Seed growth rate and carbohydrate pool sizes of the soybean fruit

The relationships between various carbohydrate pools of the soybean (Glycine max [L.] Merrill) fruit and growth rate of seeds were evaluated. Plants during midpod-fill were subjected to various CO2 concentrations or light intensities for 7 days to generate different rates of seed growth. Dry matter accumulation rates of seeds and pod wall, along with glucose, sucrose, and starch concentrations in the pod wall, seed coat, and embryo were measured in three-seeded fruits located from nodes six through ten. Seed growth rates ranged from 4 to 37 milligrams · day-1 · $\text{fruit}^{-1}$. When seed growth rates were greater than 12 milligrams · day-1 · $\text{fruit}^{-1}$, sucrose concentration remained relatively constant in the pod wall (1.5 milligrams · 100 milligrams dry weight-1), seed coat (8.5 milligrams · 100 milligrams dry weight-1), and embryo (5.0 milligrams · 100 milligrams dry weight-1). However, sucrose concentrations decreased in all three parts of the fruit as growth rate of the seeds fell below 12 milligrams · day-1 · $\text{fruit}^{-1}$. This relationship suggests that at high seed growth rates, flux of sucrose through the sucrose pools of the fruit was more important than pool size for growth. Starch concentration in the pod wall remained relatively constant (2 milligrams · 100 milligrams dry weight-1) at higher rates of seed growth but decreased as seed growth rates fell below 12 milligrams · day-1 · $\text{fruit}^{-1}$. This suggests that pod wall starch may buffer seed growth under conditions of limiting assimilate availability. There was no indication that carbohydrate pools of the fruit were a limitation to transport or growth processes of the soybean fruit.