Photoassimilate Availability Drives Sunflower Sugar Storage Capacity Through Plastic Changes in Organ Size

Sunflower (Helianthus annuus L.) capacity to synthesize and accumulate soluble carbohydrates that will ultimately contribute to grain filling, either via actual photosynthesis or previously stored reserves, has been largely neglected despite its relevance regarding crop yield. The present work is aimed at studying the effect of photoassimilate availability on the dynamics of production and distribution of soluble carbohydrates in the plant during vegetative and reproductive phases. Plant photoassimilate availability was modified from production crop level in two hybrids during two field experiments by shading or thinning plants, which resulted in a range of intercepted radiation between 20 and 300 MJ per plant and also by removal of the main sink, the capitulum. Plants under higher light availability developed larger leaves and accumulated much more biomass than shaded ones. In general, plant sugar storage increased up to flowering and was highest in the developing capitulum and upper stem internodes. Increasing light availability led to a strong growth promotion that was especially remarkable in the capitulum and stem upper internodes, which precluded an increase in sugar concentration in these parts. Capitulum removal led to sugar remobilization to the plant base, resulting in a strong growth promotion of roots, basal stem internodes, and even in leaves from the lower strata, showing an extremely high plasticity of all sunflower organs in response to photoassimilates. These results also suggest that sugars per se may drive plastic changes of organ size ultimately conditioning plant capacity to store sugars and crop yield.