Localized Photosynthate Deposition in Citrus Fruit Segments Relative to Source-Leaf Position

Photosynthate translocation into fruit segments was examinedin ‘Pineapple’ sweet orange Citrus sinensis (L.)Osbeck to determine whether previously reported patterns ofdistribution [Koch (1984) HortScience 19: 260] would changeover time or with alterations in balance between source leavesand sink fruit. In control plants, ¹⁴CO₂ was supplied to a sourceleaf nearest the fruit for 1 h, followed by 5 h translocation.Over 89% of [¹⁴C]assimilates in the fruit were localized in4 segments directly aligned with the source and 73% of thesewere in the center 2 segments. Peel, pulp and seeds showed similarpatterns. Little or no lateral spreading of [¹⁴C]photosynthatesoccurred when an additional 7 days were allowed for translocation,but distribution was slightly broader when the source leaf was8 nodes farther from the fruit. Defoliation and girdling toreduce the source/sink ratio gave variable results if done 18h before experiments, but widened the area receiving [¹⁴C]assimilatesto approximately half the fruit if done 7 days earlier. Thisoccurred only when an entire fruit, was dependent upon a singlesource, leaving the opposite half fruit without an externalsupply of photosynthates. These data show an extreme degreeof preferential translocation and inflexibility which can occurin a transport path. ¹Supported by United States Department of Agriculture CompetitiveResearch Grant 59-2121-1-1-752-0, Regional Project NC-142, andthe Institute of Food and Agricultural Sciences, Universityof Florida. (Received January 12, 1984; Accepted May 14, 1984)