Sunflower response to repeated foliar applications of Paclobutrazol

Paclobutrazol (PBZ), a well-known growth retardant of the triazole family, is mostly used for controlling plant size and growth, resulting in more desirable compact plants for floricultural purposes; however, limited experimental data are available for use of PBZ in field sunflower. The objective of this study was to evaluate the effect of repeated foliar applications of PBZ at rates of 50 + 50 g ha-1(double application) and 50 + 50 + 50 g ha-1 (triple application) on sunflower morphology and productivity. PBZ applications corresponded to growth stages of four to five, five to six, and six to eight true leaves of sunflower plants, respectively. The double foliar application of PBZ reduced sunflower plant height at maturity by 4.4% (or by 11.7 cm). The triple foliar application of PBZ reduced sunflower plant height at maturity by 14.4% (or by 49.2 cm). However, PBZ either in the double or the triple application reduced achene yield per plant by 25.6% and 22.5% and the 100-achene weight by 11.4% and 25.0%, respectively, compared with the non-treated control. Overall, the repeated foliar applications of PBZ at the rates tested in this study, apart from providing a reduction of sunflower plant height, had major adverse effects on achene yield and weight. Thus, different application schemes of PBZ or perhaps different growth regulators should be investigated to control plant height in sunflower. Paclobutrazol (PBZ) é um bem conhecido retardador de crescimento da família triazol utilizado principalmente para controlar a altura e o crescimento das plantas, resultando em indivíduos mais compactos, o que pode favorecer o seu manejo para fins de floricultura. No entanto, dados experimentais limitados são disponíveis em relação ao uso do PBZ em girassol em campo. O objetivo deste estudo foi avaliar o efeito de aplicações foliares repetidas de PBZ em 50 + 50 g ha-1 (aplicação dupla) e em 50 + 50 + 50 g ha-1 (aplicação tripla) sobre a altura de planta e os componentes do rendimento de girassol. Aplicações de PBZ coincidiram com as fases de crescimento de quatro a cinco, cinco a seis, e seis a oito folhas verdadeiras de girassol, respectivamente. A aplicação foliar dupla de PBZ reduziu a altura das plantas de girassol na maturidade de 4,4% (ou 11,7 cm). A aplicação foliar tripla de PBZ reduziu a altura das plantas de girassol na maturidade de 14,4% (ou 49,2 cm). No entanto, PBZ em dupla ou tripla aplicação reduziu a produção de aquênios por planta em 25,6% e 22,5%, e o