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growth would lead to an increased number of FF and grains in wheat. The aim of this study was: (i) to analyze in three different commercial cultivars the impact of a photoperiod extension (NP + 10) during the stem elongation phase on the number of FF and (ii) to determine whether the photoperiod extension effect might be reversed by increasing the availability of assimilates by a direct modification of the source/sink ratio at booting. The number of FF per spike was reduced by extended photoperiod in the experiment sown in an optimum date, though no differences were observed when sown much later, probably due to the poor environmental conditions of a spring sowing. The detrimental effect of lengthening the photoperiod on the number of FF per spike was reversed by trimming the spike at booting in two of the three cultivars analyzed. This reversion, together with the effect of photoperiod extension on FF associated with similar effects on spike growth, may be considered as a confirmation that, at least in part, the detrimental effects of photoperiod extension on FF would be mediated by changes in assimilate supply to the developing florets. No differences were observed between photoperiod treatments on the onset of rapid spike growth; however, extended photoperiod reduced the duration of spike growth phase without differences in the maximum rate of spike growth in two of the three cultivars. Although a positive association was found between the number of FF and spike dry weight at anthesis, differences among cultivars were observed in this relationship, suggesting different carbohydrates partitioning (i.e., FF per unit of spike weight) among them. © 2007 Elsevier B.V. All rights reserved. Keywords: Floret development; Photoperiod; Fertile floret; Wheat (Triticum aestivum L.)Floret development; Photoperiod; Fertile floret; Wheat (Triticum aestivum L.)