| Growing conditions in the Xinjiang Uyghur Autonomous Region are ideal for the productionof processing tomatoes. However, tomato yield have declined in recent years due to thewidespread use of continuous cropping practices. This paper includes results from both pot andfield experiments. The objective of the pot experiment was to assay allelopathic potential inextracts from processing tomato plants and fruit. The objective of the field experiment was todetermine the effect of continuous cultivation on the growth, photosynthetic activity, metabolicenzyme activity, dry matter production, and yield of processing tomatoes. The study increasesunderstanding about allelopathy and auto-toxicity in continuously cropped processing tomatoes.The results can be used to develop recommendations for increasing tomato production in XinjiangProvince.Aqueous extracts from processing tomatoes were applied to seeds of four indicator species:cabbage, radish, cotton and wheat. The effect of the extracts varied among the indicator species.Furthermore the negative (i.e., allelopathic) effects generally increased as the concentration of theextracts increased. Extracts from tomato fruit had the greatest allelopathic effect, followed byextracts from tomato leaves, stems, and then roots. The differences among plant parts weresignificant (P<0.05). At concentrations of0.08g/ml, the inhibitive effect of tomato fruit extract oncotton seedling fresh weight were19.7%greater than the inhibitive effect of root extracts,23.4%greater than the inhibitive effect of stem extracts, and15.1%greater than the inhibitive effect ofleaf extracts. Aqueous extracts at concentrations of0.02g/ml had significant effects on the freshseedling weight of both cotton and wheat. Aqueous extracts at concentrations of0.04g/ml hadsignificant effects on the fresh seedling weight of turnip and cabbage.The effect of aqueous extracts on tomato germination, seedling growth, and enzymeactivity varied depending on the concentration of the extract. Root length, fresh weight,dry weight, and lateral root number of tomato seedlings height all decreased gradually asthe concentration of the extracts increased. The activities of superoxide dismutase (SOD)and catalase (CAT) all decreased as the concentration of the extracts increased.Peroxidase (POD) activity initially increased and then decreased as the concentration ofthe extracts increased. The malondialdehyde (MDA) content significantly increased as theconcentration of the extracts increased.When the concentration resches0.08g/ml, theindexes reach a maximal difference processing tomato roots, stems, leaves, and fruitextracts35.5%,47.9%,56.2%, and60.3%respectively.In addition, the root activity ofseedlings was less than that in the control treatment. Auto-toxic effects varied amongdifferent plant parts, even when the concentrations of the extracts were the same.Extracts from the above-ground parts of tomato plants exhibited greater auto-toxicitythan did extracts from the below-ground parts. Specifically, auto-toxic effects weregreatest in fruit, followed by leaves and then roots. Overall, these results indicate th at allparts of processing tomato plants exhibit auto-toxicity.Continuous cropping practices reduced the growth of processing tomatoes. Plantheight, stem diameter, root cap ratio, and biomass were all significantly reduced bycontinuous cropping practices. The largest negative effects were observed at thefruit-bearing stage in the7year continuous cropping treatment. Compared with the controltreatment, the7year continuous cropping treatment significantly reduced plantheight by26%, stem diameter by28%, the root cap ratio by10.7%, and biomass by34%(P<0.01). Boththe leaf area index and the leaf chlorophyll content decreased significantly as theduration of continuous cropping increased. These observations were true at all growthstages. Intercellular CO2concentrations from the seedling stage to maturity weresignificantly higher in the3to7year continuous cropping treatment than in the controltreatment. Continuous cropping significantly reduced photosynthetic rates (Pn),transpiration rates (Tr), stomatal conductance (Gs) values, and water use efficiency(WUE)(P<0.01). Furthermore, the negative effects increased as the duration ofcontinuous cropping increased. Continuous cropping reduced tomato leaf Pn and Gs butincreased tomato leaf Ci. These results indicate that leaf Pn was limited by stomatalfactors.Some negative effects of continuous cropping began at the seedling stage. The PODactivity increased and then decreased as the duration of continuous cropping increased.Similarly, CAT activity, SOD activity, and root activity decreased steadily as the duration ofcontinuous cropping increased. In contrast, MDA contents increased significantly as the durationof continuous cropping increased. Root activities between the seedling stage and maturity were42to68%lower in the7years of continuous cropping treatment than in the control treatment. Thesedifferences were highly significant (P<0.01). The negative effects of continuous croppingpractices on enzyme activity and MDA content were greater in roots than in shoots. These resultsindicate that continuous cropping causes the greatest harm to the roots of processing tomatoes.The dry weight of the processing tomatoes decreased as the duration of continuous croppingincreased. The dry weight was significantly lower in the7year continuous cropping treatmentthan in the control treatment. Continuous cropping reduced the translocation of storage materialsby57.44%, the metastasis rate by20.96%, and the contribution rate by35.56%. At the floweringand fruit development stage, the leaf area index in the control treatment was1.4times that in the7year continuous cropping treatment. The fruit weight-leaf ratio in the control treatment was1.25times that in the7year continuous cropping treatment. Dry matter production decreased as theduration of continuous cropping increased, eventually causing a decline in tomato yield.Continuous cropping for3,5, and7years reduced tomato yields by9.46,15.61, and33.94%,respectively. The results showed that continuous cropping practices are not good for theproduction of processing tomatoes. Continuous cropping is one factor contributing to recentdeclines in processing tomato yields. |
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