C-Photosynthate Partitioning and Translocation in Soybeans during Reproductive Development

Partitioning and translocation of ¹⁴C-photosynthates were examined during flowering and seed maturation in soybean (Glycine max L.]Merr.) 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 eighth node was exposed to steady state levels of ¹⁴CO₂ for 2 hours, 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 one-third 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% to 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% to 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% to 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% to 48% of the seed N depending on the stage of pod-filling.