Critical Toxicity Level of Arsenic and Elemental Composition of Arsenic-Induced Chlorosis in Hydroponic Sorghum

Abstract

Physiological and mineralogical responses of sorghum (Sorghum bicolor L. cv. Fast sorghum) in hydroponic culture at elevated concentrations of arsenic (As) were evaluated. Seedlings were grown in the presence of 0, 6.7, 33.5 and 67 µM As levels (0, 0.5, 2.5 and 5 mg As l−1) up to 14 days after treatments (DAT). Shoot and root dry matter yield were repressed by higher As levels. At low As level (6.7 µM) shoot dry matter yield was enhanced by 2.3% but at 33.5 and 67 µM As levels, the yield decreased by 52 and 79%, respectively. The root growth was similarly enhanced (8%) at the lower As level while the growth decrement at the higher As levels were 33 and 68%, respectively for the two treatments. Considering 10% dry weight (DW) reduction, the critical toxicity level of As was calculated to be 11.7 µg g−1 DW for shoot and 367 µg g−1 DW for root, indicating that shoot was more sensitive to As-toxicity than root. A whitish chlorotic symptom was observed in the fully developed young leaves at the 67 µM As level. The lowest chlorophyll content was also observed at this As level. Arsenic concentration increased both in shoot and in root with increase in solution As concentration. The concentrations of As and Fe were about 16, 28 and 17 times; and 2, 25 and 144 times higher in root than shoot at 6.7, 33.5 and 67 µM As levels, respectively. The concentrations of K, Fe and Cu were significantly lower while Ca, Mg and Mn concentrations were higher in the shoot at the 67 µM As level compared to the control plants. On the other hand, Fe, Mn, Zn and Cu concentrations were higher in root at the 67 µM As level. In the shoot, accumulation and translocation of metal micronutrients, particularly that of Fe, decreased significantly because of the presence of As. The present observations suggested that As might induce a toxic effect on sorghum by hampering the translocation of the metal micronutrients. It is suggested that “As-induced Fe-deficiency” caused chlorotic symptoms in the hydroponically grown sorghum.