Vein Loading in Seedlings of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc

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Abstract
Vein loading in unifoliate leaves of white bean seedlings exposed to excess Co2+, Ni2+, or Zn2+ for 1 to 4 days was studied by incubating leaf discs in [14C]sucrose. The discs from plants exposed to metal exhibited an increased total uptake of radiosucrose but reduced vein loading. Differences between treatments due to infiltration of disc margins were eliminated by analyzing 7-millimeter discs cut from the center of incubated 14-millimeter discs. Uptake of radiosucrose was greater particularly in discs from seedlings exposed to excess Ni2+ and Zn2+. The effect increased as exposure of the seedlings to metal increased up to 4 days. Autoradiographs showed vein loading in control leaf tissues with most of the radiosucrose accumulating in minor veins and little remaining in the mesophyll. In discs from metal-treated plants, most of the sucrose remained in the mesophyll without accumulating preferentially in the minor veins. This effect was evident within 24 hours of exposure to excess metal and intensified with longer exposures to metal. The inhibition of vein loading was also evident in situ. Both the preferential accumulation of sucrose into the minor veins of control tissues and the accumulation into mesophyll of metal exposed tissues were sensitive to 2,4-dinitrophenol and the blockage of sulfhydryl groups. It is concluded that the inhibition of vein loading contributes markedly to the observed toxicological effects of reduced photoassimilate export and of accumulation of carbohydrates in fully expanded leaves of bean seedlings exposed to excess metal ions.