The generative area as the site of floret initiation in the sunflower capitulum and its integration to predict floret number

Abstract

The number of disc florets in the capitulum of the cultivated sunflower ( Helianthus annuus L.) varies according to genotype and to growing conditions experienced in the early life of the plant prior to floral stage 8. Disc florets are generated by the receptacle and are accommodated on its surface during the maturation of the capitulum. The influence of receptacle size upon floret number has been examined by firstly measuring the area of the receptacle and the region of floret initiation (generative area) during the period of floret production and secondly recording the number and persistence of the floret parastichies (rows). Phenotypes with contrasting floret numbers were produced by growing the F1 hybrid Hysun 30 under short or long photoperiods or under four levels of nitrate nitrogen. The influence of genotype was investigated by sampling populations of Hysun 30 and the cultivar Sunfola 68-2, together with an experimental hybrid Sirosun 25H and its male (25R) and female (89A) parental lines. It was established that phenotypic and genotypic variation in disc floret production is expressed through the regulation of receptacle expansion during floral stages 5 to 8. This controls the number of floret rows initiated in floral stage 5 and the number of florets produced by each row in floral stages 6, 7 and 8. It is shown that the total number of disc florets can be predicted either from the daily integral of the generative area or from the initial number of long floret rows and the number of florets in each using the empirical expression [ a(0.67 × f)] + [ a′(0.33 × f)] where a is the initial floret row number, f is the number of florets in a row, and a′ is the next number down from a in the Fibonacci series.