ABSTRACT:
The objective of this study was to determine the number of days and degree-days in which panicle initiation (PI) takes place before panicle differentiation (PD) for rice cultivars with different developmental cycle duration. A field experiment was carried out in a lowland area, in Santa Maria, RS, Brazil. The experiment consisted of three sowing dates and four rice cultivars ('IRGA 421', 'BRS Querência', 'IRGA 424' and 'SCSBRS Tio Taka') during two growing seasons (2011/12 and 2012/13), in a complete randomized block design with four replications. The date of PD was estimated from the tables contained in STEINMETZ et al. (2010STEINMETZ, S. et al. Uso de graus-dia para estimar a data de diferenciação da panícula (DP) de seis subgrupos de cultivares de arroz irrigado visando à adubação nitrogenada em cobertura no Rio Grande do Sul., Pelotas: Embrapa Clima Temperado 2010. 75p. (Boletim de pesquisa e desenvolvimento, 121).) and around 14 days before the estimated date of PD. Ten plants were sampled daily to determine PI in a laboratory under a 45 times magnifying glass. After PI, 10 plants were sampled daily to determine PD using the "cotton ball" stage as a marker. Thermal sum was calculated considering the basal temperature of 11ºC. The greatest differences between the dates of occurrence between PI and PD were 8 and 6 days, and 116.4 and 73.7ºC day. Most of the differences varied between 2 and 8 days, and between 23.8 and 116.4ºC day. It was concluded that the maximum period that PI takes place before the PD is eight days, corresponding to 116.4ºC day, being influenced neither by sowing date nor by the length of cultivars cycle.
Key words:
thermal time; nitrogen topdressing; Oryza sativa; plant development
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