Arsenic stress effects on root water absorption in soybean plants: Physiological and morphological aspects

Soybean (Glycine max L.) is often cultivated in areas contaminated with arsenic (As), which negatively affects plant growth and reduces crop yield. The deleterious effects may be due, at least in part, to disturbances in the water status, as was reported for some plants exposed to heavy metals. However, to our knowledge, these mechanisms have not been studied in depth in soybean plants exposed to As. The aim of the present work was to analyze possible changes in water relations and the responses developed in soybean plants under arsenate (AsV) and arsenite (AsIII) stress. We discuss physiological and morphological aspects of the As stress response, such as root absorption rate, water content, stomatal conductance, water and osmotic potential, accumulation of compatible solutes, leaf conducting tissues and stomata characteristics. AsV and AsIII caused a significant decrease in root absorption rate, which could reduce metalloid uptake. On the other hand, water content decreased at the beginning of the treatment but was re-established after 4 and 8 d. This was correlated with a decrease in stomatal conductance and a reduction in leaf water and osmotic potential due to the accumulation of proline and soluble sugars. Besides, smaller leaf xylem vessels and abnormal stomata were observed in plants under As treatment. These mechanisms increased the plant's ability to retain water and therefore to avoid dehydration. Thus, the results of the present work contribute to the understanding of how soybean responds to As, by describing key tolerance strategies to the metalloid.