Possible mechanism of medium-supplemented thiourea in improving growth, gas exchange, and photosynthetic pigments in cadmium-stressed maize (Zea mays)

Cadmium (Cd) stress is highly damaging to plant growth but its toxicity can be alleviated with exogenous supply of growth promoters. In this greenhouse study, we determined the effectiveness of medium-supplemented thiourea (TU) in improving growth, leaf gas exchange, and photosynthetic pigments contents. Pak-Afgoi (Cd-tolerant) and EV-20 (Cd-sensitive) maize (Zea mays L.) varieties were grown in pots containing sand supplemented with nutrient solution for 30 days. The plants were treated with 1,000 mu M or without Cd. On the appearance of Cd-toxicity symptoms (in about 10 days), a pre-optimized level of TU (0.25 mM) was supplemented in the rooting medium, and plants were allowed to grow for further 15 days. Data for various growths, leaf gas exchange characteristics, chlorophyll (Chl), and carotenoids (Car) contents were recorded in spring (February-April) and autumn (August-October) seasons. Results showed that Cd-stress significantly decreased length, fresh and dry weight of shoot and root, number of green leaves and leaf area per plant, net photosynthetic rate (P-n), transpiration rate (TR), stomatal conductance (g(s)), Chl-a, -b, Chl-a-to-b ratio, and Car but increased substomatal CO2 concentration (C-i) in the varieties in both the seasons. Although Cd was toxic to maize, medium-supplemented TU nullified the Cd-toxicity as was evident from improved growth, P-n, g(s), and Chl-b and Car contents in both varieties. Overall, 0.25 mM TU level was quite effective in reducing the Cd-toxicity on both the maize varieties; especially, sensitive maize more profoundly responded to Cd-toxicity under medium supplementation of TU. Cd-tolerance produced by TU was superior in the autumn-than in spring-grown maize. From the reduced Cd content of leaf and root, the results suggested a possible role of TU in reducing Cd-availability to the root and its transport to shoot. Presence of thiol group in TU appears to be important to Cd-binding/inactivation, and thus its tolerance by maize.