Darkness and low-light alter reserve mobilization during the initial growth of soybean (Glycine max (L.) Merrill)

Light is both a source of energy and a developmental signal, and therefore has a major impact upon initial seedling growth. This is particularly the case for species where photosynthesis in the cotyledons makes a significant contribution to carbon balance. In addition to containing reserves of proteins, lipids and carbohydrates the cotyledons of soybean (Glycine max (L.) Merrill) maintain sufficient photosynthetic activity to counterbalance respiratory carbon loss. Based on the hypothesis that altered light regime would affect seedling photosynthesis and development, we investigated the impact of shading and darkness on the reserves present in different soybean seedling organs. Plants were grown until the VC/V1 stage under normal light conditions (500 µmol m−2 s−1, 12/12 h) before being divided into three groups; normal light, low light (50 µmol m−2 s−1, 12/12 h) and darkness. After 3 days plants were harvested at three time points over the course of a day/night cycle and seedling organs analysed separately. Low light and darkness led to partial etiolation and the consumption of carbohydrates and fatty acids in several organs. Effects on cotyledon reserve mobilisation were modest, as darkness and low light did not lead to decreased cotyledon mass. However, metabolism in the epicotyl was more strongly perturbed, with the accumulation of branched chain and aromatic amino acids together with sugars that may be required to drive organ elongation. Darkness also served to abolish diurnal alterations in starch and asparagine concentrations detected in control plants. Overall, the data reinforce the flexibility of seedling metabolism, revealing similarities to the responses of adult plants to carbon starvation .