Artificial defoliation and its impact on the agronomic performance of sunflower in a non-preferential season

The objective of this work was to evaluate the partitioning of photoassimilates in sunflower plants sown in a non-preferential season coincident with the occurrence of fallow areas in the region, submitted to artificial defoliation with 100% intensity, in different phenological stages of the reproductive period. The experimental design used was randomized blocks with six treatments and three replications, consisting of 18 experimental units and all plants were defoliated. The treatments performed were artificial defoliation, I: No defoliation (control); II: Defoliation at the R1 phenological stage; III: Defoliation at the R3 phenological stage; IV: Defoliation at the R5 phenological stage; V: Defoliation at the R7 phenological stage and VI: Defoliation at the R8 phenological stage. Artificial defoliation performed at 100% intensity at different phenological stages influences the morphological attributes and yield components of sunflower in a distinct way. Defoliations performed at R1 phenological stage have a negative effect on most of the variables analyzed, which provided lower plant height, stem diameter, capitulum diameter, number of rows of achenes, number of achenes per row, achene length, achene width, thousand achene mass and, consequently, in grain yield. This phenological stage was marked by the beginning of the reproductive period, which has a high demand for photoassimilates to maintain plant metabolism and yield. Defoliation performed at R8 phenological stage had better performance for plant height, capitulum diameter, capitulum mass, number of rows of achenes, number of achenes per row, number of plants per hectare and grain yield (>1000 kg ha-1). The plants of experimental units without defoliation had results similar to those defoliated in the R8 phenological stage, however, they still had better performance for thousand achene mass, achene length, achene width, volume index and even a smaller diameter of missing seeds.