The effect of boron (B) deficiency in sunflower (Helianthus annuus L.) plants on early capitulum development was studied. Plants were grown under artificial conditions on an 18 h long-day photoperiod in B-deficient soil mixture (<0.1 ppm B) irrigated with B-deficient or complete Hoagland’s solution (0.27 ppm B). B-deficient plants were transferred 25 days after seedling emergence (DAE) to a B-rich soil mixture (2.0 ppm B) and irrigated with complete B Hoagland’s solution. Compared with controls, plants grown in B-deficient soil mixture had 18% less leaf area, 25% less receptacle area, and 33% less shoot dry weight; all differences were statistically significant. In both treatments the generation of new floret primordia started at the capitulum rim at 35 DAE and continued towards its center. Each floret primordium differentiated gradually and cen tripetally over space and time, first as a slight dome that became a two-part structure, the bract and the floret corolla. Capitulum damage was observed in B-deficient plants early during floret differentiation, resulting in the onset of surface splits on the receptacle at floral stage (FS) 4 (28-30 DAE). The edges of these splits acted as centres of floret primordium differentiation which devel oped centrifugally and resulted in aberrant capitula and the development of ray flowers and involucral bracts in abnormal positions on the inflorescence. This paper provides a detailed description of the beginning of floret primordium dif ferentiation during the floral morphogenetic process in sunflower capitula where damage was observed in plants grown under boron stress.