Transmission of the Monocarpic Senescence Signal via the Xylem in Soybean

Abstract

MATERIALS AND METHODS During monocarpic senescence in soybean (Glycine max IL.I Merril cv.Anoka) there is a remobilization of nitrogen from the leaves to the seeds, and it has been hypothesized that this loss of nitrogen from the leaves induces foliar yellowing. The phloem in a smaUl segment of the petiole between the pods and the target leaf can be inactivated with a jet of steam. 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, whereas most of the rest of the plant remains green. The nitrogen 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 fuly 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 Since nutrients would have to be withdrawn from the leaves via the phloem, the pods do not induce yellowing by pulling nutrients out of the leaf and must be able to exert their influence via the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.Historically, monocarpic senescence has been attributed to nutrient exhaustion (due to drain and diversion) from the vegetative parts by the developing fruit (7, 9). Because depodding (4) and deseeding (6) extend the life of soybean plants far beyond normal, the seeds must control monocarpic senescence in soybeans. For convenience, this influence of the seeds will be termed the senescence signal (6). In the soybean, the primary target of the senescence signal is the leaves (8).The theories which hold that the senescence signal is a nutrient deficiency have been questioned some years ago (4, 9-11). Recently, it has been shown that, in surgically modified soybean plants, seed growth (with the accompanying nutrient drain and diversion) can occur without producing the senescence response (12) which raises doubts about the role of nutrient exhaustion in the induction of monocarpic senescence (9, 11). Nonetheless, where senescence does occur, important constituents, especially nitrogen, move out ofthe leaves, and this seems to be an important, even if secondary, component of the senescence of attached leaves (9, 10).Photosynthates and other 'nutrients,' such as amino acids and hormones, being exported (withdrawn) from the leaf blade would have to travel down through the petiole via the phloem which is a living tissue (1, 2, 13). We decided to test the nutrient withdrawal hypothesis by killing a small segment of the petiole with steam (often called 'girdling' or 'ringing'). This does not interfere with the ability of the xylem (which is already dead at functional maturity) to supply water, nutrients, and hormones to the leaf blade. Soybeans (Glycine max [L.] Merrill) cv. Anoka were grown as described (5). Foliar senescence and fruit development were measured quantitatively following the visual procedures outlined elsewhere (5). Leaf yellowing measured in the visual procedure reflects the decline of essential functions during monocarpic s...

Sink to Source Translocation in Soybean

“…As one would expect, then, this reverse flow of radioactivity from sink to source is nutritionally insignificant. However, as discussed above and demonstrated by Nooden and Murray (9), sink to source translocation may be physiologically important to sink/source interactions.…”

Seed Production Affects Maternal Growth and Senescence in Arabidopsis

“…Mainly, two competing hypotheses have received the most attention in the past, one that invokes a simple source-sink relationship (Molisch, 1929) and one that invokes offspring-derived signals (Wilson, 1997). By far the most extensive research examining the causes of monocarpic senescence has been performed in legumes such as pea (Pisum sativum) and soybean (Leopold et al, 1959;Lindoo and Noodén, 1977;Noodén and Murray, 1982), but even in these species the debate has not been settled. A potential role of the hormones auxin and abscisic acid (ABA) in correlative control has been discussed (Tamas and Engels, 1981).…”

Only xylem-borne factors can account for systemic wound signalling in the tomato plant

“…Steam-girdling is a recognised method of killing phloem and other living tissue (Cunningham 1895;Nooden and Murray 1982;Nelson et at. 1983;Minchin and McNaughton 1987).…”