The making of giant pumpkins: how selective breeding changed the phloem of Cucurbita maxima from source to sink

Despite the success of breeding programmes focused on increasing fruit size, relatively little is known about the anatomical and physiological changes required to increase reproductive allocation. To address this gap in knowledge, we compared fruit/ovary anatomy, vascular structure and phloem transport of two varieties of giant pumpkins, and their smaller fruited progenitor under controlled environmental conditions. We also modelled carbon transport into the fruit of competitively grown plants using data collected in the field. There was no evidence that changes in leaf area or photosynthetic capacity impacted fruit size. Instead, giant varieties differed in their ovary morphology and contained more phloem on a cross-sectional area basis in their petioles and pedicels than the ancestral variety. These results suggest that sink activity is important in determining fruit size and that giant pumpkins have an enhanced capacity to transport carbon. The strong connection observed between carbon fixation, phloem structure and fruit growth in field-grown plants indicates that breeding for large fruit has led to changes throughout the carbon transport system that could have important implications for how we think about phloem transport velocity and carbon allocation. Through selectively breeding, growers have been able to produce pumpkins that can weigh over 1000 kg but relatively little is known about the anatomical and physiological changes that accompany this dramatic increase in fruit size. This study shows that this selective breeding for large fruits results in significant changes in the carbon transport pathway with giant varieties containing more phloem on a cross-sectional area-basis and a greater allocation to reproduction on a per-plant basis than non-giant varieties. In field-grown plants, there is also a tight coupling between the amount of carbon fixed on a whole-plant level and maximum fruit growth rate. This study provides critical insights into the relationship between phloem transport velocity, carbon allocation and phloem anatomy and brings to light a major gap in knowledge in our understanding of the relationship between phloem structure and function.