Exposure of white bean seedlings to phytotoxic burdens of Co, Ni, or Zn reduced the export of "C-photoassimilates from the nearly fully expanded unifoliate leaves. Little "C reached the major sink areas, the young trifoliate leaves and the root tips, of seedngs exposed to metal. The unifoliate leaves accumulated sucrose, reducing sugars, and starch. These effects were evident within 1 or 2 days.The most general symptoms of metal toxicity in plants are stunting and chlorosis (7). Yield reductions could occur by excess metal directly or indirectly inhibiting either assimilate production in source leaves, translocation from source to sink, utilization in sink regions, or several of these. Various metal ions have adverse effects on respiration and photosynthesis in intact organisms, ranging from algae and lichens to vascular plants, as well as on isolated mitochondria and chloroplasts (5,10,16,19,22). In this context, it is particularly noteworthy that carbohydrates accumulate in shoots of barley (1) and starch accumulates in leaves of white beans (23) on exposure of seedlings to excess Co, Ni, or Zn. Agarwala et aL (1) suggested that the carbohydrate accumulations were due to metal interferences with carbohydrate metabolism. Rauser (23) gave the alternative interpretation that starch accumulations reflected reduced translocation out of leaves which normally export carbon.The aim of this work was to determine the levels of sucrose and reducing sugars in relation to starch in seedlings of Phaseolus vulgaris exposed to individual phytotoxic burdens of Co, Ni, and Zn. In addition, we assessed the rate of export of '4C-photoassimilates from unifoliate leaves and their pattern of distribution in bean seedlings exposed to excess metal. Sucrose, Reducing Sugars, and Starch Contents. Unifoliate leaves were harvested between 1200 and 1300 h and quickly dropped into 40 ml boiling 13.7 mol 1-1 ethanol in water and refluxed for 30 min. After reextraction with 40-and 20-ml portions of aqueous ethanol, the leaves were completely decolorized. The three alcohol extracts of each leaf were combined and evaporated nearly to dryness by gently warming. The residue was taken up in 20 to 30 ml deionized H20 filtered quantitatively through a glass fiber filter, and the filtrate made to a total volume of 50.0 ml with water. A 10.0-ml subsample of the filtrate was pipetted into a 15-ml centrifuge tube containing 1 ml each of swollen Dowex l-X8 anion exchange resin (200-400 mesh, C1-form) and of Dowex 50-W cation exchange resin (100-200 mesh, H+ form). The contents were mixed thoroughly and centrifuged at l,500g for 2 to 3 min. The supernatant was decanted and the resin mixture was extracted once with 4 ml water. Both supernatants were combined and made to a total volume of 15.0 ml. MATERIALS AND METHODSVariable volumes of the resin-treated supernatants were used to determine reducing sugars (18). Other subsamples were digested with 11 units of invertase Sigma Chemical Co.) for 2 h at 50 C. The volumes of supernatant used we...

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 I14Clsucrose. 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'.

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In the presence of the leaf blades, excised petioles of the semiaquatic celery-leaved buttercup Ranunculus sceleratus L. elongate when submerged, or when treated in air with indoleacetic acid (IAA), gibberellin A3 (GA3), or C2H4 alone or in combination, responses that are similar to those obtained using intact plants. Isolated petiole sections do not respond to C2H4, but are responsive in the presence of low levels (10−6 mol L−1) IAA. Gibberellin A3 is less active than IAA in promoting growth or in sensitizing the petiole tissue to C2H4.

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