Abstract:
Relative growth rate (RGR) is a complex trait determined by plant morphology and physiology. Based on studies of native plants from low-nutrient habitats, a low maximum relative growth rate (RGRmax; measured under non-limiting conditions) is hypothesized to be associated with low-nutrient stress tolerance and a resource conservative growth strategy in plants. Understanding the relationships of RGR, and its component traits, to stress responses for cultivated species may provide insights on focal traits for breeding for stress tolerance. Here we examine the intra-specific variation in RGR and functional traits in cultivated sunflower (Helianthus annuus L.). In a greenhouse study, seedlings of 18 inbred genotypes were grown in non-limiting (control) and low-nutrients (stress) treatments and harvested at two time-points (7 and 20 days after treatment initiation). As expected, RGRstress (RGR under stress treatment) was lower than RGRmax (RGR under control treatment). We found genetic variation in RGR expressed under control and stress conditions, and that the genotypes differed in their RGR in response to the low-nutrients stress. Additionally, a higher RGRmax was associated with a greater decline in RGR in response to the low nutrient stress. There were also significant genotype, treatment and genotype-by-treatment interaction effects for many of the biomass allocation and leaf and root functional traits expected to affect RGR, including specific leaf area (SLA) and leaf mass ratio (LMR). RGRmax correlated positively with LMR and negatively with root mass ratio (RMR) at the first harvest, suggesting that these traits might be useful target traits for selection for greater RGRmax and RGRstress. Lack of other RGR-trait relationships suggest that we need to look beyond carbon centric trait in order to explaining maximum growth rates and growth responses to stress in cultivated sunflowers.
