Temperature is the primary factor involved in breaking of physical dormancy (PY). Depending on the species, PY-break in seeds can take place either in one step or in two steps. The process of PY-breaking in seeds of certain annual species takes place in two steps controlled by two different temperature and/or moisture regimes. During the first step, PY-seeds become sensitized to dormancy-breaking treatment(s), yet they remain impermeable. During the second step, seeds become permeable upon exposure to the appropriate environmental conditions. The concept of thermal time, i.e. the exposure to a temperature above a threshold level for a particular time period, has been successfully applied in determining and comparing the rates of various physiological events in plants and poikilothermic invertebrates. This concept has been employed in describing and quantifying physiological dormancy (PD)-break by after-ripening and single-step PY-break. However, the concept of thermal time has not been used for the explanation of stepwise PY-breaking processes.
A recent paper in Annals of Botany builds a thermal time (degree-weeks) model to explain sensitivity induction quantitatively in driving the two steps of PY-breaking in Geranium carolinianum seeds and proposes a mechanism to explain PY-breaking, focusing on the water gap region of the seed. Thus the water gap region acts as a thermal sensor that detects the onset of autumn.
Quantitative analysis of the thermal requirements for stepwise physical dormancy-break in seeds of the winter annual Geranium carolinianum (Geraniaceae). (2013) Annals of Botany 111 (5): 849-858. doi: 10.1093/aob/mct046
Physical dormancy (PY)-break in some annual plant species is a two-step process controlled by two different temperature and/or moisture regimes. The thermal time model has been used to quantify PY-break in several species of Fabaceae, but not to describe stepwise PY-break. The primary aims of this study were to quantify the thermal requirement for sensitivity induction by developing a thermal time model and to propose a mechanism for stepwise PY-breaking in the winter annual Geranium carolinianum. Seeds of G. carolinianum were stored under dry conditions at different constant and alternating temperatures to induce sensitivity (step I). Sensitivity induction was analysed based on the thermal time approach using the Gompertz function. The effect of temperature on step II was studied by incubating sensitive seeds at low temperatures. Scanning electron microscopy, penetrometer techniques, and different humidity levels and temperatures were used to explain the mechanism of stepwise PY-break. The base temperature (Tb) for sensitivity induction was 17·2 °C and constant for all seed fractions of the population. Thermal time for sensitivity induction during step I in the PY-breaking process agreed with the three-parameter Gompertz model. Step II (PY-break) did not agree with the thermal time concept. Q10 values for the rate of sensitivity induction and PY-break were between 2·0 and 3·5 and between 0·02 and 0·1, respectively. The force required to separate the water gap palisade layer from the sub-palisade layer was significantly reduced after sensitivity induction. Step I and step II in PY-breaking of G. carolinianum are controlled by chemical and physical processes, respectively. This study indicates the feasibility of applying the developed thermal time model to predict or manipulate sensitivity induction in seeds with two-step PY-breaking processes. The model is the first and most detailed one yet developed for sensitivity induction in PY-break.