Yield determinant responses to stand structure and crop population density in sunflower

M.LÓPEZ PEREIRA; A.J. HALL

Mar del Plata/Balcarce

Conferencia; 18th International Sunflower Conference; 2012

International Sunflower Association

ABSTRACT ·                     Crop population density and distance between rows used in commercial crops has not changed in during the last forty years. However, sunflower yield potential (in crops protected from lodging and disease) continues to increase at higher crop population density than currently regarded as optimal (3-5plm-2). In addition, Libenson et al., (Crop. Sci. Vol, 42: 1180, 2002) suggested that there is some intra-specific variability for tolerance to high population density associated with a differential partitioning of biomass to grain. Understanding the nature of crop responses to population density and stand structure is a requisite step on the way to obtaining sunflower cultivars intrinsically superior in terms of crop yield potential at higher density. The aims of this study were to evaluate the responses of yield determinants to different stand structures and crop population densities. ·                     Experiments were conducted in each of three separate growing seasons, using irrigated crops of a widely used sunflower hybrid, sown in arrangements covering a wide range of areas per plant (AP, 0.07-0.96m-2 pl-1) and densities (1-14.3 pl m-2) in two canopy structures, with rows at 0.70m (E0.70 , 0.07-0.49m-2 per plant) and 1.40m (E1.4, 0.14-0.96m-2 per plant). The plots were fertilized (60 kg N ha-1) and irrigated. Diseases, insects and lodging were controlled. At physiological maturity, biomass per plant (Bpl, g pl-1) and per unit area (B, g m-2), oil yield per plant (RacPL, gpl-1) and per unit area (Rac, g m-2), and yield components: flower number (Nfl, pl-1), grain set (fert, %), grain number (Ng, pl-1), grain weight (Wgrain, g grain-1), oil concentration(Coil, %) and grain oil content (Contoil , g oil grain-1) were determined. Values corrected for the cost of oil synthesis were calculated for plant biomass (Boscpl, g pl-1), biomass per unit area (Bosc, g m-2) harvest index (HIOSC, dimensionless) and oil yield per plant (YieldOSC,, g pl-1). ·                      Oil yield (Rac) and total biomass per unit area increased with crop population density across the whole range of crop population densities and across the range common to structures with rows at 0.70m and 1.40m. Corrected Oil yield per plant (YieldOSC) was linearly associated with Boscpl across the whole of the ranges of AP explored in both structures. Harvest index (HIOSC) remained relatively constant (ca.0.45) over a wide range of AP in E0.70 (0.07-0.49m-2 per plant)and E1.4 (0.14 -0.96m-2 per plant) and was not associated with Boscpl in either row separation. Relative changes in Nfl, Ng, Wgrain, and Contoil with AP were substantial, responses of fert and Coil were less marked. Grain number (Ng) explained the largest proportion of the variation of the Racpl in response to AP in both E0.7 (86%) and E1.4 (95%). Ng variation was exclusively associated with NFL in E1.4 while in E0.7 it was associated with NFL (70%) and the residual variation was explained by grain set.  ·                     We conclude that grain set and harvest index were substantially stable across a broad range of AP and that the most plastic responses to crop population density and canopy structure were the number of flowers (and, consequently, grain number), followed by  grain weight and grain oil content in decreasing order of importance. These results indicate that an exploration of intra-specific variability for tolerance to high crop population densities should pay particular attention to the responses of yield determinants we have identified as plastic.  Our results contrast, in part, with those of Villalobos et al. (FCR 36: 1, 1994) in that: a) Total biomass per unit area increased at crop population densities greater than 5 pl m-2, while Villalobos et al. found no such increase. This suggests some degree of improvement of yield tolerance to high density. b) HI was stable across the whole range of crop population density we used (which explored higher densities than theirs, 14 pl m-2 vs. 10 pl m-2) while their HI declined between 5 and 0.5 pl m-2 (0.33 to 0.16). This suggests that breeders have unconsciously achieved greater plasticity in the determinants of yield which can affect HI. Grain set was stable across the whole range densities while Villalobos et al. found a negative relationship between grain set and crop population density.