Estudos sobre as relações entre zinco e fósforo na nutrição da planta

Portuguese. Uma série de experimentos com raízes destacadas e plantas inteiras de cevada (em solução nutritiva ou solo) deu apoio à conclusão de que a deficiência de zinco induzida pelo fosfato pode ser explicada pela operação de vários processos: inibição não competitiva de absorção do zinco; precipitação de fosfato de zinco na superfície das raízes; redução na translocação para a parte aérea; efeito de diluição resultante de alta velocidade de crescimento causada pelo fósforo no meio.

English. This paper deals with the results of experiments designed to study the nature of the effect of phosphate (P) ion on the absorption of zinc (Zn) by plants. Due to the conflicting results found in the literature the following working hypothesis were raised and checked: a. R decreases Zn levels in the top by interfering with its uptake at the root level; b. decreases Zn uptake by roots due to preciptation at their surface; c. R decreases Zn content in the top by a negative effect on the upward movement of the absorbed element; d. R decreases Zn uptake in soil conditions by rendering it unavailable. Experiments with excised roots and whole plants were conducted using barley as an indicator. Experiments with excised roots and whole plants were carried out at there Zn concentraions (5 x 10-6M, 5 x 10-5M, and 5 x 10-4M) a pH 5.0 6.0 and 7.0. In the farmer trials 0,5 g of roots excised from 7 day old seedlings were used during an uptake period of 30 minutes; phosphates of Na, K, NH4, Ca and Mg were used at the concentrations of 10-5M, 10-4M and 5 x 10-3M. In order to better define a possible role of the accompanying cation same experiments were repeated replacing the phosphate by the chloride ion. The effect of the internal R status of the plant both of he upake and translocation was studied using respectively roots excised from plants previously grown in phosphate solutions (10-4M, 10-3M, and 5 x 10-3M) and whole plants (0, 10-4M, 10-3M and 10-2M). The influence of phosphatic fertilization was assessed by supplying varying rates of R (0, 200 and 400 ppm) plus other macro and micronutrients in standard amounts to three soil types (Regosol, Reed yellow Latosol sand phase and Reed Latosol) in the presence and in the abscence of lime. In all experiments 65Zn was used as tracer. The main conclusions were the following: a. all salts used in soilless experiments reduced Zn uptake by non competitive inhibition; b. the size of the effect is pH dependent; c. inhibition is greater when either R or Cl are accompanied divalent cation; d. effect is higher than Cl's; e. Precipitation of Zn by R in soilltss mtdium may take place depending upon concentration and pH, the phenomenon occurring either in them edium it self or at the surface; f. previous phosphatic nutrition did not decrease Zn uptake having however, slight, negative effect on translocation when high levels of the anion were used in the pretreament combined with low concentrations of the micronutrient during the experimental period; g. phosphatic fertilization and liming decreased Zn concentration in the olant having no lowering effect on its availability in the soil as revealed by a chemical method, the first variable actually increasing absorptionas determined by the "L" values thereof.