Estimation of base temperature for Sorghum halepense rhizome sprouting

Sorghum halepense is a rhizomatous perennial summer weed. It spreads by seeds and rhizomes that form ramets. The development of predictive models for ramet emergence (RE) from rhizomes may provide important information about their seasonal dynamics and consequently for the timing of control operations. The aim of this work was to analyse the effect of temperature on S. halepense rhizome sprouting (RS) and to estimate its base temperature, which is the first step for creating mathematical models to predict RE. Rhizomes collected from two North Italian populations were incubated at several constant temperatures. The total number of ramets, sprouted rhizomes and their ratio (ramet/sprouted rhizome) were calculated. The influence of temperature on sprouting performances was analysed. Base temperature was calculated using two methods: the reciprocal time of 50% RS (method) and RE (method). Confidence limits (at 0.95 level) were defined for each estimated base temperature using a bootstrap method. Base temperatures were compared to verify whether there were differences between populations and methods. Results indicated a significant stimulating effect of temperature on S. halepense rhizome performances. Estimated base temperatures did not differ between populations, nor between methods within a population (Padova RS 9.7 ± 1.25°C (confidence limits), RE 10.0 ± 1.28°C, Mantova RS 8.0 ± 1.89°C and RE 9.7 ± 1.23°C). The same thresholds may consequently be adopted for thermal time models for the two populations. These results also suggest that both methods (RE and RS) can be used without distinction to estimate the rhizome base temperature.