Plasticity of the thermal requirements of exotherms and adaptation to environmental conditions.

Abstract

In exothermal organisms, temperature is an important determinant of the rate of ecophysiological processes, which monotonically increase between the minimum (td min) and maximum (td max) temperatures typical for each species. In insects, td min and td max are correlated and there is a approximately 20°C interval (thermal window WT = td max − td min) between them over which insects can develop. We assumed that other exotherms have similar thermal windows because the thermal kinetics of their physiological processes are similar. In this study, we determined the thermal requirements for germination in plants. Seeds of 125 species of Central European wild herbaceous and crop plants were germinated at nine constant temperatures between 5 and 37°C, and the time to germination of 50% of the seeds D and rate of germination R (=1/D) were determined for each temperature and the Lactin model used to determine td min, td max, and WT. The average width of the thermal windows for seeds was significantly wider (mean 24°C, 95% CI 22.7–24.2°C), varied more (between 14.5 and 37.5°C) and development occurred at lower temperatures than recorded for insects. The limiting temperatures for germination, td min and td max, were not coupled, so the width of the thermal window increased with both a decrease in td min and/or increase in td max. Variation in WT was not associated with taxonomic affiliation, adult longevity, or domestication of the different species, but tends to vary with seed size. Plants are poor at regulating their temperature and cannot move to a more suitable location and as a consequence have to cope with wider ranges in temperatures than insects and possibly do this by having wider thermal windows. Synthesis: The study indicated specificity of WT in different exotherm taxa and/or their development stages.