14C Photoassimilate Partitioning in Developing Sunflower Seeds

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Abstract
Changes with age of protein and oil content in field-grown Helianthus annuus 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 approximately 18% and 50% of the dry weight respectively. In an attempt to determine the importance of the role of photosynthesis in grain-filling, 14CO2 was incorporated into sunflower leaves and translocated radioactive photoassimilates in the seed were studied. 14C-labelled products including carbohydrates, nitrogenous compounds and lipids were determined as a function of seed position, length of chase period, and seed age. Within 8 h, 14C-labelled photosynthate is detected in the seed, up to 80% of seed-incorporated radioactivity being in the form of free sugars and organic acids. The conversion rate from free sugars to storage compounds (lipids, proteins and starch) varies according to seed position and age. Lipids constitute the major photosynthetic sink, reaching levels of greater than 80% of the total seed-incorporated radioactivity. The effects of abscisic acid on uptake and partitioning of 14C sucrose into immature excised cotyledons were also studied.