Photoperiodically Induced Changes in Seed Growth Rate of Soybean as Related to Endogenous Concentrations of ABA and Sucrose in Seed Tissues

Short-day photoperiods can increase the partitioning of assimilates to filling seeds of soybean (Glycine max L. Merr.), resulting in higher seed growth rates. The plant growth substance ABA has been implicated in the regulation of assimilate transfer within filling soybean seeds. Thus, we hypothesized that an increased concentration of endogenous ABA in seeds may enhance sucrose accumulation and seed growth rate of soybeans exposed to short-day photoperiods. Plants of cv. Hood 75 were grown in a greenhouse under an 8-h short-day photoperiod (SD) until 11 d after anthesis (DAA) of the first flower, when half of the plants were transferred to a night-interruption (NI) treatment (3 h of low-intensity light inserted into the middle of the dark period). Plants remaining in SD throughout seed development had seed growth rates 43 % higher than that of plants shifted to NI (7-6 mg seed"1 d"1 vs. 5-3 mg seed"1 d"1). On a tissue-water basis, the concentration of ABA in SD seeds increased rapidly from 7-6 /tmol I"1 at 11 DAA to 65-2 /tmol T1 at 18 DAA, but then declined to 6-6 /jmol I"1 by 39 DAA. In contrast, the concentration of ABA increased more slowly in NI seeds, reaching only 47-4 /;mol I"1 by 18 DAA, peaking at 57-0 fimo\ I"1 on 25 DAA, and declining to 10-2/tmoll"1 by 39 DAA. The concentration of sucrose in SD embryos peaked at 73-5 mmol I"1 on 25 DAA and remained relatively constant for the remainder of the seed-filling period. In NI, the concentration of sucrose reached only 38-3 mmol I"1 by 25 DAA, and peaked at 61-5 /jmol I"1 on 32 DAA. Thus in both SD and NI, sucrose accumulated in embryos only after the peak in ABA concentration, suggesting that ABA may have stimulated sucrose movement to the seeds. The earlier accumulation of ABA and sucrose in SD suggests that ABA may have increased assimilate availability during the critical cell-division period, thus regulating cotyledon cell number and subsequent seed growth rate for the remainder of the seed-filling period.