Abstract
The objective of this study was to evaluate the effect of priming treatments on the upper and lower thermal limits for germination of Urochloa brizantha cv. basilisk, and testing the hypothesis that pré-imbibition affect thermal parameters of the germination. Pre-imbibed seeds both in distilled water (0 MPa) and PEG 6000 solution (-0.5 MPa) were put to germinate in different temperatures. It is suggested that U. brizantha seeds have low response to priming when they were placed to germinate in medium where water is not limiting. The response of U. brizantha seeds to priming is dependent on the temperature and water potential conditions at which the seeds are pre-imbibed, as well as on the germination temperature. The optimum temperature for germination of U. brizantha shift toward warmer temperatures in primed seeds. Priming effect was more pronounced at temperatures closer to the upper and lower limit for germination, but probably that response cannot be accounted for changes in the thermal time constant (θT(g)) and ceiling temperature (Tc(g)). Otherwise, a decrease in the base temperature (Tb) was observed in primed seeds, suggesting that the Tb distribution in U. brizantha seeds is influenced by priming.
Highlights
The seeds exhibit a minimum, an optimal and a maximum temperature for germination
The water content of Urochloa brizantha seeds soaked for 24h in polyethylene glycol 6000 (PEG) and distilled water (DW) was affect both by temperature and priming medium
That response was expected since earlier reports show that temperature affect the rates of water uptake primarily by changing the water viscosity, the increase in water uptake with temperature can be discontinuous from 5 °C to 35 °C as reported for pine embryos in which the slope of the regression line of the water uptake on temperature was significantly steeper above 20 °C (Murphy and Noland, 1982)
Summary
The seeds exhibit a minimum, an optimal and a maximum temperature for germination (the cardinal temperatures). A thermal time (or degrees-day) approach have been used to describe the distribution of the times to germination at different temperature regimes according to the models θT(g) = (T-Tb)t(g), for suboptimal temperatures, and θT = (Tc(g)-T)t(g), for supraoptimal ones, where θT(g) is the thermal time required for (g) percent of the seeds germinate, T is the temperature, Tb is the minimum or base temperature, t(g) is the time for (g) percent of the seeds germinate and Tc(g) is the maximum or ceiling temperature corresponding to a percentage fraction (g). Basilisk σθT (standard deviation in thermal time), Tc(50) (median Tc) and σTc (standard deviation in Tc(g)) are known, the germination time courses at different temperatures can be normalized on a common thermal time scale which allows the germination rate at any temperature regime can be predicted (Bradford, 1995). The germination time of a given percentage (tg) is inversely proportional to the difference between Ψ and Ψb(g) (the base or threshold Ψ capable of preventing a percentage (g) to germinate), and the variation in germination rates among seeds in the population can be accounted by shifts in Ψb(g) distributions (Bradford and Still, 2004; Finch-Savage, 2004). Alvarado and Bradford (2002) proposed that the distribution of Ψb(g) among the seeds accounts for the distribution of Tc(g) since above T optimum the accumulation of thermal time would stops and the temperature effects on the germination would be primarily due to changes in Ψb(g)
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