Significance of Photosynthate Produced at Different Stages of Growth as Carbon Source for Fruit Filling and Seed Reserve Accumulation in Lupinus angustifolius L

Growth and reproductive phenology of field-grown, non-irrigated L. angustifolius (cv. Unicrop) were studied by periodic sampling of plants. Anthesis occurred when plant carbon content was only 15% of its final maximum. Flowering and the main periods of pod growth and seed filling took place, respectively, 0-6, 5-9 and 8-12 weeks after anthesis. Over 80% of the plant's gain of carbon in dry matter occurred 0-8 weeks after anthesis, after which drought-induced defoliation curtailed dry matter accumulation. 14CO2 was fed to selected plants at weekly intervals over the period 0-12 weeks after anthesis and time courses of transfer of 14C-labelled assimilates to fruits were assayed non-destructively by periodic sampling of phloem sap from fruits. Average specific radioactivity of carbon of phloem sap samples from primary, secondary and tertiary fruits of each feeding treatment were compared with specific 14C activities of the respective fruits harvested at maturity. The agreement between data was very close with times of 14CO2 feeding up to 8 weeks after anthesis. With later times of feeding, average specific radioactivity of phloem sap was higher than that of fruits as labelled sap was collected for only the latter part of the life of the fruit and before the fruit had accumulated unlabelled carbon. Carbon from photosynthate fed before or during flowering contributed mainly to protein of the seed, that from photosynthate formed during fruiting mainly to non-protein seed components. Distribution of 14C between amino and non-amino compounds of fruit phloem sap of plants fed 14CO2 at anthesis or in late fruiting reflected this effect. Estimates of contributions of carbon of photosynthate to fruits suggested that only 2% of the fruit's carbon came from photosynthesis before flowering and only 3 % from photosynthesis 0-3 weeks after anthesis, while 18, 28 and 23% came from photosynthate formed respectively 3-6, 6-9 and 9-12 weeks after anthesis.