| Cowpea is an important crop in the south of China. Continuous cropping is becoming the bottleneck of the cowpea production. Some problems have not yet been answered, including the reasons which can account for the obstacles caused by the continuous cowpea cropping, the autotoxins which lead to the obstacles, the relations between the autotoxins and cowpea growth. The purpose of this study is to demonstrate the role of autotoxicity played in the obstacles caused by continuous cropping by using activated carbon, to identify the autotoxins in the soil under continuous cropping, to demonstrate the role of the autotoxins played in the obstacles caused by continuous cropping by the addition of activated carbon and cinnamic acid, and then to clarify the mechanism of the major autotoxins. Through addressing these issues, we will provide consultation for clarifying the mechanism of obstacle caused by continuous cropping and for controlling the obstacles. Main results of this study are as follows:Activated carbon can absorb the autotoxins in the soil, so we investigated the role of autotoxins in the obstacles caused by continuous cropping by the addition of the activated carbon into the soil. The results showed:the autotoxins in the soil under continuous cropping remarkablely decreased the root vigor, dry weight of the root and shoot dry weight and stem diameter. The autotoxins also affected the number of plants, yield per plant and total yield. When the cowpea continuous cropping for 8 years was treated by activated carbon, the yield per plant, number of trees, the total yield increased seriously compared with that not treated by activated carbon, but lower than the rotation cowpea. Total yields of the cowpea continuous cropping for 8 years, the continuous cropping cowpea treated by activated carbon, the rotation cowpea treated by activated carbon are 31.9%,68.9% and 93.6% of that of the rotation cowpea, respectively. The plant growth and the yield of the rotation cowpea were not seriously different when treated by activated carbon, which showed that the activated carbon itself did not affect the cowpea. The results demonstrated that there are autotoxins, which lead to 38.0%-47.4% of the yield loss caused by continuous cropping, in the soils. Activated carbon can partially eliminate the influence caused by the autotoxins.We looked into the phytotoxicity and chemical composition of three rhizosphere soils used in the continuous cropping of cowpea for 0,4, and 8 years, respectively. The concentration and phytotoxity of four phenolic acids identified from the continuous cropping soils were examined, and the cowpea root extracts was identified. Compared with the distilled water and extract of soils from the continuous cropping of cowpea for 0 years, the extracts of rhizosphere soils from continuous cropping for 4 and 8 years significantly inhibited seed germination rates and shoot growth of cowpea. We detected 25,27, and 28 principal chromatographic peaks in the soil extracts of continuous cropping cowpea for 0,4, and 8 years, respectively by using GC/MS. Twenty-one compounds were found in all three extracts, including phenolic acids, organic acids, esters, ketones, and hydrocarbons. With the increase in the years of cropping, the contents of phenylacetic acid, cinnamic acid,4-hydroxybenzoic acid increased, whereas that of squalene decreased. HPLC analysis showed that phenylacetic acid, cinnamic acid, 4-hydroxybenzoic acid and the phthalic acid were accumulated to the inhibitory concentrations in the soils of continuous cropping cowpea. These phenolic acids partially came from the root extracts of cowpea, though most of the phenolic acids from the root extracts were not detected in the cowpea soils. In conclusion, the existence of autotoxicity in soils under continuous cowpea cropping was verified. Moreover, the accumulation of the four phenolic acids (i.e., phenylacetic acid, cinnamic acid, 4-hydroxybenzoic acid, and phthalic acid) was one of the important reasons for the obstacles caused by the continuous cropping of cowpea.By the addition of the exogenous cinnamic acid and activated carbon, we realized the different levels of cinnamic acid in the soils to study the role of cinnamic acid in the obstacles caused by continuous cowpea cropping. The results showed:cinnamic acid lead to peroxidization and leakage of the plasma membrane, and inhibited the root vigor. When the level of cinnamic acid increased, the chlorophyll content, the Mg, Zn, B and K content of the leaf decreased, but the MDA content and the membrane permeability increased. When the level of cinnamic acid decreased afer the addition of the activated carbon, dry weight and vigor of the root, the chlorophyll content, the Mg, Zn, B and K content of the leaf increased, the MDA content and the membrane permeability decreased. Cinnamic acid decreased the number of plants, the yield per plant and then the total yield.The effects of cinnamic acid on photosynthetic and chlorophyll fluorescence characteristics of cowpea seedlings leaves were studied under nutrient solution conditions. The results showed:under treatments of 0.1 mmol·L-1 and 1 mmol·L-1 cinnamic acid, the photosynthetic rate of cowpea seedling leaves decreased significantly compared with the control. Intercellular CO2 concentration of cowpea seedlings treated with 0.1 mmol·L-1 cinnamic acid showed no significant difference throughout the whole experiment, while that of cowpea seedlings leaves treated with 1 mmol·L-1 cinnamic acid increased significantly after 5 days of the treatments. The cinnamic acid treatments reduced carboxylation efficiency, RuBPC activity and photorespiratory rate of cowpea seedlings leaves, but did not change the relative chlorophyll content, the maximal photochemical efficiency of PSII (Fv/Fm), the initial fluorescence yield (Fo), the photochemical quenching (qp) and the electron transmit rate (ETR) within 9 days after treated with cinnamic acid. It could be concluded that the decrease of photosynthetic rate under cinnamic acid treatments were attributed to the decreased RuBPC activity, not to chlorophyll content, photoinhibition or ETR.We investigated the effects of cinnamic acid (CA) on ribulose-1,5-bisphosphate carboxylase (RuBPC) activity and the endogenous polyamine levels of cowpea leaves. The results show that 0.1 mmol-L"1 CA treatment decreased photosynthetic rate (Pn) and RuBPC activity, but it did not affect the maximal photochemical efficiency of PSⅡ(Fv/Fm), the actual photochemical efficiency of PSⅡ(ΦPSⅡ), intercellular CO2 concentration (Ci), and relative chlorophyll content. These suggest that the decrease in Pn is at least partially attributed to a lowered RuBPC activity. In addition,0.1 mmol·L-1 CA treatment increased the putrescine level, but decreased spermidine (Spd) and spermine (Spm) levels, thereby reducing the (Spd+Spm)/Put (PAs) ratio in the leaves. The exogenous application of 1 mmol·L"1 Spd markedly reversed these CA-induced effects for polyamine and partially restored the PAs ratio and RuBPC activity in leaves. Methylglyoxal-bis (guanylhydrazone) (MGBG), which is an inhibitor of S-adenosylmethionine decarboxylase (SAMDC), results in the inability of activated cells to synthesize Spd and exacerbates the negative effects induced by CA. The exogenous application of 1 mmol·L-1 D-arginine (D-Arg), which is an inhibitor of Put biosynthesis, decreased the levels of Put, but increased the PAs ratio and RuBPC activity in leaves. These results suggest that 0.1 mmol·L-1 CA inhibits RuBPC activity by decreasing the levels of endogenous free and perchloric acid soluble conjugated Spm, as well as the PAs ratio.Conclusion:autotoxins inhibit the growth and yield of the continuous cropping cowpea; the autotoxins include phenylacetic acid, cinnamic acid,4-hydroxybenzoic acid, and phthalic acid; cinnamic acid decreases the Pn, by inhibiting the RuBPC mediated through decreased spermine and changes in the ratio of polyamines in cowpea.
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