14C-Photosynthate Partitioning and Translocation in Soybeans during Reproductive Development

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Abstract
Partitioning and translocation of 14C-photosynthates were examined during flowering and seed maturation in soybean plants to quantify allocation to sugars, amino acids, organic acids and starch, and to study transport of C and N from leaves to reproductive sinks. The trifoliolate leaf at the 8th node was exposed to steady state levels of 14CO2 for 2 h, followed by immediate extraction and identification of radioactive assimilates in the fed leaf blade, tissues of the transport path (e.g., petiole and stem), and fruits if they were present. About 1/3 of the total 14C recovered from the leaf blades was in starch until late pod-filling, after which the proportion dropped to 16%. Sugars comprised 70-86% of the recovered 14C from soluble assimilates of the source leaf, with highest proportions occurring during late flowering and early pod-filling. Amino acids accounted for 8-17% of the 14C recovered from the soluble fraction, and were most evident during early flowering and mid to late pod-filling. The 14C-organic acids comprised from 3-14% of the soluble 14C-assimilates in leaves. Petioles consistently contained a higher percentage of recovered radioactivity in sugars (87-97%) and a lower percentage in amino acids (3-12%) than did leaf blades. 14C-Amino acids in petioles attained their highest levels during mid and late pod-filling, while 14C-organic acids comprised 2% or less of the recovered radioactivity after pod initiation. The distribution of 14C-assimilates in the internode below the source leaf was similar to that found in petioles. A comparison of the above data to calculated C and N requirements for seed development suggests that 14C-amino acids derived from current photosynthesis and translocated from source leaves supply at least 12-48% of the seed N depending on the stage of pod-filling.