germination.