14C Photoassimilate Partitioning in Developing Sunflower Seeds

Changes with age of protein and oil content in field-grown Helianthusannuus seeds were followed during the grain filling period.Seeds were sampled from different zones on the flower head:peripheral, intermediate, and central. Regardless of seed position,at maturity protein and oil content accounted for approximately18% and 50% of the dry weight respectively. In an attempt todetermine the importance of the role of photosynthesis in grain-filling,¹⁴CO₂ was incorporated into sunflower leaves and translocatedradioactive photoassimilates in the seed were studied. ¹⁴C-labelledproducts including carbohydrates, nitrogenous compounds andlipids were determined as a function of seed position, lengthof chase period, and seed age. Within 8 h, ¹⁴C-labelled photosynthateis detected in the seed, up to 80% of seed-incorporated radioactivitybeing in the form of free sugars and organic acids. The conversionrate from free sugars to storage compounds (lipids, proteinsand starch) varies according to seed position and age. Lipidsconstitute the major photosynthetic sink, reaching levels ofgreater than 80% of the total seed-incorporated radioactivity.The effects of abscisic acid on uptake and partitioning of ¹⁴Csucrose into immature excised cotyledons were also studied. Key words: Sunflower, photosynthate, abscisic acid