Assessment of daily heat pulse regimes on the germination of six Amaranthus species

In Mediterranean environments, thermal requirements for seed germination of Amaranthus species are met in spring. Nonetheless, seedlings of these species start emerging in the field from late winter, although the theoretical heat sum thresholds required for germination are not likely to be reached under field conditions. We hypothesised that soil thermal fluctuations can reduce the heat requirements of these species. Cardinal temperatures and thermal times of six Amaranthus species (A. albus, A. cruentus, A. deflexus, A. graecizans, A. retroflexus and A. viridis) were determined on nondormant seeds at constant temperatures ranging from 10 to 40°C. Subsequently, germination response to heat pulses was studied by imposing a thermal fluctuation regime of 32/8°C with two different thermoperiods of 3/21 and 6/18 h. In the two thermoperiods, exposure to 32°C was imposed for a different number of days: from 1 to 12 and from 1 to 6 heat pulses cycles in the 3/21 and 6/18 h thermoperiods respectively. Cumulative germination, germination rate and mean germination time were evaluated. Heat sum requirements and final germination percentage were affected by thermoperiod and number of thermal cycles. Amaranthus spp. germination was higher and faster when seeds were submitted to 6 h compared with 3 + 3 h of heat pulses. Our data showed that heat sum requirements for germination may change depending on the way in which varying temperatures are imposed on germinating seeds.