Carbohydrate Levels and Photoassimilate Export from Leaves of Phaseolus vulgaris Exposed to Excess Cobalt, Nickel, and Zinc

Abstract

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...

Effect of excess iron and copper on physiology of aquatic plant Spirodela polyrrhiza (L.) Schleid

“…The low content of soluble carbohydrate suggested that plants consumed a large amount of soluble sugar to maintain the basic physiological functions, such as photosynthesis and respiration. Our results were in agreement with the report of Samarakoon and Rauser (1979) that converse changes of carbohydrate and photosynthesis in leaves of Phaseolus vulgaris under excess Ni stress.…”

Modulation of jasmonic acid and polyamines by 24- epibrassinolide in Brassica juncea L. under copper stress

“…The results are consistent with the results of Aly and Mohamed [29], who observed an enhancement in total sugars in Zea mays under Cu(II) stress, and with Samarakoon and Rauser [30] who found that reducing sugars increased in Phaseolus vulgaris plants under Ni and Zn stress. Sugars play role in osmoregulation and act as storage reserves to support the basal metabolism of plants under stress [31].…”

Toxidez de zinco no crescimento e nutrição de Eucalyptus maculata e Eucalyptus urophylla em solução nutritiva

“…Agarwala et al (1977) também observaram que o excesso de Zn 2+ reduz a translocação de Fe em cevada, sem, entretanto, isto estar relacionado com a ação induzida do Zn. É sabido que o excesso de Zn pode interferir no metabolismo de carboidratos, inibindo o transporte de fotoassimilados (Samarakoon & Rauser, 1979), e afetando o crescimento da planta.…”