Seasonal variation of photosynthetic carbon flow rate into young walnut and its partitioning among the plant organs and functions

Summary Daily fluxes of photoassimilates into young, branched walnut trees were investigated over the whole season through gas exchange measurements. Their partitioning among the different plant organs and functions was assessed by quantitative 14 C labelling in August and October. At harvest, 7 hours or 5 days after labelling, the labelled compounds were partitioned among 4 fractions: starch, sugars, lipids, and the «residue» fraction containing all other compounds, mainly cell walls and proteins. It took ca. five days for the bulk of exportable photoassimilates to be actually exported and metabolized in sink areas. Daily fluxes of recent photoassimilates were derived from applying the 5-day partitioning pattern of the 14 C recovered in the evening of the labelling day to the total C gained over the whole day by net assimilation (daily 43g CO 2 in August, and 10g in October, for a leaf surface area of 3.8 m 2 ). The translocation pattern, which was already highly basipetal in August, became even more so in October; in the same time, the daily fluxes of recent photoassimilates incorporated into all biochemical fractions decreased significantly at the whole plant level. Furthermore, the relative part of sugars in the total exported C doubled, which indicated a slower metabolic rate. However, whereas some organs like the tap-root apparently just slowed down but did not qualitatively change their activity — growth vs. reserve storage, as indicated by the starch : residue labelling ratio —, others like the fine roots exhibited a considerable shift from quasi-exclusive growth to a much more balanced activity. In contrast to the fluxes of recent assimilates incorporated into the plant dry matter, the fluxes allocated to respiration in October were only slightly lower than in August; expressed relative to the total incoming C, they appeared much greater. However, in both situations the part of recent (i.e., up to 5 days old) photoassimilates in the total plant respiration did not exceed 25 %, which stressed the importance of older carbon remobilization.