Heat shock effects on germination and seed survival of five woody species from the Chaco region

Abstract

In tropical dry forests, fire is a common disturbance and seed traits may drive regeneration of plant populations during these events. In this study, we aimed to (i) investigate whether the seed coat and/or fruit endocarp impose physical dormancy, (ii) check whether physical dormancy confers seed tolerance to heat shocks and (iii) verify whether heat shocks break physical dormancy among woody species. Seeds were collected from five native species of dry forests occurring within the Argentine Chaco: Aspidosperma quebracho-blanco, Sarcomphalus mistol, Schinopsis lorentzii, Prosopis nigra and Vachellia aroma. Initial seed viability was checked by the tetrazolium test before treatments. Heat shocks of 80, 110, 140, and 170 °C simulating a fire event were applied to the seeds for five minutes. Controls consisted of untreated seeds subjected or not to scarification. After heat shock treatments, the seeds were incubated in a germination chamber set at 30 °C under a photoperiod of 12 h (white light). After 35 days, non-germinated seeds were scarified, and after more 55 days the remaining seeds were checked for their final viability. Seed coat does not arrest germination in seeds of A. quebracho-blanco and showed germination after heat shock of 80 °C. Seeds of S. mistol present a fruit wall-imposed dormancy, but barely tolerated heat shock of 80 °C. S. lorentzii present seed dormancy, but not imposed by the seed coat since scarification was ineffective to promote germination; its seeds do not tolerate heat shock. Seeds of P. nigra present endocarp-imposed dormancy, and they germinate after heat shock of 80 °C. Seeds of V. aroma present coat-imposed physical dormancy, and they tolerate heat shock of 80 and 110 °C, but these treatments did not alleviate dormancy. Seed viability among non-germinated seeds after heat shocks were close to zero, revealing a low tolerance to high temperatures. Overall, seed coats did not protect seeds from the damaging effects of heat shock, and heat shock treatments did not break coat- or endocarp-imposed dormancy among the selected species. The heat sensitivity presented by the diaspores suggests that frequent wildfires may reduce their recruitment rates, compromising the maintenance of genetic diversity and the current distribution of their natural populations over the Chaco region.