ABSTRACT The least limiting water range (LLWR) has been extensively determined, but evaluating if LLWR can indeed indicate soil physical stress on plant growth is still a controversial issue. In this study, we used the Hydrus-1D hydrological model to simulate root water uptake (RWU) to analyze if RWU and LLWR are correlated under stress conditions. The LLWR was determined in a sandy-loam Ultisol and a clayey Oxisol. In both soils, RWU extracted by plants (leaf area index set as 3) from a rooted layer of 0.4 m was simulated over 20 days under a potential evapotranspiration rate of 6 mm day-1. For each soil, RWU was simulated over the same range of soil compaction in which LLWR was determined. The cumulative RWU over the 20 days varied between 23 to 58 mm in the Ultisol and 20 to 48 mm in the Oxisol, indicating that plants were able to take up only a small part of the cumulative potential transpiration (93 mm) and experienced severe water stress in some soil conditions. However, RWU under water stress was poorly correlated with both bulk density and LLWR. The correlation between RWU and LLWR was 0.5 (p<0.01) for the Ultisol and 0.22 (p<0.19) for the Oxisol, suggesting that LLRW has little (for Ultisol) or almost no (for Oxisol) ability to indicate soil quality related to plant water availability. Our simulations suggest that RWU in the water availability range (between field capacity and wilting point) may be little affected or even improved by light soil compaction. Studies to elucidate this phenomenon would contribute to the understanding of the compaction effect on RWU and the weak correlation between RWU and LLWR.

A alface (Lactuca sativa L.) destaca-se como a espécie mais produzida em sistema hidropônico. Nesse tipo de cultivo, a maior parte do nitrogênio é fornecida na forma de nitrato, o que pode acarretar acúmulo deste íon nos vacúolos celulares. O teor de nitrato nas plantas depende de fatores genéticos, da disponibilidade desse nutriente no sistema radicular e de fatores ambientais. Diversos estudos indicam que o consumo de nitrato pode ser nocivo à saúde humana, causando metahemoglobinemia e câncer gástrico. Porém, pesquisas recentes discorrem que não há ligação entre nitrato e as doenças citadas, destacando ainda o papel benéfico do nitrato à saúde humana, como protetor contra gastrenterites. Existem indícios de que a presença de certas vitaminas em vegetais inibe uma possível ação negativa do nitrato. Altas concentrações de nitrato em alface cultivada sob hidroponia em países do norte europeu causaram apreensão da comunidade científica e de consumidores brasileiros. Porém, as pesquisas realizadas no Brasil demonstram que, por haver maior disponibilidade de radiação solar, devido às menores latitudes, os teores de nitrato encontrados em alface produzida em hidroponia são bastante inferiores aos limites máximos preconizados pela união européia, indicando não haver risco à saúde humana.

Lettuce (Lactuca sativa L.) is the main vegetable crop produced hydroponically. In this kind of cropping, the majority of nitrogen is provided as nitrate, which may lead to the accumulation of this ion in cells. Nitrate amount in plants depends upon genetic factors, the availability of this nutrient in roots and environmental factors. Several studies have indicated that the consumption of nitrate may be harmful to human health, causing methaemoglobinaemia and gastric cancer. However, recent studies have proposed that there is no link between nitrate and these diseases, and that nitrate may be even good for human health as a protector against gastroenteritis. Some other studies have indicated that the presence of certain vitamins in plants is likely to inhibit possible negative effects of nitrate. High nitrate concentrations in lettuce grown hydroponically in northern European countries have been a matter of concern to the scientific community and Brazilian consumers. Nevertheless, studies have demonstrated that in Brazil, because of higher solar radiation at low latitudes, nitrate amounts found in lettuce produced hydroponically are much lower than the amounts given by the European community, indicating no risk to human health.