The distribution of metals from wastewater in the Mexican Valley of Mezquital

Since the beginning of this century, wastewater has been utilized for crop production in the Mexican Valley of Mezquital; however, few data are available for evaluating the level of heavy metal pollution in that region. Waters, soils and plants were sampled in different sites, representing three specific irrigation sources in the valley. Trace metals were determined in all samples. Total amount of most trace metals in raw municipal wastewater, and that of Ni, Cr, Pb and Zn in diluted water were quite high. Wastewater irrigation markedly increased the amounts of trace elements extracted from soils by 4M HNO₃; the amounts of metals were proportional to the amount of wastewater applied. Copper, chromium, and lead concentrations in soils roughly paralleled their organic matter contents. A wide range in tissue concentration of metals was observed. Zinc was the most metal absorbed by alfalfa and oats. Levels of Ni and Pb in plants, and to some extent, those of Co and Cu, were increased by raw wastewater irrigation. Chromium and lead accumulations in alfalfa were well above the tolerance levels suggested by some workers and the high Co concentration found in this legume may be a health risk for animals and human beings.     

Epicuticular wax content and morphology as related to ethylene and storage performance of ‘Navelate’ orange fruit

The effect of ethylene (2 μL L⁻¹) on total and soft epicuticular wax content and wax morphology has been investigated in mature ‘Navelate’ (Citrus sinensis, L. Osbeck) oranges held under non-stressful environmental conditions (22 °C and constant high relative humidity (90–95% RH)). In addition, the objective of the study was to understand whether the ethylene-induced changes in epicuticular wax might participate in the beneficial effect of ethylene reducing non-chilling peel pitting, by modifying peel water, osmotic or turgor potential, or disease incidence caused by Penicillium digitatum (Pers.:Fr.) Sacc. Ethylene increased total and soft epicuticular wax content in ‘Navelate’ fruit and induced structural changes in surface wax that might be related to the formation of new waxes. Changes in epicuticular wax morphology, but not in its content, might be involved in the protective role of ethylene reducing non-chilling peel pitting, although the beneficial effect of the hormone is not related to water stress. Cell water and turgor potentials in freshly harvested fruit and fruit stored in air under non-stressful conditions suggest that water stress is not a limiting factor leading to the development of this physiological disorder. In addition, the results indicated that formation of new waxes in fruit treated with ethylene may partially cover stomata, cracks or areas lacking wax occurring in stored fruit and is likely to improve physical barriers to P. digitatum penetration.