| Benazolin-ethyl is a kind of herbicides used to control the broadleaf weeds in the field of oilseed rape(Brassica napus L.), developed by a company named Agrevo in Germany. It is widely used to control the broadleaf weeds in domestic market. However, there are no systematic and profound researches on its effects on the growth of B. napus L. and the tolerance of different cultivars of B. napus L. Therefore, it is necessary to know the physiological mechanism of this herbicide and its safety effect on B. napus L.In this study, two cultivars of rapeseed (B. napus L. cvs. Zheda619and Zheda622) were investigated to clarify their different tolerance mechanism to benazolin-ethyl stress by spraying various concentrations [0(control),2.5,5.0(recommended concentration sprayed in the field),10,20g/L] to rapeseed seedlings, in order to understand physiological mechanism of benazolin-ethyl regarding various physiological measurements, such as chlorophyll fluorescence measurements and microscopic analysis. The main results are as follows:low concentration (2.5g/L) of benazolin-ethyl had no effect on B. napus L. cvs. Zheda619and Zheda622. The recommended concentration (5.0g/L) of benazolin-ethyl had slight inhibiting effect on the B. napus L. plants, especially on cultivar Zheda622with its leaves slightly being curly. The10g/L concentration of benazolin-ethyl revealed markedly adverse effect on B. napus L. cvs. Zheda619and Zheda622, with their leaves being seriously curly. High concentration (20g/L) of benazolin-ethyl exhibited terribly toxic effect on these two rapeseed cultivars, with their leaves and stems being seriously curly and wilting, their tissue swelling. In addition, the toxic symptoms on cv. Zheda622were heavier than cv. Zheda619.We found that it gradually inhibited the growth of B. napus L. plants, decreased the net photosynthetic rate, chlorophyll content, the chlorophyll fluorescence value, antioxidants enzymes (SOD, POD, APX, CAT) activities, reduced the content of soluble protein, glutathione (GSH), and increased the content of oxidized glutathione (GSSG), auxin (IAA), malondialdehyde (MDA), hydrogen peroxide (H2O2), and superoxide anion free radical (O2-) when the concentration of benazolin-ethyl ranged from2.5g/L to20g/L. Thus we can conclude that the stressful mechanism is that excessive benazolin-ethyl disturbed the photosynthetic system, chlorophyll fluorescence system, antioxidants enzymes system. Consequently, MDA, GSSG and reactive oxygen species such as H2O2and O2-were accumulated, thus damaging the protein, membrane lipid and other compositions of the cells, making the content of soluble protein and GSH decreased and eventually inhibited the seedling growth.In order to further study the effect of benazolin-ethyl on ultrastructure of leaves of rapeseed seedling, we found that the structure of the nucleus, chloroplast, mitochondria was clear in the control mesophyll cells, and the membrane of the chloroplast was also undamaged, thylakoid was lined up and fold tightly, starch grains were abundant, mitochondria appeared to be relatively sphere or oval, the construction of the membrane of the mitochondria was integral, the cristae of the mitochondria were clearly visible. However, the chloroplast distort, its membrane system was damaged and broken up, vacuolization aggravated, its thylakoid arranged disorderly and patchily, starch grains become less in the mesophyll cells when were sprayed20g/L concentration of benazolin-ethyl, moreover, the mitochondria expanded deformatively, its outer membrane looked fuzzy, the number of the crest was decreased. Thus the high concentration of benazolin-ethyl exhibited significantly toxic effect on the organelles of mesophyll cell in B. napus L. |
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