EFFECTS OF FOLIAR-APPLIED GIBBERELLIC-ACID AND BENZYLADENINE UPON YIELD COMPONENTS IN SUNFLOWER (HELIANTHUS-ANNUUS L)

Sunflower yield is determined by seed number/m(-2) and by achene weigh t. Frequently, a high percentage of empty achenes in the inner portion of the capitulum, probably due to a reduced vascularization of that s ection of the flower head, decreases final yield. The objective of the present research is to determine if foliarly applied gibberellic acid (GA) and benzyladenine (BA) can enhance the vascularization in the in ner portion of the capitulum, improving photoassimilate translocation. Field experiments were conducted during 1989/90 with hybrid SPS 894 a nd during 1990/91 with hybrid ACA 882. GA (150 mg/l(-1)), BA (150 and 250 mg/l(-1)) and GA 150 + BA 150 mg/l(-1) each were foliarly applied 20, 40, or 60 days after emergence. For both seasons and hybrids plant growth regulator (PGRs) applications significantly reduced the percen tage of empty achenes, increased achene weight, achene weight (x 1000) and achene number in the inner portion of the capitulum and in the mi ddle and outer portion during 1990/91. A 25% increase in seed yield wa s achieved due to PGR application and the capitulum partition index (a chene weight/receptacle weight(-1), CPI) was significantly increased d ue to an improvement in photoassimilate distribution. A distribution m odel was derived showing that preferential allocation of photoassimila tes in the outer portion of the capitulum can be modified by PGR appli cation, demonstrating that photoassimilate distribution is under hormo nal control.