Abstract
Few works report the use of degree-days (DD) - used in crops to predict events and schedule management activities - to describe the germination of tropical trees. The cardinal temperatures (base, optimum and ceiling temperature) for germination of the species may vary depending on the seed provenance. Peltophorum dubium (Spreng.) Taub. is an early successional leguminous tree widely distributed in South America, often occurring as cultivated or naturalized trees, thus considered to be a good example for testing DD model in tree species. The main objective of this study was to describe the seed germination response of different populations of P. dubium as function of DD accumulation during germination assays in semi-controlled (fluctuating temperatures) conditions. Germination assays with manually scarified seeds sown in aluminum sheet trays filled with a composed substrate were performed under greenhouse conditions at different times. Three methods were employed in order to describe the accumulation of thermal time throughout the assays and, considering the seed lot and sowing time, a trapezoid area method was relatively more effective in describing the germination. The germination curves of P. dubium seeds from different populations, expressed in degree-days estimated directly from temperature records schedules, tend to be more clustered suggesting little variation among thermal time requirements in different seed provenances. Otherwise, the thermal time requirement can vary depending on the time of sowing, and any increase in DD requirement when the assays were performed under higher mean temperatures can be related to a thermal effect on the germination of scarified seeds.
Highlights
Temperature is the most important environmental factor regulating seed germination, when there is a reasonable water supply
Seeds from different provenances did not differ in final germination percentage in the greenhouse, except in Assay IV (Figure 1g) where the germination of Porto Velho (PVH) seeds was lower than that observed for other populations (p
Germination percentage of P. dubium seeds from different provenances was similar to each other in the assays carried out in July, August and September, whereas the germination percentage of the PVH seed batch was lower than others in the assay IV performed in December, at higher mean temperatures as compared to previous assays
Summary
Temperature is the most important environmental factor regulating seed germination, when there is a reasonable water supply. Germination responses to temperature can be characterized by cardinal temperatures, that are the minimum or base (Tb), the optimum (To), and maximum or ceiling (Tc), with Tb and Tc being the lowest and highest temperatures, respectively, at which a given percentage of seed can germinate (Alvarado and Bradford, 2002) Between these extremes (Tb and Tc), temperature primarily influences the germination rate (i.e. reciprocal of the time taken for radicle emergence to occur), and for many species this rate increases linearly between Tb and To, decreases linearly between To and Tc. Between these extremes (Tb and Tc), temperature primarily influences the germination rate (i.e. reciprocal of the time taken for radicle emergence to occur), and for many species this rate increases linearly between Tb and To, decreases linearly between To and Tc Such germination rate dependency on temperature is one of the most important factors controlling the emergence timing of a seed (Bewley et al, 2013; Simão et al, 2010). Such corrected time is commonly referred to as “thermal time”, or “heat units”, and basically it is the accumulated product of time and the difference between actual temperature (T) and Tb for each day (or hour), calculated in degree-days or degree-hours units (UC-IPM, 2014)
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