Mineral Distribution In Relation To Fruit Development And Monocarpic Senescence In Anoka Soybeans

The changes in distribution of several important mineral nutrients (N, K, Ca, Mg, Mn, and Fe) were studied in relation to monocarpic senescence (measured as leaf yellowing) and fruit development in hydroponically-grown (and to a lesser extent field-grown) Anoka soybeans with particular emphasis on the leaves and seeds. Only N shows a clear redistribution from the leaves to the seeds as the seeds grow, and this transfer starts before visible leaf yellowing. K, Ca, Mn and Fe do not seem to redistribute, but Mg may undergo limited redistribution. Depodding prevents the drop in the amounts of minerals in attached leaves by blocking leaf shedding and/or redistribution and also creates some quantitative changes in mineral distribution. On a g fresh weight basis, only the N content of leaf blades decreases during yellowing; the K, Mg, Ca, Mn and Fe contents do not decrease. Therefore, depletion of the latter minerals from the leaves cannot be responsible for their yellowing. Although N deficiency alone could cause foliar chlorosis, the monocarpic yellowing pattern is distinctly different from that induced by N deprivation. THE CONTENT of at least some minerals (particularly nitrogen) appears to decrease in senescing plant organs (see Gregory, 1937; Miller, 1938; Williams, 1955; Howell, 1960; Ohlrogge, 1963; Hanway and Weber, 1971; deMooy, Pesek, and Spaldon, 1973; Kollman et al., 1974; Nooden and Leopold, 1978). Often, this loss from senescing organs is counterbalanced by a corresponding increase in storage tissues of developing fruit and therefore seems to represent withdrawal from the senescing organs. In general, these studies are concerned with the relationship of mineral distribution to fruit development rather than with senescence. From these data, one cannot determine how the start of changes in mineral distribution relates to the start of senescence or how the rates of these two processes are related. Furthermore, the apparent decrease in mineral content may in many cases be the result of shedding of the senescing organs rather than redistribution. This apparent redistribution of minerals from the vegetative parts to the developing fruit of monocarpic plants has given rise to the suggestion that the fruit kill the plants by depleting their vegetative parts (especially the leaves) of needed minerals (see Nood6n and Leopold, 1978). This theory is supported by the observations that removal of the flowers or young fruit (the apparent sinks) can prevent or at least delay senescence in l Received for publication 11 May 1977; revision accepted 18 July 1977. Supported in part by research grant number 416-15-79 from the USDA Cooperative State Research Service under P.L. 89-106. 2Present address: Department of General Botany, University of Istanbul, Suleymaniye, Istanbul, Turkey. many monocarpic plants including soybeans (Leopold, Niedergang-Kamien, and Janick, 1959; Lindoo and Nooden, 1976; Nooden and Leopold, 1978). Furthermore, any of several mineral deficiencies can produce leaf yellowing (Sprague, 1964), though no single deficiency syndrome precisely matches the yellowing patterns which develop during monocarpic senescence of soybeans. Because it is essential to have a clear picture of mineral withdrawal or redistribution in relation to senescence, we have analyzed the distribution of certain essential minerals in relation to the time course of fruit development and senescence of soybeans and also the effect of depodding (which prevents monocarpic senescence in soybeans) on mineral distribution as part of a study on the physiology of monocarpic senescence in soybeans. MATERIALS AND METHODs-Plant culture and measurement of development-Soybean (Glycine max (L.) Merrill cv. Anoka) seeds were sown in vermiculite, and when the primary leaves had unfolded (at approximately 10 days), the plants were transferred to plastic freezer containers with about 1 liter of full strength, modified Hoagland's solution plus iron EDTA (Arnon and Hoagland, 1940). Two plants were placed in opposite corners of each container. The level of the solution was restored daily with distilled water, and every 14 days, the solution was replaced with fresh Hoagland's solution. Solutions were aerated at a rate of 30 ml/min with air previously bubbled through water. The plants grew vigorously under these conditions with no sign of min-