Excised soybean (Gbyciae nyx ILl Merrill) cv Anoka leaf discs tend to remain green even after the corrponding intact leaves have turd yellow on fruiting plants. We have found that explants which Include a leaf along with a stem segment (below the node) and one or more pods (maintaied on distDled H2O) show shiilar but accelerated leafyellowing and abscission compared with intact plants. In podded explants excised at preodf, the leaves begin to yellow after 16 days, whereas those excised at late podfl begin to yellow after only 6 days. Altough stomatal resistances remain low during the first lHt period after exciso, they subsequently increase to levels above those in leaves of intact plants. Explants The yellowing and abscission of the leaves during monocarpic senescence of soybean plants generally occur during the completion of seed growth (2,11,17,19,21). When the seeds or pods are removed before seed maturity, the leaves do not senesce (10, 12). Similarily, leaves excised from soybean plants show abnormal and delayed senescence. We have observed recently that when single leaves are excised along with one or more pods on a subtending stem section and maintained in distilled H20, they show a pattern of yellowing and abscission which is accelerated but similar to that observed in intact plants. These explants appear to offer a convenient new experimental system for investigating the role of mineral nutrient and hormone supply in the regulation of pod development and monocarpic senescence in soybeans. As a start in the study of these problems, we report here on some character- Preparation of Explants and Leaf Dbcs To maximize uniformity, explants were excised above the third node from the bottom and below the third node from the top. Except where indicated, explants were cut at about mid podfill, when the carpels were still green and the seeds had elongated to fill the seed cavity (candled), SD 45 ± 5 d. Unless otherwise stated, the number of pods was reduced to one (with three seeds) per node 24 h before excision.Explants with 10-cm-stem sections were placed in 125-ml Erienmeyer flasks filled with distilled H20 or some other test solution.Drawing on procedures for maintaining cut flowers (4), quinoline hydroxide (20,tM in 60% v/v ethanol) was added to all solutions to give a final concentration of 0.2 ,UM (30 ,tg/ml); this suppressed microbial growth in the treatment solutions. As a further precautionary measure, solutions were replaced every 48 h, and the stem bases were trimmed weekly.Mite infestations can also accelerate yellowing of leaves on explants or intact plants, thus care must be taken to detect and exclude infested material.For comparison with the explants, leaf discs (2.2 cm) were excised from the leaf blades with a sharp cork borer and placed on moistened filter paper discs in Petri dishes. Submergence of the discs or the cut edges must be avoided since this induced necrosis.Explants and leaf discs were cultured in environmental control chambers with the SD light and temperature regime desc...
