Transmission of the Monocarpic Senescence Signal via the Xylem in Soybean

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Abstract
During monocarpic senescence in soybean (G. max [L.] Merrill cv. Anoka) there is a remobilization of N from the leaves to the seeds, and this loss of N may induce foliar yellowing. The phloem in a small segment of the petiole between the pods and the target leaf can be inactivated with a jet of stream. When a plant is depodded, except for a single pod cluster in the center of the plant, the pod cluster induces yellowing of the nearest leaf even if the petiole contains a zone of dead phloem; most of the rest of the plant remains green. The N content of these leaves with a dead phloem zone in their petioles does not decrease greatly, even though the leaves turn yellow. A similar treatment of a single leaf on a fully depodded plant (leaves stay green) does not cause that leaf to turn yellow. Since nutrients would have to be withdrawn from the leaves via the phloem, the pods do not induce yellowing by pulling nutrients out of the leaf and must be able to exert their influence via the xylem.