To determine whether seeds of the weedy shrub Wigandia urens, from the Valley of Mexico, undergo natural priming when buried in soil, comparative experiments were performed with seeds: (1) harvested directly from the plants; (2) buried in three natural habitat conditions; and (3) laboratory primed with polyethylene glycol. Seeds were sown in a growth chamber and in a shade house. Final germination percentages, emergence, germination and emergence rates, survival and initial growth were determined. Burial and priming enhanced the germination and emergence parameters evaluated in the laboratory and in the shade house. Effects of treatments on survival were not significantly different. Nevertheless, burial improved emergence and mean survival, and induced differences in specific leaf area of seedlings that could have ecological significance. Heat-stable proteins were extracted and electrophoresed. Proteins formed in W. urens seeds during burial had molecular weights (14–21 kDa) similar to those reported for late embryogenesis abundant (LEA) proteins induced by priming in other species. Nevertheless, the presence and abundance of proteins expressed (14–23, 36 and more than 45 kDa) differed among control, primed and buried seeds. During soil burial, molecular and physiological responses were induced that were similar to the effects of priming.

Germination of nondormant seeds of Manfreda brachystachya (Agavaceae) was analyzed at temperatures ranging from 11–35°C. Maximum germination (95%) occurred at 25°C. An exponential sigmoid relationship was found between time and cumulative germination. Germination rate for every subpopulation (10–90% germination) was estimated by means of a normal distribution analysis. The kurtosis indicated die amplitude of the range of temperatures where the highest germination rates were concentrated, and the skew indicated sharply inhibitory temperatures in the range of temperatures used. Based on analysis of the normal distribution models for each subpopulation, we calculated a theoretical function which described germination rate over the temperature range considered: F(T,χ) = A × exp[−B(C−1)2], where A is the function that describes germination rate for each subpopulation (characterized by the percentage [χ] at optimal temperature); B is a shape parameter, 1/(σG2); and C is the ratio between each germination temperature (T) and the optimal germination temperature. The Gaussian curves were used to calculate thermal time, and base and ceiling temperatures. Germination thermal time ranged from 1 333 to 2 373°C h, and base and ceiling temperatures were 10.44 ± 0.7°C and 39.54 ± 0.7°C, respectively. There was a linear relationship between thermal time and cumulative percentage of germination of the subpopulations. Based on fitted curves for each subpopulation, the use of a general model for all the subpopulations has been proven: F8 = A × exp[−5.9437(C−1)2], where changes in the curves for each subpopulation depended on temperature only.

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