Effects of Herbicides, Associated to a Brassinosteroid Analogue, on the Photosynthetic Apparatus of Eucalyptus grandis Seedlings

The effects of the application of imazapyr and glyphosate and their interactions with a spirostanic analogue of castasterone (BB16), on the growth of seedling clones of Eucalyptus grandis were evaluated. Applications of BB16 (at concentrations of 0.08 and 0.16 mg L-1: BB16 (-) and BB16 (+) respectively). and of imazapyr (0.750 kg ha(-1)) and glyphosate (1.440 kg ha(-1)) were carried out. The treatments were the control, BB 16 (-), BB16 6 (+), glyphosate, imazapyr, besides the hormone-herbicide interaction, at an interval of 12 h between the BB16 and each herbicide application and vice versa. Chlorophyll a fluorescence was measured daily on the third completely - expanded leaf, as well as height, diameter and dry matter of the shoot (DMS) and roots (DMR). The control, BB 16 (-) and BB16 (+) differed from the other DMS treatments; however, for DMR, height and diameter did not show any significant difference among the treatments. The fluorescence parameter means obtained for the herbicide application treatments were higher than the control, BB16 (-) and BB16 (+) until the seventh day after treatment (DAT). The seedlings that received the glyphosate application associated, or not, to BB16(-) or BB16(+), exhibited necrosis before the seventh day, while the seedlings that received imazapyr associated to BB 16 showed only necrotic lesion at the extremity of the lateral branches. The herbicides analyzed caused deleterious effects on the photosynthetic apparatus of E. grandis. However, since the seventh DAT the seedlings that received the imazapyr application (with or without BB 16) did not differ from the control, BB16 (-) and BB16 (+) treatments, in relation to the fluorescence parameters.