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 Since nutrients would have to be withdrawn from the leaves via the phloem, the pods do not induce yellowing by pulling nutrients out of the leaf and must be able to exert their influence via the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of attached leaves (9, 10).Photosynthates and other 'nutrients,' such as amino acids and hormones, being exported (withdrawn) from the leaf blade would have to travel down through the petiole via the phloem which is a living tissue (1, 2, 13).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...
Applying criteria used for higher plants, phytochrome mediation. of uredospore germination in the stem-rust fungus, Puccinia graminis f. sp. tritici (Eriks & Henn) Guyot is established. A 1 min red irradiation at 660 nm promotes uredospore germination and this potentiation of promotion is photoreversible by a 1 min far-red irradiation at 730nm.
Normally, starch (sugars) and minerals are redistributed from the leaves to the pods during monocarpic senescence in maturing soybean plants. Petiole phloem destruction (steam girdling), which blocked this redistribution by interrupting export through the petiole, altered the foliar senescence pattern producing a distinctive interveinal yellowing with green areas along the veins on pod-bearing plants. This suggests that blockage of the petiole phloem may cause nutrients to accumulate in the green zones along the leaf veins instead of being redistributed to the pods. In the leaves of untreated plants, starch showed the same distribution pattern as chlorophyll; however, starch was preserved in yellow areas as well as the green zones of the steam-girdled leaves. Mineral analyses ofthe veinal and interveinal zones oftreated leaves and controls showed that the veinal green zones and interveinal yellowing in treated plants were not respectively enriched and depleted in minerals corresponding to a redistribution of minerals within the leaves. Depodding also blocked leafyellowing, net mineral redistribution and starch breakdown. Thus, the pods are able to induce chlorophyll breakdown without net mineral redistribution or starch loss in leaves with petiole phloem destruction. This shows that chlorophyll breakdown is not obligatorily coupled with mineral redistribution or starch breakdown.
Normally, starch (sugars) and minerals are redistributed from the leaves to the pods during monocarpic senescence in maturing soybean plants. Petiole phloem destruction (steam girdling), which blocked this redistribution by interrupting export through the petiole, altered the foliar senescence pattern producing a distinctive interveinal yellowing with green areas along the veins on pod-bearing plants. This suggests that blockage of the petiole phloem may cause nutrients to accumulate in the green zones along the leaf veins instead of being redistributed to the pods. In the leaves of untreated plants, starch showed the same distribution pattern as chlorophyll; however, starch was preserved in yellow areas as well as the green zones of the steam-girdled leaves. Mineral analyses ofthe veinal and interveinal zones oftreated leaves and controls showed that the veinal green zones and interveinal yellowing in treated plants were not respectively enriched and depleted in minerals corresponding to a redistribution of minerals within the leaves. Depodding also blocked leafyellowing, net mineral redistribution and starch breakdown. Thus, the pods are able to induce chlorophyll breakdown without net mineral redistribution or starch loss in leaves with petiole phloem destruction. This shows that chlorophyll breakdown is not obligatorily coupled with mineral redistribution or starch breakdown.
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