Floret development and spike growth as affected by photoperiod during stem elongation in wheat Fernanda G. Gonza Âlez a,* , Gustavo A. Slafer a,b , Daniel J. Miralles a a Ca Âtedra de Cerealicultura, Departamento de Produccio Ân Vegetal, Facultad de Agronomõ Âa, Universidad de Buenos Aires, Av. San Martõ Ân 4453, Buenos Aires C1417DSE, Argentina b Instituto de Investigaciones Fisiolo Âgicas y Ecolo Âgicas Vinculadas a la Agricultura (IFEVA), Facultad de Agronomõ Âa, Universidad de Buenos Aires, Av. San Martõ Ân 4453, Buenos Aires C1417DSE, Argentina Received 20 June 2002; received in revised form 25 September 2002; accepted 4 October 2002 Abstract A longer duration of stem elongation (from terminal spikelet initiation to anthesis) would result in a higher wheat yield potential as a consequence of the increase in both spike dry weight and number of fertile ¯orets at anthesis. It is not clearly understood,however,whichprocessesareinvolvedintheincreasedspikedryweightandwhetherthisvariableaccountsforthe higher number of fertile ¯orets when duration of stem elongation is modi®ed by photoperiod treatments. As vernalization interactswithphotoperiodduringthestemelongationphase,abetterunderstandingoftheroleofvernalizationisalsorequired. A®eldexperimentwascarriedoutusingtwowheatcultivars(BuckManantial(BM)andEurekaFerrocarrilSur(EFS))subjected to diverse periods of vernalization (V 15 andV 50 , plants vernalized for 15 and 50 days, respectively) and exposed to different photoperiods (NP  0andNP  6,naturaland6hextendedphotoperiod,respectively)onlyduringthestemelongationphase. Floret development together with stem and spike growth during the stem elongation phase were studied. When the stem elongationphasewasexposedtoshortphotoperiod(i)durationofspikegrowthincreasedwithoutanychangeinitsgrowthrate, and (ii) most of the spike growth occurred during minimum stem growth rate as the onset of maximum stem growth rate was delayed.InEFS,thevernalization-sensitivecultivar,theformer(i)and(ii)wastrueonlywhenvernalizationrequirementswere met. For both cultivars, the survival of the initiated ¯oret primordia under short photoperiod increased, independently of vernalization.Bothpartitioningofassimilatestothespikeandadirectphotoperiodeffectseemedtohavebeeninvolvedinthe survival of ¯oret primordia when the stem elongation phase was exposed to short photoperiod. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Photoperiod; Vernalization; Floret development; Spikegrowth; Wheat 1. Introduction Wheat yield potential is determined during the last part of stem elongation phase before anthesis, when thenumberoffertile¯orets,andtherebythenumberof potential grains, is set (Fischer, 1975, 1985). In gen- eral, between 6 and 12 ¯oret primordia are initiated perspikeletdependingonspikeletpositionwithinthe spike (Langer and Hanif, 1973; Sibony and Pinthus, 1988;Yousse®anetal.,1992).Nevertheless,normally lessthanfourto®ve¯oretprimordiasurviveandreach the fertile stage at anthesis (Langer and Hanif, 1973; Kirby, 1988). Floret death or degeneration coincides with the period of maximum stem and spike growth Field Crops Research 81 (2003) 29±38 * Corresponding author. Tel.: 54-11-4524-8039/8053; fax: 54-11-4514-8737. E-mail address: fgonzale@agro.uba.ar (F.G. Gonza Âlez). 0378-4290/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-4290(02)00196-X