A morphological and anatomical study for tracking the growth and development of individual flowers and pods in soybean (Glycine max L.)

A morphology-based growth stage system should describe the growth and development of a crop and thereby help farmers and agronomists in formulating reasonable management measures conducive to the development of marketable products. However, existing stage systems for soybean are either based on plant growth or covered a particular phase of flower or pod development, making it difficult to use for tracking the entire growth period of individual flowers and pods. Therefore, the first flower and pod, located at the base of the primary raceme in the eighth trifoliate node of the main stem, was chosen to illustrate growth dynamics during the full reproductive period. The size and fresh weight of the primary raceme in the eighth trifoliate leaf axil, the first flower and pod, the pistils in the first flower, and seeds in the first pod were examined, and the growth of these organs was depicted. Integrating the morphological characteristics and the growth features of flowers and pods, as well as existing growth stage systems, the development was delineated in 13 stages. In detail, we classified the flower phase based on the relative positions of floral components, inspired by the ratio of bract to flower used for staging, refined the lag phase proposed previously, retained the use of pod length to define the early pod phase, and innovatively described the late pod phase by the seed appearance. The developmental events in each stage of flower and pod development were distinctive and closely connected to the corresponding morphology. Taken together, a more detailed growth stage system for describing individual flowers and pods in soybean was established. This system will serve as a valuable research tool for describing the development, gene expression, and cellular metabolism associated with the formation of flowers, pods, and seeds.