Influences of genotype (cultivar), temperature, light intensity, gibberellic acid (GA 3 ) and daylength on stem elongation and flowering of spinach were investigated. Most cultivars reacted quantitatively to long days (LD) both for stem and flower formation. Daylength requirements varied from almost dayneutral to LD.Cold, 2-8°C, decreased daylength requirement; 9-12° only accelerated stem growth. All cvs reacted similarly to temperature. Dimmer light decreased daylength requirement, especially in later cvs. GA 3 accelerated stem growth strongly, flower- formation only slightly.There was no juvenile stage for temperature or daylength response. Regression of flower formation may occur in a qualitatively LD cv after transfer to SD. Flower formation was most inhibited with photoperiods of 6-10 h; it increased with shorter or longer photoperiods. A qualitative LD cv as 'Nobel' remained vegetative only in SD, if stem growth was inhibited. In SD differentiation in axillary primordia and the consequent formation of flower clusters occurred much slower than in LD and stopped early unless caulescent tissue grew at low temperatures, dim light or GA 3 in accordance with the hypothesis of Chailakhyan that both gibberellins and floral stimulus were needed for stem growth and flower differentiation, respectively, of LD rosette plants. Formation of both hormones would depend on daylength.Cvs were earlier where they required shorter daylength and where they grew faster. By studying the influences of temperature, vernalisation, light intensity and GA 3 on yields, through their influence on developmental rate and growth rate, several practical conclusions could be drawn about optimum sowing date.
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