Probing Monocarpic Senescence and Pod Development Through Manipulation of Cytokinin and Mineral Supplies in Soybean Explants

Abstract

Defined solutions containing cytokinin and/or mineral nutrients were supplied in lieu of the roots through the cut stem base of soybean explants (a leaf with associated pod and subtending stem segment) in order to analyze the roles of cytokinin and mineral nutrients from the roots in pod development and foliar maintenance. In explants cut at early-mid podfill, supplying only H2O accelerated leaf senescence and pod maturation and decreased seed d. wt relative to comparable parts of intact plants. Zeatin (Z) and/or minerals not only delayed leaf yellowing and the decline in foliar chlorophyll levels and photosynthetic rates but also inhibited leaflet and petiole abscission relative to H2O controls. Even large declines in foliar assimilatory processes did not necessarily lead to abscission. Z and/or minerals also increased stomatal conductivity throughout podfill. Z showed some positive synergistic effects with minerals on leaf maintenance. Pod wall, cotyledon and radicle yellowing were delayed by Z and/or minerals but not as much as leaf senescence. Minerals only or Z +minerals prolonged seed d. wt accumulation and increased final dry seed wt to a level similar to that for intact plants. Seed growth showed a complex interrelation with pod wall and leaf f. wt and d. wt changes. A decline in cytokinin and mineral flux from the roots appears to be important for pod-induced leaf senescence; however, pod development, foliar senescence and their component processes may be affected differently. Thus, even though the explant is a physiological/nutritional module of the whole plant, it is influenced by cytokinin and minerals from the roots and therefore only semiautonomous.