Brassinosteroids protect Lycopersicon esculentum from cadmium toxicity applied as shotgun approach

Surface-sterilized seeds of two tomato cultivars (cv. K-25 and Sarvodya) were soaked in 100 mu M CdCl2 for 8 h (shotgun approach). The resulting 59-day-old seedlings were sprayed with 10(-8) M of 28-homobrassinolide (HBL) or 24-epibrassinolide (EBL) to their foliage. Both cultivars showed significantly different response to Cd stress. Cadmium severely restricted the growth, photosynthetic efficiency, and activity of nitrate reductase (E.C. 1.6.6.1) and carbonic anhydrase (E.C. 4.2.1.1) in Sarvodya as compared to K-25. However, the activities of antioxidative enzymes were significantly higher in K-25. This result may be considered an indication of better tolerance of the K-25 cultivars to Cd stress. Moreover, the spray of both the brassinosteroids (HBL/EBL) were found very effective in neutralizing the adverse effects generated by metals that reflect in better photosynthetic performance by the cultivars. An interesting aspect of this study is that HBL or EBL spray caused a further increase in proline content and antioxidative enzyme activities, which were already enhanced by Cd stress. This effect of brassinosteroids (HBL/EBL) was more pronounced in K-25 than in Sarvodya, representing the tolerance and adoptable behavior of K-25.